IRON CONSERVATION

The processes by which any metal artifact is cleaned is determined by the preliminary evaluation and only then can the appropriate conservation measures be taken. The conservation treatments accorded an object of iron will be discussed under the five main categories:

1. Electrochemical cleaning
a. Galvanic cleaning
b. Electrolytic reduction

2. Alkaline Sulfite

3. Chemical cleaning

4. Annealing

5. Water diffusion in alkaline solutions

ELECTROCHEMICAL CLEANING

The corrosion of metal, as explained earlier, is an electrochemical reaction. In turn, electrochemical and electrolytic reduction cleaning processes are the most common techniques utilized to halt, stabilize, and even reverse the oxidation of the metal. See Hamilton (1976:30-49) and North (1987:223-227) for a detailed discussion of electrolytic reduction. In the conservation literature, but not in the field of electrochemistry, it has been the general practice to distinguish between electrochemical and electrolytic cleaning. Both techniques, in fact, are electrochemical reactions based on the couple of two metals occupying different positions on the galvanic or electromotive series of the metals without an externally applied electromotive force (EMF) and the latter is an electrochemical reaction maintained by an externally applied EMF or electric current. Breaking with the convention established in the conservation literature, this publication distinguishes the two by the terms galvanic cleaning and electrolytic reduction or electrolysis. Both cleaning techniques are described in the literature. Plenderleith (1956) is a source often cited. While useful information is provided by Plenderleith and Torraca (1968) and Plenderleith and Werner (1971), a more thorough coverage is desirable to realize the maximum potential and benefits of these techniques.

GALVANIC CLEANING

This technique is the one most often described in the literature on the conservation of shipwreck material. For example, in two frequently cited works (Marx 1971:125; Peterson 1969:8384) on underwater archeology, it is the only technique recommended. For marinerecovered iron artifacts which are severely chloridecontaminated, electrochemical cleaning is not a desirable alternative. Even for metals from terrestrial sites, where chlorides present no real problems, it is often not desirable. I need only to repeat No‰l Hume's (1969:276)comment: "Those amateurs who have been told that it is a simple method that can readily be performed on the kitchen stove are advised to forget it."

To be effective, galvanic cleaning requires that a substantial metal core be present in the object being treated. In most circumstances this process can be recommended only if a few small objects are to be cleaned and if equipment for electrolytic cleaning is not available. Except for limited use, galvanic cleaning is best considered as an obsolete technique. Since it is used in some laboratories, however, it is briefly discussed. Some of its advantages and its major disadvantages are presented.

Galvanic cleaning involves placing the iron object in a vat and surrounding it with a much more active anodic metal, such as zinc or aluminum, and filling the vat with an electrolyte. In this process, nascent hydrogen acts as the reducing agent as it evolves from the surface of the metal. The chlorides are removed and reduced metal is left by the reaction. To be effective it is necessary to control the electrode potential of the artifact being treated. In galvanic cleaning this is determined by the couple between the two metals and by the electrolyte; unfortunately, it cannot be manipulated.

The simplest form of galvanic cleaning consists of wrapping the object loosely in aluminum foil and placing it in a glass beaker of 10% sodium hydroxide, NaOH (caustic soda), or a 10 to 20% solution of sodium carbonate, Na₂CO₃ (soda ash). No‰l Hume (1969:283) suggests using an electrolyte of baking soda (sodium bicarbonate, NaHCO₃) but tests in our laboratory using this solution have not been effective. The object is left in the caustic solution until the aluminum foil completely oxidizes. The reaction can be speeded up by heating the solution. The object is then rinsed and the process is repeated until satisfactory results are achieved. Only small, lightly corroded specimens can be treated in this fashion.

The most commonly used form of galvanic reduction differs from the above only in that granulated zinc or aluminum granules are used to cover the object and a 10 to 20% solution of sodium hydroxide is employed (Plenderleith and Torraca 1968:241; Plenderleith and Werner 1971:194197). Since heating accelerates the process, metal pans, or heat resistant glassware, are recommended. Ideally the solution is heated to boiling, and the level of the solution is maintained with the addition of distilled water. The cleaning continues until the electrolyte is exhausted or the activity of the zinc diminishes. The process is repeated with fresh or cleaned zinc and fresh caustic soda until all the materials are reduced and all traces of chlorides have been eliminated. (See section on qualitative test for chloride described later.) If high levels of chlorides are allowed to remain, future outbreaks of corrosion are inevitable.

The process requires facilities to heat the container, and an adequate ventilation system, such as a fume hood, to exhaust the caustic vapors. In the reduction process the caustic solution is exhausted and has to be periodically discarded, and considerable zinc is lost through oxidation. The activity of the remaining zinc is considerably reduced by an accumulated film of oxychloride and carbonate. To be renewed, the zinc must be cleaned with dilute hydrochloric acid solutions rinsed in distilled water and then dried (Plenderleith and Werner 1971:196) or melted in a ladle under a reducing flame and regranulated by slowly pouring into water (Organ 1973:193). In the latter, the reducing flame is going to have to be very good to remove oxychloride and carbonate and each time the zinc is reclaimed, slag formation will further reduce the quantity of zinc.

For iron artifacts, or any other metal with extensive chloride corrosion compounds, the galvanic cleaning process is impractical. The artifact is always obscured and it is not possible to follow visually its progress. Even under ideal conditions the process requires constant supervision and is messy. The fumes produced are obnoxious and are irritating to the skin, the eyes, and the throat. Also, it is often difficult to provide simultaneously adequate ventilation and adequate heat. Any large artifact, especially chloridecontaminated ones, require a long processing time and a prohibitive amount of zinc, which has to be constantly recleaned and replenished. For example, just the technical grade mossy zinc needed to clean a large iron object would be well in excess of $1,000. The caustic soda and, most importantly, the labor, would easily triple this figure.

In sum, galvanic cleaning of most metal artifacts should be considered only if equipment for electrolytic reduction is unavailable, and even then it may be a waste of time for a lot of iron artifacts, especially if they are very large.

ELECTROLYTIC REDUCTION CLEANING

The ease of setting up, maintaining and longrun economy of an electrolytic unit along with the versatility of electrolytic cleaning makes it one of the conservator's most valuable tools. It can be selected exclusively for its mechanical cleaning action of the evolved hydrogen, for the reduction process or, as usually is the case, a combination of the two. It can be used for most metal objects. Efficient electrolytic reduction, however, involves more than wiring up artifacts for electrolysis. A knowledge of corrosion processes and electrochemical thermodynamics is essential. One must be familiar with electrode potentials and pH, and how these variables relate to electrode corrosion, passivation, and immunity. These factors are particularly crucial when dealing with chloridecontaminated metals. This is not to say that satisfactory results cannot be obtained by the novice, but rather, that a good knowledge enables the conservator to understand and better control what is going on in the electrolytic cell and to correct adverse conditions.

The essence of the technique involves setting up an electrolytic cell with the artifact to be cleaned as the cathode. An electrolytic cell consists of a compartment or vat with two electrodes, the anode, and the cathode, and containing a suitable electrically conducting solution called the electrolyte. An electric current from an external direct current power supply is applied to cause oxidation and reduction. The anode is the positive terminal of the electrolytic cell, to which electrons, negatively charged ions, or colloidal particles, travel when an electric current is passed through the cell. Oxidation occurs at the anode and oxygen is evolved. The cathode is the negative terminal of an electrolytic cell to which positively charged metallic ions travel. At the cathode, reduction takes place and hydrogen is evolved. In the reduction process some of the positively charged metal ions in the compounds on the surface of the artifact are reduced to a metallic state in situ. In addition, chlorides and other anions are drawn from the specimen and migrate toward the positively charged anode by electrolytic attraction.

The chief advantage of electrolytic reduction is that the externally applied electromotive force (EMF), or electric current, provides the conservator with considerable flexibility. It allows him to control the current density. This control enables the conservator to select a low current density that creates a preselected electrode potential conducive for the consolidation and/orreduction of some mineralized metals. When there is an underlying metal core it is theoretically possible to reduce enough of the ferrous corrosion compounds back to a metallic state through electrolytic reduction. This will consolidate the corrosion layer, while eliminating the chloride components of the compounds. Likewise a high current density can be selected so that the evolved hydrogen will mechanically remove completely oxidized crust.

When using electrolytic reduction cleaning, the procedural factors to be considered are the equipment and the experimental variables.

1. Equipment

a. Power supplies

b. Terminal wires and clips

c. Anode material

d. Chloride monitoring

e. Vats

2. Experimental variables

a. Types of electrolytic setups

b. Electrolytes

c. Current densities

d. Electrode potentials

EQUIPMENT

D.C. Power Supplies

The regulated D.C. power supply requirements for electrolytic reduction are wideranging and a wellequipped laboratory should have several units of varying current capacities, each of which is capable of continuous operation. For example, the power units fall into four general current ranges and are capable of cleaning any object from the size of a small spike to a large anchor or cannon.

Most small D.C. power supplies have an output current with less than 0.1% ripple. The larger power supplies, battery chargers, have 0.5% or more ripple. For wellcontrolled reduction, the low ripple power supplies are recommended, however a little ripple is not harmful. The choice of a power supple depends upon the desired control, the size of the artifact, the number of artifacts on any one unit, the amount of current controls and an amperage meter. These provide a means of determining and adjusting the current as the treatment progresses. During electrolysis the current increases as the metallic species are reduced and the resistance of the object and the electrolyte decreases. The IR (current resistance) drop in the electrolyte is due to an increase in chloride and other ions. This is the main reason for the current increase; the decrease in the resistance of the object plays only a minor role. Therefore, variable adjustments are necessary if an object is to be electrolytically reduced at a fixed current density or a predetermined electrode potential.

Because of the expense of regulating power supplies it is not surprising that many conservation laboratories build their own power units. Foley (1967) and Organ (1968:291308) give directions on how to build an inexpensive power supply. Alternatively, battery chargers can be employed. Battery chargers, however, are not designed to run continuously for electrolytic cleaning. If they are to be utilized in electrolysis it is usually necessary to remove timers, relays and charging rate devices to make them serviceable. Anyone with a basic knowledge of electrical circuits can easily alter most direct current battery chargers for electrolytic cleaning of metal artifacts.

Since an electrolytic cell for cleaning artifacts lacks the resistance of a battery, and the battery chargers generally do not have the necessary internal resistance controls to compensate for it, the chargers run well above their maximum safe operating amperage. Additional resistance must be added to the circuits to keep them from overheating. Variable autotransformers such as Powerstats on the input alternating current line and variable rheostats or line resistors on the outgoing direct current negative terminals serve this purpose well.

Anyone with a basic knowledge of electronics can successfully modify battery chargers and other D.C. power supplies can be modified easily for the purposes of conservation. Over the years, I have modified a number of large 220V, threephase airplane battery chargers that were obtained through government surplus into serviceable electrolytic reduction power supplies. The easiest procedure is to add a variable autotransformer to the inputalternating current circuit. This allows the full amperage range of the unit, from 0 to 200 amps, to be used.

Terminal Wires and Clips

For most objects U.S. National Electric Code Standard 16 AWG, separation 2, 300V maximum rating insulated copper wire can be used. This wire is the standard twoply multistrand wire commonly found on many electrical appliances. The wire is quite flexible and easy to use. One strand of the wire is used for the negative connection, the other one for the positive connection; or they can be joined at the terminal ends to make one pole connection if a larger size wire is required.. For larger artifacts needing more current, No. 2 to No. 0 AWG multistrand wire is required. In all cases multistrand copper wire is recommended because it has a larger current capacity than comparable size solid wire and is more flexible and easier to manipulate. Before using any wire, one should check the amperage capacity of the wire, or, if in doubt, consult an electrician. A good rule of thumb to follow is that during electrolysis the wire should not heat up; if it does, the wire is not heavy enough.

To attach the terminal leads to the artifact and the anode, it is suggested that steel alligator test and battery clips (Mueller clips) be used. Appropriate size clips on the terminals facilitate setting up and taking down artifacts in electrolysis. These clips come in a variety of shapes and sizes. The size of the clip selected is determined by the current to be used, the size of the artifact, and placement of attachments. Mueller clips Nos. 25, 27, 48 and 85 are the most useful sizes and should be kept in stock. Steel clips usually are cadmium or zincplated and should be stripped of this plating by a quick bath in a dilute solution of hydrochloric acid. If left too long in the acid, the tempered steel spring is weakened and breaks when depressed. The removal of the cadmiumor zinc coating prevents any plating of this metal from the anode clips onto the artifact. For this same reason, copper clips should not be used. Copper clips on the positive terminal, like the exposed copper wire attached to the anode clips, eventually go into anodic dissolution when submerged in the electrolyte, and plates onto the cathode. In order to prevent the anodic dissolution, the exposed copper wire should be coated with an acrylic, polyvinyl acetate or silicone rubber.

Anode Material

For electrolytic cleaning of iron, 16gage expanded mild steel mesh with halfinch openings is an inexpensive but efficient anode material. This steel mesh is readily cut, is relatively flexible, is easy to formfit around the artifact, does not trap any gases, does not conceal the artifact from view, and permits free circulation of the electrolyte. Expanded mild steel mesh is preferable to solid mild steel sheets. While mild steel plates, and even sheets cut from mild steel drums, make serviceable and cheap anode material, their rigidity makes them more difficult to formfit around an artifact. Specially constructed mild steel vats, or even 55gallon steel drums, can serve as the container and the anode material. When a formfitted anode is desired, however, only expanded steel is cheap enough and flexible enough to use regularly.

Mild steel anodes are surprisingly durable. As long as an adequate alkaline pH (minimum of 8.5) is maintained at the surface of the mild steel anode, it is less susceptible to chloride corrosion and will even outlast stainless steel. To maintain this level of alkalinity, it may be necessary to circulate the electrolyte. Therefore, mild steel anodes are commonly used as the oxygenhydrogen cell electrodes (Worth Carlin 1975, personal communication).

It is claimed that stainless steel makes a superior anode and it is often recommended because it is relatively inert and seldom needs to be replaced. All stainless steels, however, do not make equally suitable anode material. A stainless steel with a high percentage of chromium and nickel or even titanium needs to be selected; particularly serviceable is type 316 stainless steel, which is composed of 16 to 18% chromium and 10 to 14% nickel and 2-3% molybdenum.

Only type 316 stainless steel resists Cl corrosion and is a good alternative to mild steel anodes in an alkaline electrolyte. All the other stainless steel anodes present problems, some considerably greater than that of mild steel. The expanded stainless steel mesh, like the expanded mild steel, is preferable. Stainless steel is very expensive and its use as anode material for large objects is usually prohibitive. Because of the expense, it is impractical to cut and formfit it to clean a single artifact; therefore, certain setup alternatives described below, for treating a number of artifacts at one time is the most practical way to use stainless steel anodes.

As long as the hydroxyl ion concentration in the electrolyte is kept high, one will get better service from mild steel anodes than stainless steel. In any case, when either is used, it is much simpler and more economical to change the electrolyte before the chlorides build up to such an extent that they alter the pH and electrode potential at the anode, destroying the anode's passivity and causing it to go into dissolution. When this happens, the anode has to be replaced. Regardless of whether stainless steel or mild steel anodes are used, the electrolyte should be regularly replaced, using a chloridemonitoring system as a guide.

Vats

A wide variety of containers can be used in an electrolytic setup. Many kinds of nonconductive vats of various caustic and acidresistant plastics such as polyvinyl chloride (PVC), polypropylene (PP) and polyethylene (PE) are also widely used. Also PVC plastic pipes with the ends sealed up make excellent vats for long slim artifacts such as rifle barrels. Fiberresistant plastics (FRP), unless one knows positively that they are alkaliresistant, should be avoided. Many FRP vats are not alkaliresistant and will break down. Even wooden vats or frames lined with sheets of PVC plastic can be quite serviceable as long as care is taken not to puncture the plastic. Glass containers are also often used. The variety and range of potentially satisfactory vats are obviously broad. With a little improvisation a number of containers for electrolysis can be readily and inexpensively obtained. For small artifacts, a common coffee can, with the can used as the vat and the anode, is as effective as anything much more elaborate and costing considerably more money.

In addition to nonconducting containers, conducting mild steel vats have a definite place in electrolyte cleaning. The metal vat serves as all or part of the anode and may be substituted for any of the electrolytic setups described in a section which follows. As an example of what can be done, we have constructed a twopiece 15 footlong mild steel vat designed to be used to clean anchors and other very large specimens. This "T"shaped vat is constructed of two parts, the stem and the cross, each of which is open at one end. When the parts are joined, the vat is used to clean anchors with auxiliary sheets of expanded mild steel near the top surface to achieve a better distribution of current. Separated, the two are employed to clean an assortment of large iron artifacts. After all of these objects were cleaned, the two vats were bolted together, placed on a "T" arrangement of stoves to rinse the guns, other breechblocks, and an anchor in alternate boilingcooling deionized water. The water was drained and replaced with microcrystalline wax for the final sealant. It was not necessary to dry the artifacts before heating up the wax, for the temperature of the wax is taken to over 300ø F which is well above the boiling point of water; thus all residual moisture is vaporized in the wax impregnation step.

Mild steel vats can be constructed in various gauges and are surprisingly durable and versatile, even in the lighter gauges. However, for maximum life, use a gage that provides the strength required, and does not increase the weight beyond your ability to handle it. Mild steel 55gallon drums, cut lengthwise or in half, make readily available, cheap vats which can be employed in any of the described setup alternatives in combination with auxiliary anodes to assure a more even distribution of current. Metal vats have a distinct advantage over plastic vats in that all stages of the conservation process can be carried out in them. This is especially advantageous for very large pieces, where it is not economically feasible to have different vats for electrolysis, rinsing/dehydration and wax impregnation.

For large artifacts, such as anchors, cannon, and many other artifacts, welded mild steel vat work quite well. They can be constructed very economically, and will last for years. Mylaboratory has several welded mild steel vats that have been in use for over 10 years with only minor repair of minor leaks. Several anchors and canons have been treated in them over the years, and in every instance the mild steel vat was used as the anode. In contrast, North (1987:225) states, "If mild steel tanks are being used on no account should these be made the anode, in the presence of Cl^- ions, this causes corrosion particularly at weld lines and bends in the metal." I agree that when corrosion takes place it is going be at the stress points such as weld seams and bends. After about 6 years, one welded mild steel vat, developed a series of leaks along the seams. I repaired it easily several time, and eventually gave it to another lab, who continued to use if for several more years. I am presently using several welded mild steel vats that are more than 10 years old. Considering the cost of acquiring a comparable vat made of stainless steel vat (which has the same problem as mild steel in the presence of Cl^- ions, as discussed above under anode material.) or of even various plastic vats, and rubberized liners, welded metal vats are clearly the better choice. In my opinion, if a mild steel vat self destructs in 10 year from using it as the anode, it can be easily replace many times and still save money over the alternatives. I can only speak from experience and I have been successfully using welded mild steel vats as the container and as the anode for nearly 20 years with great success and I recommend them to any one looking for a cheap, dependable vat. For the same reason, I also use and recommend to others the use of 55 gallon steel drums or barrels, and even coffee can or paint cans, for use in electrolysis. Thus North's recommendation should be ignored, but the precautions of not using a sodium carbonate electrolyte and using 5% sodium hydroxide instead of 2% sodium hydroxide for the first 2 or 3 baths when cleaning a large steel object with high chloride content should be taken.

In addition, it is difficult to understand how a metal vat can be isolated from the anodes, and even if it is done, the Cl^- ions are still present which will still attack the metal vat, but if the vat is not hooked up as the anode, it is not anodically passivated which affords it some protection. The recommendation by North is totally disregarded by most conservator responsible for treating large iron artifacts from marine sites.

Care must be taken to insure that the metal anode vats remain passive during electrolysis; otherwise, the metal will go into anodic dissolution and create perforations, which are difficult to repair. This is sometimes difficult when using low current density electrolysis in the presence of high levels of Cl^- ions; however most of the difficulties can be overcome if a 5% sodium hydroxide electrolyte is used until the chloride levels decrease or the current density can be increased to keep the anode passive.

A few comments should be made concerning the safety of using metal vats as the container and the anode. Most direct current power supplies used in electrolytic cleaning operate in a 6 to 12 or a 24 to 32volt range and a 0 to 50 or 0 to 200amperage range, but the actual voltage utilized is only 3 to 6 volts. At this voltage there is little personal danger in using metal vats. A good rule of thumb is that less than 32 volts is not hazardous because the IR (current resistance) drop in the human body is such that little or no current would pass. Care should be taken, however, to avoid shorting the two terminals of the higher voltage power supplies against each other.

CHLORIDE MONITORING

Crucial to the efficiency and success of electrolytic reduction of seabed antiquities is a system to monitor quantitatively the chloride concentration in the electrolyte. Several such tests exist. The Mohr method and ion specific electrode are often used, but the mercuric nitrate titration method is suggested because of its simplicity and low cost.

Mercuric Nitrate Method of Chloride Determination

The mercuric nitrate test is a quantitative method used to determine Cl or NaCl in parts per million in an aqueous solution. It is a quick and simple test to perform and gives accurate repeatable results. The following procedure is a modification of the "Compleximetric Titration of Chloride" as described in Volume 1 of Standard Chemical Analysis, edited by N. Howell Furman (1962: 331332).

Equipment

1.Two 25 ml. burets; preferably one automatic.

2.Two small amber bottles with droppers.

3. 250 ml. beakers.

4. A magnetic stirrer.

5. Tefloncoated stirring bars.

6. Two 500 ml. amber plastic bottles.

Chemicals

1. Diphenylcarbazonebromophenol blue indicator (keep a supply of No. 1 and No. 2 in amber dropper bottles).

2. 02N mercuric nitrate solution.

3. 18N sulfuric acid (keep a supply of No. 3 and No. 4 in amber plastic bottles to refill each buret).

4. Nitric acid to clean the stirring bars.

Procedure:

1. Take a 20 ml. sample of electrolyte or solution to be tested, and place it in a 250 ml. glass beaker.

2. Place the beaker on the magnetic stirrer and put a Teflon stirring bar in the beaker.

3. Adjust the stirrer until the liquid is in a steady swirl.

4. Add five drops of diphenylcarbazone-bromophenol blue indicator. This will change the color of the solution to blue.

5. Add drops of sulfuric acid (usually 18N for NaOH electrolyte, 9N for sodium carbonate electrolyte, or 4.5N for water solutions) from the buret or with an eye dropper until an acid end point is reached. The acid end point is indicated by a color change form blue to clear. The amount of sulfuric acid does not need to be measured, it only acidifies the sample for the next step.

6. Titrate .02N mercuric nitrate, drop by drop, from the automatic buret until the sample reaches a violet end point. The color changes gradually from clear to violet. Near the end point, each drop will show a flash of color. Continue until a single drops swirls into a single even violet color through out the solution. Additional drops make the color more intense, but do not change it.

7. Note the amount of mercuric nitrate titrated to reach the end point.

8. The concentration of chloride or sodium chloride in parts per million is calculated by the following formulas.

T x N x .05846 x 1,000,000

20 = T x N x 2923 = ppm NaCl

T x N x .03545 x 1,000,000

20 = T x N x 1772.5 = ppm Cl^-

T = amount of mercuric nitrate titrated.

N = normality of mercuric nitrate

The sensitivity of the titration can be increase by using a smaller normality solution or decreased by using a larger normality solution of mercuric nitrate. Adjustments can also be made for larger or smaller sample sizes. After mixing the new solutions, they should be calibrated against a known standard.

In order to facilitate the calculations a table, such as the example below, can be established using the formula above, and save much time in the future.

The NaCl readings are used to check the reagents against a known solution of sodium chloride.

The test gives the total amount of Cl or NaCl in the electrolyte. However, the unused alkaline electrolyte has a certain amount of chlorides in it. What needs to be measured are the chlorides expelled from the artifact. To do this, the same test is run on a sample of unused electrolyte. This provides a blank which is subtracted from the sample from the electrolytic bath.

Example:If a sample takes 4 ml. of mercuric nitrate to reach the end point, then 23.4 ppm Cl is present; if the blank is 17.5 (as in our lab) then the amount of chlorides derived from the artifact is 5.8.

Since the reagents are unstable they should be tested weekly against a known sodium chloride solution to assure consistency.

Comments on the Mercuric Nitrate Test

In taking the electrolyte samples, separate clean beakers should be used for each sample or the beakers should be rinsed with deionized water between each sample. Different stirring rods should be used with each sample or rinsed between samples. The stirring rods become stained purple but can be cleaned by putting them in dilute solution of nitric acid. Through this whole process, it should always be kept in mind that the glassware must be kept clean and uncontaminated at all times. After thoroughly washing the glassware, they should be rinsed with deionized water to avoid any cross contamination.

Two notes should be added in regard to the chloride testing procedure. First, the diphenylcarbazone bromophenol end point is to some degree subjective, but most individual are consistent about their end point. Therefore, more reliable and consistent results are obtained if only one individuals familiar with it should the tests.

All the chemicals at the exact concentration required for the chloride test can be purchased from a chemical supply house. Considerable expense can be saved, however, if the chemicals are mixed in the lab as follows:

18N Sulfuric Acid, H₂SO₄

Dilute reagent grade sulfuric acid with an equal volume of distilled or deionized water. Slowly add the acid to the water, never the water to the acid. Extreme heat is generated. Let cool.

.02N Mercuric Nitrate Solution, Hg(NO₃)₂ ^. H₂O

Dissolve 3.42 grams of reagent grade mercuric nitrate to 1 liter of distilled or deionized water.

Diphenylcarbazonebromophenol blue indicator

Dissolve .5 grams of reagent grade crystalline diphenylcarbazone and .05 grams of crystalline bromophenol blue in 100 milliliters of 95% ethanol. While this may be mixed, the weight are so small, that I always purchase it already prepared.

1000 ppm Sodium Chloride Solution to test reagents

Dissolve 1 gram of reagent grade sodium chloride in 1 liter of distilled or deionized water. Dilute in half for 500 ppm NaCl, dilute a second time for 250 ppm, etc.

It is possible to determine how the chloride removal is progressing by quantitatively measuring the chloride concentration in parts per million in a sample of the electrolyte, or any other solution. The chloride level should be regularly and frequently (at least once a week) calculated and recorded. These data can be used to make a graph that visually depicts at a glance the cleaning progress of an artifact. This will save much valuable time and enable the conservator to determine when to change a chloridecontaminated electrolyte and when all the detectable soluble chlorides have been removed from the object. Systematic chloride monitoring assures that each artifact remains in electrolysis the minimum amount of time.

Chloride monitoring is an essential aid in evaluating the efficiency of chloride removal from the artifact during treatment. It measures the chlorides going into solution from the artifact and is used to determine when this process is completed. Chloride monitoring cannot be used to determine the efficiency of the reduction of the iron compounds. This can only be confirmed with analytical tests on samples taken from the artifact before and after treatment.

The graph in Figure 12 presents the progress of a typical iron artifact recovered from a marine environment. It clearly depicts the high initial rise in the chlorides coming from the guns and the subsequent decreases as the electrolyte is periodically changed. Drops in the graph to the zero line represent electrolyte changes. The two week gap in the graph in November was when the guns were taken out of electrolysis to mechanically clean the remaining encrustation and loose corrosion products. Occasionally a drop from a previous high is seen on some chloride graphs, especially when the chloride level is high. Perhaps this is caused by chlorides reacting with corrosion products from the anode, chlorate formation, or the actual liberation of chlorine which causes the chlorides to be undetectable by taking them out of solution. Electrolysis is continued until the chloride concentration levels off for several days and does not increase above that present in a blank of the electrolyte.

EXPERIMENTAL VARIABLES

Types of Electrolytic Setups

The manner in which artifacts are set up for electrolysis is dependent upon 1) the size and condition of the specimens, 2) the backlog of artifacts waiting to be processed, 3) the number of available regulated direct current power supplies, 4) the current capacities of these units, and 5) the number, size and nature of the vats. Each alternative has its disadvantages and advantages (Hamilton 1973. 1976).

The "Ideal" electrolytic setup (Figure. 13, A) consists of one artifact, surrounded by a close, formfitted anode that is equidistant from all surfaces of the artifact, in one vat and on one regulated direct current power supply. With this setup a conservator is able to control precisely the current to the artifact and to maintain a predetermined electrode potential conducive to metal reduction on the surface of the specimen. The setup also allows one to monitor the chloride level in the electrolyte as it comes out of a single artifact. This option is selected when it is important to the optimum conditions to preserve those artifacts that are especially significant and need to be conserved as carefully as possible. Because a separate power supply is necessary for each artifact, this setup is not used for processing the average artifact.

The Type 2 electrolytic setup (Figure 13, B) has several artifacts in one vat, but each is surrounded by its own close, formfitted anode, and each is on a separate direct current power supply. When this arrangement is used, it is important to make sure that the distance between the different anodes is greater than the distances between the artifact and its anode in order to prevent any crossover current. (This point is not graphically depicted in Figure 13, B.) With this setup the current flow to each artifact can be carefully controlled and the correct electrode potential maintained. Since the chlorides present in the electrolyte come from all the artifacts in the vat, it is not possible to determine exactly when any one is free of chlorides. The chloride test, however, does tell the conservator when to change a chloridecontaminated electrolyte and when all specimens have been cleaned.

If a specimen requires close supervision, for example, to consolidate the metaloxide interface, or to preserve some surface or structural detail, it is advisable to hook it up in one of the two methods described above. The most critical variable for precision control is the ability to maintain an even current density on the cathode surface by positioning the anode equidistant from all parts of the artifact while maintaining a steady reduction electrode potential. Monitoring the chlorides during electrolysis is of less importance.

There are any number of ways of connecting multiple artifacts to a single power supply and some form of this setup is the one most commonly used in electrolytic reduction. Regardless of how it is done, Type 3 electrolytic setups (Figure 13, C & D) are the least desirable from the standpoint of control, but they have the advantage of processing a number of objects at one time on one power supply in one vat. In one variety (Figure 13, C), each piece is individually connected to the negative terminal of a single power supply. The artifacts share common anode sheets placed above and below the specimens. An alternative variation of this setup has a common bottom anode and individual topform-fitted anodes to assure a more even distribution of the current to each artifact.

The most popular variety of this same setup (Figure13, D) is to suspend artifacts from a brass cathode rod conductor, with adjustable vertical anode sheets hung to either side, and preferably another anode along the bottom of the vat. This setup is sometimes referred to as the "sandwich setup". The oxygen evolved off the bottom anode sheet assures that the solution is continually mixed, thus preventing chlorides from concentrating along the bottom of the vat. The increased circulation also helps to maintain the anodes in a passive state by preventing strongly oxidizing, acidic hypochlorite from forming at the anode.

The Type 3B setup (Figure 13, D) is the one most commonly described in the literature (Plenderleith 1956:194196; Plenderleith and Torraca 1968:243; Plenderleith and Werner 1971:198). This setup has all the disadvantages discussed above. In addition to the varied resistance of each artifact, the proximity of the object to the negative terminal connection is a factor in the current flow. The closer an artifact is to the negative terminal the more current it receives. If one uses this setup, it can be improved by regularly repositioning the artifacts so that for the duration of the treatment, each object receives an average current. This alternative has the advantage of making it possible to process a number When of specimens on one power supply in one vat. This consideration is important when limited facilities are available to clean a great many small articles.are available to clean a great many small articles.

When a metal vat is used as the anode, it is often advantageous to have auxiliary, adjustable expanded steel anode sheets hung along the sides and at the top to adjust for differences in the size of artifacts. When plastic vats are used, it is desirable to have a bottom anode because the oxygen evolved from the bottom assures that the solution is continually mixed, thus preventing the tendency for molecular chlorides and hypochlorite to concentrate along the bottom or at the anodes.

Other refinements can be made to improve this workhorse alternative setup for electrolytic cleaning. Most descriptions (Plenderleith 1956:195) recommend that three brass rods be suspended across the top of the vat, vertical sheets of steel be hung by copper wire from the side rods, and artifacts be suspended by copper wire from the center rod. It is much simpler to use expanded steel sheets or stainless steel sheets which extend up to the top of the vat and are bent and extended over the ends of the vat. This eliminates the two brass anode bars and the copper suspension wires which would in any case go into anodic dissolution and plate the cathode. The anode sheets are attached to the positive terminal with a Mueller clip. Since the copper wire attached to the clip is out of the solution it does not go into anodic dissolution and disconnect from the clip, which is a common occurrence on anode connections submerged in the electrolyte.

Utilizing the technique of suspending the artifacts by copper wire wrapped around then and the cathode rod is not effective. Inevitably, at some point electrical contact will be lost. It is more desirable (at least for most small specimens) to attach them with doubleended clips. Such clips can be purchased or made by bolting the ends of two clips together. The clips apply a constant pressure and assure maintenance of a secure contact on the cathode rod and on the artifacts. With wrappedcopper wire, a good contact is difficult to maintain. The clips also facilitate attaching and removing artifacts without unnecessary difficulty.

All of these varieties have the disadvantage of being unable to regulate the current to each artifact. Therefore, since the electrode potential or the current density cannot be controlled, the possibility of reducing the appropriate corrosion compounds back to a metallic state is considerably lessened. It is also impossible to monitor the chloride loss from any one artifact.

There is also a number of setups that involve a number of different artifacts, each in its own vat, but all connected to a single power supply. In Figure 5, E an example is shown using a mild steel or stainless steel vat with compartments; the vat is connected to the positive terminal and serves as the anode. An artifact can be placed in each of the compartments. When more than one compartment is used, the current to each artifact cannot be controlled, but the chloride level of the electrolyte in each of the compartments can be monitored. This approach takes advantage of limited facilities by using one direct current power supply for several artifacts. It also enables the conservator to determine exactly when each object is cleansed of chlorides, thus keeping the length of treatment of any one to a minimum. When an artifact is completed, any compartment can be taken down and set up without disturbing the electrolytic treatment in the other cells.

Another variation, being used by a number of conservation laboratories, is a single power supply hooked to a control panel consisting of a number of amperage gauges and rheostats. Each artifact is placed in a separate vat, and the current to it from the single power supply is regulated by means of the rheostat wired in line to it. Each artifact or vat has a gage that shows the amperage going to it. This particular setup can be used to regulate the current to a number of artifact from a single power supply and it has no particular disadvantages if the number of artifacts being treated is kept reasonable. However, a number of laboratories have adopted a practice of treating a number of marine iron (20 or more) from a power supply that can only put out 20 amps. The current to each artifact is in the milliamp range. The result is that is largely cosmetic; a lot of artifacts ar put into treatment at the same time, but the time to finished a single artifact is dramatically increased. In fact the situation or question arises of whether or not all the chlorides can actually be removed during this exclusively low current density electrolysis. I strongly recommend, from a multitude of experience, that whatever setup is utilized, that a range of current densities be used, and at some point in the treatment, it necessary to have a steady evolution of hydrogen from the artifact. Otherwise, this setup is a waste of time. See the discussion on current densities.

ELECTROLYTES

The only two electrolytes commonly used in conservation for treatment of iron objects are alkaline solutions of sodium carbonate (Na₂CO₃) and sodium hydroxide (NaOH), the most alkaline electrolyte one can get. In all cases it should be kept in mind that the alkalies (and acids) used in conservation should be concentrated enough to do the required job, but no stronger. This avoids over cleaning the specimen and helps keep the operating cost as low as possible. In recent years there has been a switch from a general use of sodium hydroxide to sodium carbonate. In his first description of electrolytic reduction, Plenderleith (1956:194) recommended a 5% solution of sodiumhydroxide, but in the recent revision of the book, Plenderleith and Werner (1971:198), only a 5% sodium carbonate solution is recommended. No discussion is presented as to the advantages and disadvantages of each electrolyte and under what circumstances one is preferable over the other. In proper conservation procedures, both electrolytes have their uses and the conservator needs to know when to choose the one best suited for the object at hand.

Sodium Carbonate

Generally speaking, a 5% sodium carbonate electrolyte with a pH of 11.5 will suffice for the electrolytic cleaning of most iron artifacts if maximum reduction is not the objective.. In terms of safety, it is much less caustic than sodium hydroxide and is much safer to handle. It is less conductive than sodium hydroxide and has to be used in stronger concentrations, 5 to 10% versus 2 to 5%. It is less soluble, which makes it more difficult to mix, but it does not generate the extreme heat of sodium hydroxide when mixed in concentrated solutions. When expense is considered, the stronger percentages of sodium carbonate are only slightly less costly, and sodium carbonate is usually easier to obtain from chemical supply houses than sodium hydroxide.

Preliminary experiments comparing 5% sodium carbonate electrolyte to 2% sodium hydroxide produced some interesting data. One set of experiments (Locke, ms) compared artifacts treated in 5% sodium carbonate mixed in tap and deionized water and 2% sodium hydroxide mixed in tap and deionized water. In all cases chlorides seem to diffuse out of the artifacts and reach higher Cl concentration in the sodium carbonate electrolyte more quickly than comparable artifacts in sodium hydroxide electrolyte. The major problem encountered with the sodium carbonate is the cathodic precipitates of insoluble carbonate on artifacts. The carbonate precipitates from the electrolyte and is more apt to happen at high current densities and when certain tap water is used to mix the electrolyte .

The marked carbonate deposit on the artifacts treated in 5% Na₂CO₃ is much more prevalent if the tap water has a large amount of carbonate in it. Artifacts from salt water, encrusted with calcium carbonate, magnesium hydroxide, and other minerals can provide the necessary elements to react with the carbonate in the electrolyte to form insoluble carbonates such as calcium or possibly magnesium carbonate. Once an article is plated with a carbonate deposit, it can seal off the surface and chlorides can be trapped inside, misleading the conservator as to when treatment of the artifact has been completed.

If a white, insoluble carbonate precipitate occurs when using sodium carbonate as the electrolyte, then gluconic acid, sodium gluconate or sodium glucoheptanate can be added as a sequestering agent to the electrolyte. With the addition of gluconic acid there was a decrease in the deposit, but it was still very apparent on iron objects in Na₂CO₃ in tap water, and slightly noticeable in Na₂CO₃ in deionized water. By adding 2% of the weight of the sodium carbonate in the electrolyte as gluconic acid or sodium gluconate, the tendency to deposit carbonate is reduced but not always eliminated. For maximum effectiveness gluconic acid or sodium gluconate require an excess of a free base (hydroxide). The pH of 5% sodium carbonate (11.5) is too low for maximum effectiveness. Tests with sodium glucoheptanate as a sequesterant appears to eliminate the carbonate deposit. If the tap water in an ares contributed to the formation on the precipitate, then onlydeionized water should be used with Na₂CO₃. In the precipitate is not noticed, then there is no reason to go to the trouble of adding a sequesterent to the electrolyte.

If a carbonate deposit does precipitate on an object it is usually impossible to brush off or remove by electrolysis. The carbonate deposit can, however, be eliminated by soaking several days in a 5% solution of sodium sesquicarbonate or sodium hexametaphosphate. Sodium sesquicarbonate complexes with the insoluble calcium or magnesium salts to form soluble salts in the same manner as sodium hexametaphosphates (Plenderleith and Werner 1971:253).

Another major problem commonly encountered with a sodium carbonate electrolyte is that the pH and conductivity of the solution are not adequate to keep the mild steel anodes passive in the presence of high chloride levels. In sodium carbonate (OH) ions discharge as oxygen at the anode more readily than the dissociation products of carbonate ions (CO₃)². The anode becomes acidic by the accumulation of hydrogen from the oxygen evolution reaction; therefore, anodic dissolution is more prevalent than when NaOH, with its surplus of hydroxyl ions, is used. To avoid this the mild steel anodes must be cleaned and replaced more often than in sodium hydroxide, especially if the electrolyte is not circulated. It has also been noted that takes longer to rinse out all residue of a sodium carbonate electrolyte than to it does to rinse out all residue of a sodium hydroxide electrolyte.

The most important disadvantage of Na₂CO₃ relates to its pH and cathode reduction potentials. Theoretically, more efficient reduction of a ferrous corrosion compound is possible using 2% to 5% NaOH with a pH of 12.9 than 5% Na₂CO₃ with a pH of 11.5. More details in regard to this are discussed subsequently under electrode potentials. In general, sodium carbonate can be used as an electrolyte if reduction is not the objective, but when it come to treating metals from marine site, sodium hydroxide is preferred.

Sodium Hydroxide

The shortcomings of 5% Na₂CO₃ are overcome by using 2 to 5% NaOH with its higher pH. However, this electrolyte being much more caustic, constantly presents a potential hazard to those working with it. Caution has to be exercised, and adequate safety equipment such as gloves, eye shields, eye washes, and safety showers should be available. In spite of these problems, a 2% solution of NaOH is the only option is the objective is to reduce corrosion products. In most laboratories that treat iron recovered from marine sites, it the standard electrolyte.

As a precautionary measure, gluconic acid, sodium gluconate, or sodium glucoheptanate in the amount of 2% of the NaOH or Na₂CO₃ in the electrolyte can be added as a sequestering agent to prevent the precipitation of insoluble calcium carbonates (which come from residue of the encrustation and/or the electrolyte) onto the objects being electrolytically cleaned is a problem. In our operations 50% aqueous sodium glucoheptonate has been found to be the superior sequesterent in both NaOH and Na₂CO₃ and can be stored for longer periods than 50% aqueous gluconic acid. In addition to preventing undesirable deposits on the cathode, the gluconate ions act as rust inhibitors which keep the steel anodes clean by sequestering dissolved ferric ions which would ordinarily precipitate as ferric hydroxide or oxide. They also prevent the specimens from rusting during rinsingand drying. More enduring protection is suspected but not substantiated.

EFFECTS OF ELECTROLYTES ON METAL VATS

When iron objects from the sea are cleaned there is a very large buildup of chlorides in the electrolyte for the first few baths. The high level of chloride (2,000 to 18,000 ppm) and lack of external circulation can cause the anode (the metal vat) to corrode extensively through anodic dissolution. The problem is worse with 5% Na₂CO₃ so it should not be used in vats that are not easily replaced. Corrosion is still encountered with 2% NaOH even with its higher pH of 12.9. By using a 5% NaOH electrolyte with its surplus of hydroxyl ions, anodic dissolution is more easily prevented. After the chloride levels drop below 1000 ppm, 2% NaOH can be safely used.

WATER IN ELECTROLYTES

It is common to see the statement that only distilled or deionized water is to be employed in all electrolytes. The exception to this general rule occurs when the objects to be cleaned are heavily contaminated with chloride. It is more economical and safer to use tap water in the electrolyte until the chloride level approximates the level of the local tap water. Then deionized water is substituted. The use of tap water in the first electrolytic baths may actually reduce the electrolysis time. Our preliminary experiments indicate that chlorides are removed more rapidly when tap water rather than deionized water is used in the electrolyte, Na₂CO₃ or NaOH. While there was a problem of selecting truly comparable artifacts for the experiments, the same conclusions have been derived from four tests on eight apparently analogous specimens.

In summary, when searecovered artifacts are cleaned by electrolytic reduction, it is recommended that the artifact be started in 2 to 5% NaOH in tap water. During this early period, at low current density, the reduction of ferrous compounds is attempted and high levels of chloride are removed. Reduction is continued in the tap water electrolyte until the chloride measurements approximate the level found in the tap water. Tap water is then replaced by deionized water, electrolysis is continued with 5% NaOH or lowered to 2% NaOH. A low current density is maintained until the electrolyte is again changed, then a medium current density is used.

The higher pH of NaOH is preferable when the objective is the reduction and/or consolidation of ferrous corrosion products. If this is the case, NaOH should be used as the electrolyte. If the corrosion products are in a ferric state, there is no advantage in using NaOH, and Na₂CO₃ can be used from the beginning. This is applicable to many iron objects that are airoxidized in archeological sites, but this generalization should not be carried out too far. There will almost always be some ferrous compounds that can be successfully reduced.

CURRENT DENSITY

The current density used in electrolytic reduction is expressed as the number of amperes per unit of square area, introduced in the electrolytic cell by external D.C. power supply, such as one ampere per square centimeter (1 amp/cm2). Any given current density should have a given objective, butin the literature these objectives, if any are intended, generally are not stated. Current densities from .001 to 1 amp/cm2 have been proposed in the literature (Plenderleith 1956:195; Plenderleith and Torraca 1968:242; Plenderleith and Werner 1971:198; Townsend 1972:252; Pearson 1972a:12), but guides to their use are seldom given.

Plenderleith and Werner (1971:198) suggested approximately 10 amps per square decimeter (1 amp/cm2) of cathode area so as to give a steady, vigorous evolution of hydrogen. They say, however, that this density is not critical for iron and steel artifacts. This statement is misleading. If the artifact is solid, wellconsolidated metal, or has only ferric corrosion compounds, the current rate is not critical. If the object has ferrous corrosion compounds, an initial high current density will quickly flake off the corrosion layer and significantly alter the shape of the specimen. In addition, an initial high current density can disfigure the surface of iron and also seal off the object, preventing the removal of deeply seated chlorides. By using a low current density in the early stages these problems can be avoided. Even more important, it is possible to reclaim enough of the metal through a reduction process to consolidate the metal oxide interface and thus preserve a closer approximation of the object's actual dimensions. This is true even of wrought iron objects whose corrosion layers are often nonadherent.

In Hamilton (1976:41) I proposed the following current densities and objectives:

1. Low current density .001.005 amp/cm2 attempts to approximate the reduction range of ferrous corrosion compounds.

2. Moderate current density, .05 amp/cm2 attempts to approximate the optimum conditions for chloride removal without undue evolution of hydrogen.

3. High current density of .1 amp/cm2 has as its major goal the vigorous evolution of hydrogen for mechanical cleaning.

These were recommended as rough guidelines for treating most iron artifacts from under the sea. These current densities were established by measuring the electrode potential established first on the surface of a rectangular bar of steel and then verified on a number of small artifacts. They do work as rough guidelines. If iron reduction is the objective, the specimen should be started off at a low current density. It is followed by a medium current density for the long chloride removal stage. A continuation of low current density would only lengthen the process, while the vigorous production and evolution of hydrogen at high current density interferes with the efficient removal of chloride from the metal and its corrosion products. At both low current density and medium current density the marine encrustation, some corrosion products and scale are slowly removed from the object by the mechanical action of the evolved hydrogen. For maximum efficiency, however, the artifact should be finished at a high current density. This insures the complete removal of any remaining marine encrustation and loose corrosion layers or scale, as well as any remaining chlorides. When high current density electrolysis is used only in the last state of electrolysis, the metal corrosion products that are capable of being reduced have been reduced and it is less likely that they will be removed by the vigorous evolution hydrogen. While the above densities serve as useful guidelines, they cannot be adhered to rigidly. The conservator should respond to the individual peculiarities of the objects and to the facilities available. Thus, in the case of very large objects, these calculations might require current in excess of the maximum capacity of an available or practical power supply.

Anyone who has used current densities knows the difficulty of determining the square area of many metal artifacts because of their irregular shapes. Although it is seldom stated, the calculated area should be concerned only with the cathode area that is exposed opposite an anode. The total area of the cathode is used only when an anode is formfitted around all sides of the cathode; otherwise, only that area exposed to an anode is used. It must be understood that any given current density determines the electrode potential established between the electrodes and the electrolyte and the rate of hydrogen evolution at the cathode. For reduction of metallic corrosion compounds, the establishment of a certain electrode potential on the cathode is essential. Since there is not a onetoone relationship between a given current density and a corresponding electrode potential on differentially corroded iron artifacts and different iron alloys, current density alone is not the ultimate guide. Calculated current densities are simply not a fine enough tool for precision treatment.

For these reasons, North (1987:226) recommends starting electrolysis off at an applied voltage of 1.8 volts D.C. and to continue treating the artifact at this voltage for at least one week. This voltage is usually not sufficient to cause hydrogen to be evolved, but it does reduce the more reactive specie present in the artifact and sets up the artifact so that a hydrogen potential graph can be developed for that particular artifact. After the first week of electrolysis the point, as indicated by voltage, is found where the current, as indicated by amperage starts to increase rapidly. The chart is made starting with points much lower than the point of hydrogen evolution and increasing the voltage well above the point of hydrogen evolution. By graphing the intersection points, with amperage as the x-axis and volts as the y-axis, it is easy to see where the voltage that corresponds to the point at which the amperage starts to increase rapidly. This is the hydrogen evolution voltage. The electrolysis is then conducted at this voltage. By using this technique, some of the difficulties of using current densities are eliminated, and its use is recommended.

In practice, I discovered years ago, that neither measuring current densities or establishing hydrogen evolution voltage is necessary for most artifact. One can merely "eye ball" the artifact. Start the electrolysis off by slowly increasing the voltage or amperage, until you first began to observe a few bubbles of hydrogen evolving. In theory, for metal reduction you do not need any hydrogen evolution at all, but the irregular evolution of hydrogen from the surface of the metal gives one a visual indicator that the current is flowing. By this technique, neither voltage or amperage is the indicator, it is the evolution of very first evidence of hydrogen evolution. Run the artifact at this setting. However, after a short time, one will observe the evolution of hydrogen increases as the resistance of the artifact is broken down and there is some reduction of the corrosion products in the metal. So, you usually have to decrease the setting back to the point where hydrogen first starts to evolve. After going through this low hydrogen evolution, where the objective is to reduce as much of the corrosion products as possible, the current is increase so that there is a steady, not vigorous, evolution of hydrogen. This is necessary if one wants to remove the maximum amount of chlorides within a reasonable length of time. Maintaining a very low current density current will only prolong the process an unacceptably long time and not contribute any other benefit. As an option, one may choose to run the artifact for a few hours or days at a high current density that is for maximum mechanical cleaning. In general, I have found that surfaces of the artifacts that might come off at this point during the high current density treatment, have little change of staying intact as you carry it through the rest of the treatments. This process is a simplification over that proposed by North (1987:226) for it depends on: 1) visual evolution o slow, irregular evolution of hydrogen formaximum reduction, 2) visual evidence of a steady evolution, not vigorous, of hydrogen for the much longer process of chloride removal, and optionally, 3) a vigorous evolution of hydrogen for final mechanical cleaning to remove any last remaining encrustation or loose corrosion products. In practice, this is how most of the experiences conservators determine the current density at which to treat iron artifacts, as well as artifacts of other metals and works well for most artifacts. .

At times, a more precise alternative, than that proposed by North, or that just described above is required for treating those one of a kind artifacts, for which there is no room of guessing. In those cases, it may be necessary to actually measure the electrode potential established at the surface of the artifact being treated. By treating taking a series of carefully measured artifacts at different voltage/amperage rate and measuring the resultant electrode potential is how I derived the current densities I recommended in 1976. Over a period of time, I found, and electrode potential measurement validated it, that all that was necessary was a visual indicator of the rate of hydrogen evolution through treatment. I will briefly discuss electrode potential measurements, for the objective of current densities, hydrogen evolution potential, and visual observation of rate of hydrogen evolution is to establish at the metal surface and corrosion layer the voltage necessary to reduce the corrosion products present. In the case of iron, most of the corrosion products, if they can be reduced, are reduced to magnetite. It has not been shown that any of the corrosion products actually reduce back to metal, as in the non-ferrous metals. By considering electrode potential, it also clearly shows why one electrolyte is preferred over another, when maximum reduction is the objective.

ELECTRODE POTENTIALS

The normal electrode potential, or relative activity, of each metal is ranked in an electromotive series based on the hydrogen scale (Table 1). The normal electrode potential of each metal is in comparison to a normal hydrogen electrode since there is no practical means of measuring a single electrode potential without reference to another single electrode potential. The hydrogen electrode is given an arbitrary potential of zero and all metals are negative or positive to it. The normal electrode potential of a metal represents the EMF required to balance the cell formed by a particular metal electrode immersed in a solution of its salt of normal cation activity and a hydrogen electrode (Evans 1963:231).

The potential series of the metals represent their equilibrium values at a pH of 0. These potentials change with pH value. In order for a metal to be corroded anodically, a potential more positive than the equilibrium value must be established. For cathodic reduction a more negative potential must be created. Cathodic reduction of iron becomes pronounced only if the potential is made considerably more negative than the equilibrium value. Dissolution of the anode becomes significant when the potential is made considerably more positive.

The potentials of the metals . . . are altered if the [ionic] activity (effective concentration) is not normal. Clearly, if the solution is diluted, the passage from the ionic to the metallic state will be slower, whereas the passage from the metal into solution remains unaltered. Thus the balance will be upset, but a fresh balance may be obtained at a more negative potential . . . This means that at 18ø C., if activity and concentration are regarded as identical, every tenfold dilution (say from Nto N/10, or from N/10 to N/100 will shift the potential in a negative direction by about 0.058 volt for a monovalent ion, or 0.029 volt for a divalent ion. The theoretical shift of potential is approximately realized in the case of the more noble metals; but on some of the less noble metals [iron] the potential actually measured is found to be almost independent of the concentration of the metallic salt in the original solution; this occurs if the metal is capable of reacting with the solution, so that the concentration in the layer next to the metal becomes different from that in the body of the liquid (Evans 1963:234). The potential of iron immersed in an aqueous solution originally free from iron ions depends on the hydrogenion concentration, becoming steadily more negative as the pH value rises (Evans 1963:235).

From the Electromotive Series in Table 1 it can be seen that the reduction reaction of ferrous ions, Fe⁺² ---> Feo
-2e, occurs at -.409 volts to -.440 volts at a pH of 0. The reduction potential increases -.029 volts for each pH increase. The reduction of ferric ions, Fe⁺³ ---> Feo -3e, occurs at -.036 and increases -.019 volts per pH increase. The hydrogen discharge potential 2H⁺ ---> H_{2^ -}2e occurs at a voltage potential of 0 and increases -.058 volts per unit pH increase. The reduction potential of ferrous, ferric and hydrogen ions at different pH's and standard temperature of 20o C. are graphically presented in Figure 14. Utilizing this graph it is possible to determine the theoretical electrode potential voltage necessary for reduction of iron oxides for any given pH.

In electrolytic cleaning of iron the cathode electrode potential is critical because as the hydrogen discharge potential is reached, hydrogen ions are formed, which are utilized to reduce adjacent ferrous compounds. At more negative potentials molecular hydrogen is formed so rapidly that the molecules combine with each other and evolve as hydrogen gas. Any hydrogen evolved as a gas does not reduce oxide compounds. Rather, it acts as a mechanical cleaning agent that physically removes any unconsolidated corrosion layers. The theoretical point for the electrolytic reduction of ferrous ion corrosion compounds is at the intersection of the hydrogen discharge line and the ferrous ion reduction line (Figure 14). Using the formula Fe⁺² + 2e ---> Feo and substituting values for the Fe⁺² reduction potential (-.409) and the correction for pH ( -.029 pH) the intersection appears at a pH of 14.1 and an electrode potential of -.82 (-.409v minus - .029v per pH). This potential is reached just prior to the evolution of hydrogen. It is possible to establish or at least approach a maximum pH of 14 on the surface of a cathode in dilute aqueous caustic soda. At low current density electrolysis, little or no hydrogen is evident; however, the objective is to establish the electrode potential of the intersection of the discharge potential of hydrogen and the reduction potential of ferrous ions or immediately to the left of this intersection on the cathode surface. It is then theoretically possible to obtain maximum reduction and/or consolidation of the ferrous corrosion compounds.

It has already been stated several times that it is thermodynamically impossible to reduce ferric oxide and other ferric iron corrosion compounds in an aqueous solution. This point is graphically depicted in Figure 14. The reduction electrode potential is well out of the range possible for electrolytic reduction in an aqueous alkaline electrolyte and an external EMF. By measuring electrode potentials it is possible to insure that a desired electrochemical reaction will occur on a metallic surface in contact with an electrolyte of known pH and ion content. Controlling the electrode potential is the main objective and advantage of electrolytic reduction of metal artifacts. The procedure for measuring electrode potentials is described in Hamilton (1976:103-105).

By using electrode potentials it is possible to determine what current density in an electrolyte of known pH is required to establish the condition most conducive to the reduction of ferrous compounds on the metal. Figure 15 gives the measured potentials of an artifact in an electrolyte of 2% NaOH with a pH of 12.9 at different current densities over an 11day period. The initial test was taken before the surface of the artifact was thoroughly saturated with hydrogen, the IR of the electrolyte had dropped, and the resistance of the artifact was broken down. After a period of 20 minutes, and throughout the 11day test period, the measurements fell within a rather narrow range. Utilizing the data presented in Figure 6, the potential range for ferrous ion reduction at a pH range of 13 to 14 can be determined as -.79 to -.85 volts. On Figure 15 this potential range is achieved at a current density below and up to .005 amps/cm2, which is barely within the capability range of 2% NaOH. As can be seen in Figure 8, which compares the potentials of the same artifact in 2% NaOH and 5% Na₂CO₃, the sodium hydroxide is in the more favorable position. The pH of sodium carbonate is not high enough to establish the desired theoretical electrode potentials for the reduction of ferrous compounds. On theoretical grounds a sodium hydroxide electrolyte is clearly superior when reduction of the ferrous corrosion products is the objective. Electrode potentials are affected by temperature, pH value and electrolyte composition. In the conservation of iron it is the pH that is most important. There are several ways of measuring the pH of the electrolyte, but no simple way of determining the pH at the surface of the electrodes. Unfortunately, this is the pH reading of concern. It is known that the pH of the catholyte (the solution at the surface of the cathode where hydrogen is evolved) is higher than the remaining electrolyte. In order to arrive at a reasonable estimation of the prevailing pH at the cathode, the pH of the electrolyte was taken and a maximum increase of one pH for each electrolyte was assumed, for a maximum pH of 14 for NaOH and 12.5 for Na₂CO₃. This approach is imprecise, but no other means was perceived for ascertaining the pH. These adjusted pH ranges are those shown in Figures 14, 15, and 16.

Once an electrode potential has been established on the surface of the cathode, periodic measurements and adjustments have to be made in the direct current to maintain that potential. After electrolysis has begun, several hours to a full day are required for the cathode to adjust to an equilibrium state. At the metal surface a metal/hydrogen (MH) bond is established and the surface becomes saturated with hydrogen, followed by the evolution of hydrogen. Until the metal surface becomes saturated with hydrogen the potential shifts. Once equilibrium is established the potentials even out. After establishing an electrode potential on an artifact, daily adjustment may be necessary to maintain the potential as the IR of the electrolyte drops with the addition of dissolved salts and the resistance of the metal and its corrosion products are altered. Before adjusting back to the predetermined potential, one has to be sure that any shift is due to changes in the cathode, and not due to the transport system, i.e., theelectrolyte through lack of circulation. In Figure 15 the six series of tests were taken over an 11day period without circulating the electrolyte. On the last day the electrolyte was airlanced and the test run again. It can be seen that there are negligible differences in the potential readings in the last two tests except for the potentials at high current densities.. The transport system is still adequate, especially at the reduction potentials of ferrous compounds. If any changes occur in the transport system it will become visually apparent at the anode. Even though an alkaline electrolyte is used, acidic areas can form at the anode. In these areas chloride ions react with the positively charged anode to form hypochlorite ions (ClO) or hypochlorous acid (HClO) which oxidizes the anode. Adverse reactions are easily detected at the anode, and when this occurs, the cathode is also affected, although it is not as apparent or as detrimental to the operation of the electrolytic cell. The potential at both electrodes is altered, however. Should a change in the potential occur, the electrolyte should be stirred or circulated and the measurements taken again. If the electrode potential remains the same, then the amperage may be adjusted back to reestablish the optimum range.

From the standpoint of efficiency, electrode potential measurements are necessary for precise control and quality processing of very select artifacts. The areas of advanced or retarded corrosion will be reflected in the electrode potentials, making it possible for the conservator to identify those areas requiring more extensive reduction and a way to measure the progress of the electrolyte reduction. The data acquired from electrode potential measurements was used to arrive at realistic current densities that established the approximate electrode potentials for the bulk of the iron artifacts that will require treatment but do not warrant the close supervision or the time required for taking potential measurements. Still, there is the difficulty of arriving at current densities for given artifacts, especially irregular shaped artifacts and differentially corroded artifacts. For these reasons, except for those special artifacts, using either hydrogen evolution potential voltage or merely visually observing the rate of hydrogen evolution is sufficient.

OUTDOOR ELECTROLYSIS

Any electrochemical reduction cleaning process results in irritating caustic vapors and hydrogen gas, which have to be properly exhausted from any room or building for the safety and comfort of personnel. Expensive fume hoods or sealed rooms with exhaust systems are required, or the electrolytic vats can be placed outside. In practice, most laboratories will utilizes variations of all three methods. The smaller artifacts are cleaned in the main laboratory under fume hoods. A fewlarge artifacts are cleaned in a ventilated room in a separate building. Other large artifacts and some small ones, are electrolytically cleaned yearround in outside vats made of mild steel or plastic.

Working outdoors overcomes the problems of expensive air exhaust systems. The solutions are changed often enough so that any airblown dirt and sand present no problems. Rain is of no concern, unless it is heavy enough to dilute the solution substantially. This problem can be determined easily with a hydrometer to test the specific gravity, and remedied by adding more sodium hydroxide or sodium carbonate. In many cases, rain will only decrease the amount of deionized water that needs to be added to the vats daily to compensate for evaporation, caustic vapors and electrolytic reduction.

If the area is fairly isolated (safe from human traffic) and has adequate utilities, it is strongly recommended that outdoor electrolytic reduction be seriously considered. The power supplies must be kept out of the weather, but sheds over the vats are optional. Although lower winter temperatures do not decrease the chemical activity significantly, there can be an IR drop in the electrolyte that will increase the cell voltage. In the Conservation Research Laboratory, at Texas A&M University, by far, the majority of electrolysis is carried out in various metal vats place outside.

GENERAL OBSERVATIONS ON CLEANING IRON OBJECTS BY ELECTROLYTIC REDUCTION

These observations are directed primarily toward large iron objects, but are not restricted to them. Recommendations for the improving the procedure and reducing the time in electrolysis are given.

Although conservators often avoid cleaning large iron objects by electrolysis because of their size, there are no insurmountable problems. In order to better understand the success or failure of electrolytic cleaning, several case histories are reviewed.

The earliest attempt to clean electrolytically a large, seabed iron artifact was by Lieutenant Nielsen of Norway. The specimen was a wrought iron gun recovered in 1942 from a ship dating to the 15th century (Eriksen and Thegel 1966:100102). The conservation was attempted under the adverse conditions of World War II, with inadequate facilities and supplies. The attempt was unsuccessful for a number of reasons, the most important being:

1. Failure to remove the wooden undercarriage shielded a large portion of the cannon and prevented an even distribution of the current.

2. Treatment was too brief to remove the chlorides. It lasted a total of 69 discontinuous hours spread over a 384hour period.

3. The placement of the steel anode sheet along only one side of the vat failed to assure an even distribution of current.

4. After the brief electrolytic treatment, the wooden undercarriage was swabbed with hydrochloric acid to neutralize the 1% sodium hydroxide used as the electrolyte. This acid introduced additional chlorides to attack the iron.

Ora Patoharju (1964, 1973) reports one of the first successful attempts to clean a large castiron cannon. In 1963 two guns from a ship sunk in 1790 were treated. One had been recovered many years before treatment and had been allowed to dry out; the second cannon had recently been recovered and had been kept wet. Both served as the cathode with three stainless steel sheet anodes in a 10% Na₂CO₃ electrolyte. They were run at a rate of 20 amperes at 4 volts (current density ca. .027 amps/cm2) for one month and a rate of 150 amperes at 4 volts (current density ca. .2 amps/cm2) for five months.

Not enough details on all aspects of the electrolytic treatment are presented to evaluate adequately the methods used, but after being displayed outdoors for two years the treatment of the wet gun was judged to be successful, although the dry gun continued to deteriorate as rapidly as before electrolysis. Regardless, the treatment appears to have been successful, for the wet cannon continues to be stable. Perhaps the main reason for the successful treatment of one gun is that is was kept wet, preventing further corrosion, and the six months of electrolysis assured the complete removal of the chlorides in the metal.

The preliminary conclusion of Patoharju is that wet castiron objects coming directly from the sea can be conserved by electrolysis in a liquid phase. If the object is allowed to dry out before treatment the specimen will undergo further oxidation and any conservation attempt may be futile. A similar problem has been noted in my work. A couple of large breechblocks had been successfully treated, leaving the majority of the corrosion layer intact on the specimens. A month after completion of the treatment it was decided to put them back into treatment in order to darken the surfaces. The wax was melted off in an oven, which probably caused increased conversions of FeO to Fe₂O₃ on the breechblocks, and they were put back into electrolysis. In both cases the corrosion layer completely detached, leaving a badly disfigured surface. These examples suggest that if there are thick layers of corrosion, the treatment must be successful the first time. Wrought iron corrosion layers are nonadherent and after the water is removed from the corrosion layers during the drying and sealant steps, they apparently undergo additional oxidation and may also lose the electrical couple with the metal core. During subsequent electrolysis, hydrogen will evolve at the surface of the metal and slough off the corrosion layer. Perhaps this can be alleviated by soaking the object in electrolyte for a long time to reestablish the electrical couple prior to any further electrolysis.

Pearson (1972a, 1972b) reports the conservation of six castiron cannons and other relics jettisoned in 1770 by Captain Cook on Endeavor Reef off the Australian coast. Pearson's very thorough and detailed report on the conservation of the cannon is summarized here.

Upon arrival at the laboratory the cannons were stored in 2% solution of sodium hydroxide and each cannon placed in an epoxycoated wooden support cradle especially constructed to handle it throughout preservation. The coral encrustation was mechanically removed with hammers and the cannon surfaces carefully scrubbed with water to remove the loose black corrosion products. Each cannon and its support cradle was then set in a fiberglasslined wooden tank to be electrolytically cleaned in a 2% NaOH electrolyte. The cannon was set up as the cathode and two anodes, a single mild steel sheet along one side of the vat, and a steel rod in the bore, were used. A current density of 10 amps/m2 (.001 amps/cm2) was applied. Three cannons were simultaneously treated inseparate vats hooked up in series on one power supply, analogous to the type 4 setup described earlier. Each week the third cannon was removed from its vat and the electrolyte discarded and replaced with fresh electrolyte. Cannon No. 1 was then placed in this vat; Cannon No. 2 was placed in the vat from which No. 1 had been removed, and Cannon No. 3 was put in the vat formerly occupied by Cannon No. 2. In this way the first cannon, the one in electrolysis the longest, was always placed in the fresh electrolyte. This rotation was continued weekly until analysis showed that there was no increase for one week in the chloride content of the bath containing the first cannon. This required six to eight weeks of electrolysis for each cannon.

A current density of .001 amps/cm2 was chosen by Pearson because through experiments, it proved to be the optimum value for the removal of chlorides. Pearson found that higher values blistered the graphitic surface layers of the cast iron and lower values only prolonged the time to remove chlorides. More importantly, this current density is within the range most efficient for the reduction of ferrous iron compounds. A series of experiments (Hamilton 1976:40-46) on wrought iron indicate that .001 amps/cm to .005 amps/cm2 is theoretically most efficient for metal reduction (as is shown in Figures 9 and 10), .05 amps/cm2 to .1 amps/cm2 is most efficient for chloride removal and the vigorous evolution of hydrogen produced by a density of .1 amps/cm2 and above is used for mechanical cleaning.

EXPOSURE TO ANODE SURFACE

When Pearson (1972a) cleaned Captain Cook's cannons, he used a single mild steel anode along one side of the vat and used immovable wooden support cradles on the cannons. The placement of the anode failed to assume an even current density and the cradles shielded two areas from what current flow was present. This shielding interfered with the removal of the chlorides and the reduction process. Pearson (personal communication) reports that minor corrosion has occurred at these two support points because the cannons were not rotated sufficiently during the treatment. We have found similar occurrences at support points and even on small surface areas covered with plastic identification tape. From these examples it is clear that no portion can be covered throughout electrolysis, for even a small piece of narrow tape can shield the area beneath sufficiently to prevent complete removal of the corrosive chlorides. To avoid this, cannons should be rotated frequently and the position of movable supports, such as bricks, should be shifted each time the electrolyte is changed. When numbered plastic tape is placed on artifacts for purposes of identification, the position of the tape should be moved at least once during electrolysis.

More than any one factor, it is unfortunate that the conservation literature has failed to emphasize the importance of form fitted anodes when electrolytically cleaning artifacts. Anodes placed to assure an even current density over all the surface of the artifact improves the over-all efficiency of the process.. Now by form-fitted, I do not mean that there is an exact distance maintained between the artifact and the anode. This is nearly impossible for irregular shaped artifacts. I simply mean for artifact, especially irregular shaped artifacts, bend and shape the anode so that it is some what evenly distanced from the anode. North (1987:225) states that the use of form-fitted anodes has been suggested, citing Hamilton (1976) but he goes on to state that these have been shown to be unnecessary as long as the artifact-anode distance is between 20-80 cm a satisfactory current distribution will be achieved. However, in support of my recommendation he then goes on to say that when a rod is used as the anode in the barrel of a cannon, this does not apply, for the current will be concentrated in the bore because of the small distance between the rod and the cannon. Then while discussing the establishment of hydrogen evolution potential voltage (North 1987:226) he states that if the hydrogen gas evolution is coming from only one area of the artifact, that there is either a poor electrical contact at this point or that the anode is too close to the artifact at that point (emphasis mine). Obviously, my recommendation stands; a anode shaped to conform to the shape of the artifact (even roughly) is recommended even if we do not do it in everyday conservation, usually placing artifacts between two vertical anodes. In any case a form-fitted anode, if I may call it that, is intended to maintain somewhat of an equidistance between the artifact and the anode. If used, it will maximize iron reduction and chloride removal, and may even cut down on the electrolytic time. Since all areas of the objects should be exposed to anode surface, immovable supports, lifting frames, and such things as wooden undercarriages on cannons should be avoided.

DURATION OF ELECTROLYSIS

Another disturbing aspect of the preservation of Cook's cannons was the short electrolysis time, six to eight weeks, and the long rinse period, up to five months. The latter indicates that the full potential of electrolysis was not achieved. If time had permitted a longer electrolysis period, it is probable that the rinsing time could have been substantially cut.

Ora Patoharju (1964) reported an electrolysis time of six months to clean two cannons recovered from a 1790 wreck. This is similar to our experience, as every wrought iron object approaching the size of a cannon has required between six and twelve months of electrolysis. As an example of the length of time required for large wrought iron objects, one fully armed swivel cannon, 6'7" (198 cm) long, required 20 days to remove all the encrustation and to dismantle the component parts, 251 days of electrolytic cleaning at 20 to 50 amps, seven days of rinsing in several changes of alternate boiling and cold deionized water, 15 days dehydrating in alcohol, and one and a half days submerged in molten microcrystalline wax. Three hooped barrel cannons processed together in one vat required 480 days of electrolysis, three months of rinsing (rinsing period was prolonged while awaiting delivery of microcrystalline wax), five days of dehydration in alcohol, and five days submerged in microcrystalline wax. Even small wrought iron artifacts such as spikes often require 60 to 90 days of electrolysis.

Reducing Electrolysis Time

It is clear that when artifacts are electrolytically cleaned in small vats with a low volume of electrolyte to artifact volume, the length of electrolytic time is considerably extended. The larger the volume of electrolyte to artifact being cleaned, the shorter the period of electrolysis required. Two wrought iron breechblocks that were cleaned in two small vats required 25 months of electrolysis. It follows that more frequent changes of the electrolyte will considerably speed up the process also.

Several aspects of shortening the length of electrolytic treatment have been worked out. It has been concluded that the problem of reducing the chloride (Cl) level in the electrolyte below 50 to100 ppm Cl requires the greatest amount of electrolytic time. Below 100 ppm Cl the artifact is presumably in the final stages of cleaning. There are at least two explanations for this long electrolytic time: 1) The electrolysis is carried out at high current densities which have a tendency to repress Cl migration in preference for H₂ evolution.2) The Cl concentration in the electrolysis is governed by the Donnan equilibrium equation.

The Donnan theory pertains to the unequal distribution of ions on two sides of a membrane (Kunin 1958:1416). Although no membrane exists between the artifact and the electrolyte, the interface between the solid and liquid phases may be considered as a membrane. An exchange of Cl^- ions continues until the concentration ratios are equal in both phases. The lower the Cl ratio in the electrolyte, the more effective the diffusion of the Cl ions from the artifact to the electrolyte. Using the Donnan equilibrium theory, it can be generalized that the rate of Cl transfer from the artifact to the electrolyte is negated despite the electrolytic field when the Cl content of the electrolyte is higher or equal to the Cl level in the artifact. This results, according to the Donnan theory, because Cl1 = Cl2, i.e., the chloride ion concentration of two solutions in contact will equalize. As a result, chloride tests can be misleading under equilibrium conditions. Negligible or no increase in the chloride reading in the electrolyte over several days may lead the conservator to believe that the chloride removal has been accomplished and terminate electrolysis, leaving deepseated chlorides in the artifact.

For optimum efficiency in electrolysis, the chloride level of the electrolyte should be less than the chloride level of the artifact. When the Cl content in the electrolyte maintains a steady level in a controlled constant volume of electrolyte, it is apparent that: 1) Equilibrium conditions have been obtained and unless the electrolyte is renewed to lower the Cl concentration, little or no Cl removal from the artifact can be achieved. 2) The chloride removal process has been completed. By changing the electrolyte this can be easily validated. By frequently changing the electrolyte and using as large a vat as possible, principles of the Donnan equilibrium equation can be used to the conservator's advantage. The lower the Cl level in the electrolyte, the greater the driving force for Cl removal, which translates as Time and Efficiency.

For example, with more frequent changes of electrolyte, one wrought iron anchor required six months of electrolysis in contrast to 11 months for the first anchor cleaned. Three wrought iron cannons required 16 months of electrolysis at 100 to 150 amperes and three volts to remove the chlorides. A second group of three hooped barrel cannons were completed in 11 months. Similar decreases in required electrolytic time have been achieved with other specimens treated in larger volumes of electrolyte and when the electrolyte is changed often. Crucial to the electrolytic process is a satisfactory quantitative chloridemonitoring test. The chloride tests allow the conservator to determine the rate and efficiency of the chloride removal.

SODIUM SULFITE TREATMENT

The alkaline sulfite treatment was developed by North and Pearson (1975) to stabilize marine cast iron, but is also used on wrought iron. Bryce (1979:21) found that the treatment is effective oniron objects that are moderately to heavily corroded, but they must have a metallic core. Otherwise, the iron object breaks up in treatment. The procedure is as follows:

1.Once the objects have been mechanically cleaned, they are immersed in a solution of 0.5 M (20 g. per liter of water) of sodium hydroxide and 0.5 M (126 g. per liter of water) of technical grade sodium sulfite. Tap water can be used for the first one or two baths but deionized or distilled water should be used in the final baths. The container should be filled as full as possible and sealed to prevent any access to air. The solution is mixed and the object placed in it as quickly as possible to avoid any oxidation of the solution. The container is placed in an oven and kept heated to a temperature of 60 degrees C. The object is processed through several baths until chlorides are eliminated; this may take from a week to several months and numerous baths. The solution does not attack any residual metal so there is no danger of too many baths.

When a marine iron object is immersed in this hot reducing solution, the iron corrosion compounds are converted to magnetite and the chlorides are transferred to the solution where they are discarded with each bath change. The objects come out of the treatment with a very black surface coloration. Since the solution is strongly alkaline, contact with the skin should be avoided.

2.Once the alkaline sulfite stabilization treatment is completed, the objects are washed for one or more hours in several baths of deionized water and then placed in a 0.1 M solution of barium hydroxide (32 g. per liter of water). Barium hydroxide is slightly poisonous, so contact with the skin should be avoided. If the object is intensely rinsed in several baths of deionized water following the alkaline sulfite stabilization, the barium hydroxide baths can be eliminated -- thus it is optional.

The alkaline treatment has been very effective for conserving iron recovered from a marine environment. The main drawbacks of the treatment are that it has to be carried out in an air tight container and the solution should be kept heated.

CHEMICAL CLEANING

A number of chemical cleaning procedures are used for iron artifacts from nonmarine environments with negligible chlorides present. The most common chemicals are oxalic acid, citric acid, phosphoric acid, ethylenediamine tetraacetic acid (EDTA), and other complexing agents. Because of the problems of chlorides in iron from the sea, the exclusive use of any of these may improve the appearance of an object, but they do not remove chlorides and hence cannot prevent subsequent corrosion. Therefore, they are not considered as conservation alternatives for treating iron from salt water. Details concerning the use of these and other chemicals are described by Plenderleith and Werner (1971).

Two chemicals, phosphoric acid, along with its derivatives in commercial rust removers, and tannin solutions, are often used to form a corrosionresistant film of phosphate and tannates on thesurface of treated iron pieces. The corrosionresistant significance of phosphate and tannate films was first made apparent when iron articles in a possible 2,000 yearold Roman tannery in England were found to be in an excellent state of preservation (Farrer, Beck and Wormwell 1953). Before either chemical can be used, however, the chlorides must be removed by electrolytic reduction, alkaline sulfite, or water diffusion.

TANNIC ACID

The corrosionresistant nature of tannate films on iron was investigated by Knowles and White (1958) and later by Pelikan (1966). In accelerated exposure tests it was found that tannate films on iron were more corrosionresistant and lasted twice as long as a phosphatecoating. Solutions of hydrolysable tannins such as chestnut, myroblans, or valonea extracts with a pH of 2 to 2.5 provide the most weatherresistant protection (Knowles and White 1958:16). If the tannic acid mixture has to high of a pH, phosphoric acid should be added to bring it down to a pH of 2.4. I have found that it is important that the right tannic acid be selected as many are not effective. Tannic acid sold by Baker Chemical has a pH of 2.5 to 3 and has been found to be the most effective (Hamilton 1976:50). See Argo (1981) for a discussion of the requirements and benefits of tannic acid solutions. Tannic acid solutions (Baker reagent tannic acid, C₇₆H₅₂O₄₆) with a pH of 2.53.0 provide good, weatherresistant tannate films. Tannic acid solutions are a standard part of the conservation of all iron artifacts in most conservation laboratories. Although some use the tannic acid coating as the final step, I recommend that an additional sealant, such as microcrystalline wax, be applied over the oxidized tannate film for maximum protection.

For maximum protection, several coats of 20 tannin solution (200 g. tannin, 1 liter water, 150 ml. ethanol) is applied with a stiff brush. A brushedon film provides better protection than a dipped or sprayed application because the brushing assures that the solution has access to the metal in areas of loose rust and eliminates the polarization of cathodic areas by the formation of hydrogen (Pelikan 1966:112). Nevertheless, cast iron cannon balls have been successfully treated by vacuumimpregnating with a tannin solution. After applying, the object is allowed to completely airoxidize between each application of the tannic acid. Allow it to dry one to two days after the final application.

The tannin solution reacts with the iron or iron oxide to form a ferrous tannate with oxidizes to a mechanically strong, compact blueblack colored ferric tannate. In order to assure a continuous film. Knowles and White (1958) recommend that all the iron oxide be removed, otherwise, they contend, there is a possibility that corrosion may start at the junctures of the cathodic iron oxide and the tannate film. Good results can be achieved even if this recommendation is not followed, for Pelikan (1966:110111) found that tannin solutions react directly with the metal base and with the rust if the solution is sufficiently acid, pH 2 to 3. In addition to forming a corrosionresistant film, tannin solutions can be used to impart an aesthetically pleasing black color to iron.

A phosphate film is formed on iron objects by impregnating them with a 20% solution of phosphoric acid (H₃PO₄). Impregnation under a vacuum is recommended to assure completepenetration into all the porous areas of the metal. The acid complexes with the iron to form an inert film of ferric phosphate film on the surface of the metal. ReesJones (1972) describes the above procedure and reports that porous cast iron cannon balls from a 1588 Spanish Armada shipwreck were successfully treated by this method after removing the chlorides by water diffusion. Similar results is achieved on wrought iron or steel.

Data reported by Pelikan (1966:112113) indicate that a mixture of phosphoric acid and tannin solution can be used on badly rusted iron and it appreciably improves the corrosion resistance of a phosphate film. One hundred milliliters of 8085% phosphoric acid solution is added to the 20% tannin solution and several coats are brushed on the artifact, then at least four coats of the standard 20% tannin solution is brushed on the object. Following the treatment of an object with the mixed solution, phosphoric acid or tannin, a final sealant should be applied to seal off the tannate or phosphate film. I do not generally use this phosphoric acid/tannic acid solution because it does not form the dense black coloration that Baker Chemical tannic acid does by itself.

More recently, Logan (1989) recommends mixing up 1 liter of a 10% tannic acid solution of tannic acid (100 g. tannic acid, 50 ml ethanol, 900 ml deionized water) and if necessary, add sufficient concentrated phosphoric acid so that the solution has a pH of at least 2.4. She also cautions that different brands of tannic acid react differently and recommends BHD Chemical tannic acid as working consistently well. She differs primarily in that she states that a 10% solution of tannic acid is much too concentrated and recommends that several coats of diluted 2-3% tannic acid be brushed on the object. She differs also in that she does not recommend using a sealant over the tannic acid, but instead recommends controlled storage.

For the past 20 years, I have used only the 20% mixture of tannic acid on marine

iron that has been treated, and I have had consistently good results. I have never found this concentration to be too concentrated. However, I have never attempted to use a 2-3% solution of tannic acid on marine iron, and it may well be that this strength works quite well. Otherwise, we agree on the advantages of the treatment. I still recommend that a sealant, such as microcrystalline wax be applied over the ferric tannate film formed over the object, The wax will provide a vapor barrier, which the ferric tannate film does not, and will also contribute some strength to corrosion layers on the metal. Regardless, of the exact procedure, I consider the application of tannic acid to be an inherent part of the treatment of any piece of iron, and especially iron recovered from marine environments.

ANNEALING

Oxidizing Atmosphere

Treating searecovered iron objects by heating to a temperature of 850ø C. was first attempted in Denmark in 1955 (Eriksen and Thegel 1966). It was proposed that at this temperature the chemically bound water associated with the ferrous chloride corrosion products in the porous matrix of the metal would be driven off, leaving inactive anhydrous ferrous chloride. It should be noted, however, that just leaving chlorides in an anhydrous state will not prevent subsequent corrosion. Both ferrous and ferric chloride are deliquescent, capable of absorbing water from the atmosphere and reinitiating the corrosion process unless a perfect airtight, atmosphereproof coating is applied. The success of annealing in the air is probably due to the sublimation of the chlorides. Experiments conducted by Pearson (1972a:25) indicate that a temperature of 850ø C. is well above the melting point of ferrous chloride, and both ferrous and ferric chloride sublimed strongly from a piece of test iron at 700ø C. Therefore, it is probable that the sublimation of ferric and ferrous chlorides occurs at a temperature of 700ø C. and higher.

The apparent absence of corrosion during 20 years of outdoor exposure since some of the cast iron guns were treated in this manner is testimony to its success. Notwithstanding, considerable sacrifices have to be mad

1. There is no opportunity to preserve the iron oxide interface. The corrosion layers spall and crumble, obliterating any decoration or identification markings on the cannon, leaving a badly disfigured surface.

2. The remaining surface is oxidized to an unattractive red iron oxide color.

3. The elevated temperatures alter the metallurgical microstructure of the metal, making it useless for future metallurgical examinations. Because of these shortcomings, the technique of annealing in an oxidizing atmosphere is not recommended.

Reducing Hydrogen Atmosphere

Work by Barkman in connection with the preservation of the Wasa, the 1628 ship raised intact from the Stockholm Harbor in 1961 (Barkman and Franzen 1972), has demonstrated that annealing iron at a temperature of 1060ø C. in a reducing atmosphere successfully stabilizes and preserves it, returns it to a metallic state, and eliminates by sublimation the corrosive chloride compounds.

The iron to be treated is placed in a hydrogen furnace and heated in the presence of hydrogen gas. The temperature is slowly elevated to 1060ø C. over a period of one week. At this elevated temperature, all the moisture is driven off and all the chloride corrosion compounds are volatilized. The hydrogen reduces the iron corrosion compounds back to a metallic state and combines with the oxygen in the corrosion products, forming water which is driven off by the heat.

Annealing in a reducing atmosphere is said to result in very little damage to the surface of the metal. More experiments are needed, however, to determine the extent of any undesirable surface alteration. From the standpoint of time and final results, the technique appears to be a satisfactory, efficient alternative for cleaning iron recovered from the sea.

The primary drawbacks to the technique are the lack of hydrogen kilns sizeable enough to treat large objects and the expense of the equipment even for small objects. Presently, a few conservation laboratories use annealing to treat very numerous, small artifacts, such as cannon balls, with aminimum amount of hands-on handling. One drawback to the treatment is the large amount of HCl produces when the chlorides are driven off and which attack any exposed metal in the kilns or furnaces. When one, some companies who owned industrial furnaces allowed archaeologists to use these furnaces to treat cannons, the detrimental effect the HCl fumes on the metal hardware has discouraged most from volunteering their furnaces.

Generally speaking, objects ranging in size up to about 10 inches by 4 feet is the size of object usually treated by this technique, because most laboratories can only afford the smaller size furnace, which have to be changed all too often. Cannons and other similar size artifacts have been preserved by this process and in most instances they remain stable, even though in most cases they have been annealed in an oxidizing furnace and the surface are badly oxidizing. In general I am not a proponent of this treatment and do not recommend it. Considerable interest has been manifested in objects that have been annealed in a reducing atmosphere. Still, there remains some controversy over possible side effects the heating and cooling have on the surface corrosion layers and the morphology of the metal.

Over the years the temperature to which the hydrogen oven is taken to has been decreased. As recently as 1978, objects undergoing hydrogen reduction were placed in a special furnace with 100% dry hydrogen gas, or a mixture of hydrogen and nitrogen, and heated to a temperature of 300ø C and over a period of days taken to 1000ø C (Barkman 1978:155-166). During the treatment, all the moisture is driven out of the artifact and the chloride corrosion compounds are volatilized. The hydrogen reduces the iron corrosion compounds back to a lower oxidation state or metal. Hydrogen also combines with oxygen in the corrosion products, forming water which is driven off by the heat. This treatment, while successful, has several drawbacks. First it requires rather expensive and sophisticated equipment that is outside the financial capabilities of most laboratories. Also the larger the object, the more expensive the treatment. Second, there is the problem of the changes in the metallurgical characteristics in the metal when heated to high temperatures. Recent information by Tylecote and Black (1980:95) reports:

The loss of information by the treatment of totally rusted marine cast iron at 800 degrees C will not be great and there seems to be little objection to the use of the hydrogen reduction process at 800 degrees C for this purpose. The reduction of rust on wrought iron is a different matter. the main problem is knowing whether the residual metal contains, either intentionally or unintentionally, enough carbon to give useful information to the archaeometallurgist. If Carbon is absent then treatment at 380 degrees C is acceptable although some change will occur. The slag inclusions will suffer very little microscopic change and no macroscopic change. To ensure the removal of chlorides at 380 degrees C the treatment time must exceed 60 hours.

As long as the conservator follows the recommendations cited above, annealing in a reducing atmosphere of hydrogen, the major objections are overcome. The main limiting factor is the high cost of this type of furnace, which also limits the size one can afford to purchase and the inherent safety concerns and potential danger of heating hydrogen to these temperatures. The treatment does result in stable, chloride free artifacts.

HYDROGEN PLASMA REDUCTION

Hydrogen plasma reduction is a new technique being experimented with in a number of laboratories (Patscheider & Vepøek 1986). In this treatment, iron artifacts, as well as those of copper and silver, have been conserved by placing them in a quartz discharge tube surrounded by hydrogen gas under a low pressure which is ionized into plasma by the introduction of high frequency radio waves. In the process iron is in the center of the hydrogen plasma and the magnetite and ferric oxide on the piece are converted to metallic iron. Because the treatment is carried out at a temperature of less than 400^o C. there is no change in the metallic structure of the iron.

The primary disadvantage of this conservation technique, although preliminary results have been encouraging, is the high cost of the equipment and the small size of the artifacts that can be treated in all but the most expensive units. A unit capable of treating a single artifact in a chamber about 30 cm X 10 cm. costs in the neighborhood of $15,000.00. Thus far, the technique has not seen use in most conservation laboratories.

WATER DIFFUSION

In any archaeological excavation of a ship, there will always be some artifacts that can not be conserved by any the treatments discussed above. Any electrochemical or chemical treatment would considerably alter the form of the object. Little or no metal may remain. Three main choices are possible. The chlorides can be removed by a process referred to as water diffusion, the article can be embedded in plastic, or the artifact can be cast. The lastnamed choice is discussed under Casting and Molding.

The only way an iron artifact recovered from the sea can be stabilized is to remove the chlorides from the metal. This is most easily accomplished by electrolytic reduction techniques, but if the artifact is very badly oxidized, and the overall form and dimensions of the object are to remain intact, the only alternative is to remove the soluble chlorides in the much slower process of water diffusion (Oddy and Hughes 1970). The name accurately describes the process. The artifact is placed into a container filled with water, and the water is changed frequently as the soluble chlorides diffuse out of the metal into the solution. The water should be changed weekly or as often as necessary as determined by a qualitative or quantitative chloride test. However, for all intent and purpose, water diffusion is not an option when it comes to treating iron recovered from a marine environment. It just does not remove the chlorides within in any accepted time frame when any other option is available. It is only a consideration, when one is attempting to conserve an artifact that is so badly corroded that it chances of being destroyed if it were cleared by electrolytic reduction or by the alkaline sulfite treatment discussed elsewhere.

Since water diffusion requires a long time, the water must be inhibited to prevent the metal from rusting. Alkaline chemicals, such as a 5% sodium sesquicarbonate, 5% sodium carbonate, or 2% sodium hydroxide solution, serve only as inhibitors to prevent rusting; it is the water that removesthe soluble chlorides. For chloridecontaminated iron, the solution is made with tap water until the maximum chloride level approximates that of the tap water; then deionized water is substituted. Uninhibited deionized water should not be used in water diffusion because it is very corrosive.

Alternating hot and cold temperatures, suggested by Organ (1955) is said to speed up chloride removal by alternately expanding and contracting the capillaries in the metal and the corrosion layer, causing a flushing action of expelling and drawing in fresh water. From a standpoint of coefficients of thermal expansion, however, the alternate heating and cooling probably changes the diffusion gradient of the solution rather than significantly changing the size of the metal capillaries. In some instances this will decrease the time required to remove the soluble chlorides, but Oddy and Hughes (1970:187) found that there was no significant difference between the time required to wash similar objects at room temperature and at ca. 50ø C. for both iron and bronze. The alternated heating/cooling cycle may facilitate the treatment, however, when significant levels of chloride are present and months to years are required to remove them. After the soluble chlorides have been eliminated, the artifacts need to be carried through the same final steps as iron treated by other methods.

FINAL CONSERVATION STEPS

Rinse After Treatment

Following any conservation treatment electrochemical, electrolytic, chemical, or water diffusion it is necessary to remove insoluble oxide sludge, metallic powder, residual chlorides, and all the chemical residues through an intensive rinsing (Plenderleith and Werner 1971:20). In electrolytic reduction or water diffusion the artifact is removed after establishing that the chloride count in the solution has leveled off and ceases to rise when it is changed. The artifact is then removed and rinsed thoroughly in several changes of alternate boiling and cold deionized water to get rid of any residual electrolyte and chlorides. By rinsing in boiling water the surface of the metal oxidizes to a flat black color that provides a pleasing appearance. Since large objects may require two to four weeks of rinsing, the iron may rust in the deionized water. This can be prevented by adding gluconic acid, sodium gluconate or sodium glucoheptanate, The gluconates act as rust inhibitors during any washing and continue to serve in this capacity during solvent dehydration, heatdrying or air drying. Still, in practice, these are not commonly used unless there is some problem. Pearson (1972a:1314) prevented Captain Cook's cannons from rusting during the rinse process by washing with a potassium chromate solution (1000 ppm chromate) with a pH not lower than 8.5. But the strict disposal requirement of chromate solutions prevents their being used on a large scale. Neither the gluconates or chromate solutions are used widely, and satisfactory results are achieved without them. Still, it is worth know of the protection that they can provide, when and if their use is required.

The artifact is allowed to stand in the last vat of rinse water for a minimum of 24 hours. A sample of the bath water is taken and acidified with nitric acid and tested with .2 N silver nitrate for the presence of chloride. The silver nitrate test is suggested because it is quick, qualitative, and quitesensitive to minuscule amounts of chlorides. If the test is positive the artifact is returned either to electrolysis or further rinsing. If the test is negative the artifact is ready to be dried and sealed with microcrystalline wax.

Specimens treated by water diffusion are put through a similar rinsing process. Since, however, many of the objects treated by water diffusion are in very fragile condition, they may not be able to stand the mechanical action of boiling water. The rinse water, if heated, is kept below the boiling point. Sodium glucoheptanate is added to the rinse water as a rust inhibitor.

Qualitative Test for Chlorides

The presence or absence of chlorides is determined by the silver nitrate test (Plenderleith and Werner 1971:201). The artifact is placed in distilled or deionized water for a few hours or overnight. A 10 to 20 milliliter sample of the solution is placed in a test tube and acidified with a few drops of dilute nitric acid (ca. 10%). The solution is mixed and five drops of .2 N silver nitrate (17 grams of AgNO₃/1 liter of H₂O) is added. The test tube is held against a black background with good side lighting. If any chlorides are present a white opalescence will be apparent. Under ideal conditions, with clean glassware and uncontaminated reagents, the test provides a good qualitative indicator for the presence or absence of chloride.

Drying

After rinsing, the moisture absorbed by the artifact must be removed before any sealant, except certain waxes which are heated above the boiling point of water, can be applied. When specimens can be immersed in any wax heated above the boiling point of water, drying is an optional step. Artifact drying can be accomplished by heat, vacuum desiccation, or dehydration in watermiscible alcohol or acetone. After treating iron, the metal surfaces are in a reactive state and quickly rust on exposure to air. Contact with air should be minimized until tannic acid can be applied to the surface, if the black coloration is desired, or the final sealant or insulating coating is applied to the artifact in order to prevent superficial rust that quickly forms.. Some exposure to air is inevitable and it is particularly troublesome when drying by heat (ovens or infrared lamps) or vacuum desiccation. (Using gluconate rinses could be helpful here to prevent rust). Also, infrared lamps are not very effective on dense objects and it is expensive to obtain ovens or vacuum chambers to accommodate very large specimens.

An alternative is to use a watermiscible solvent, ethanol, methanol, isopropanol, or acetone. Isopropanol is recommended because it is nontoxic, has a higher flash point, and does not have an obnoxious odor. Ethanol, and acetone are as effective, or even more effective, but suffer from one or more of the above disadvantages. Each of these solvents surmounts the problems of rusting when exposed to air and can be used on objects of any size. For objects with little metal remaining, drying in an oxygenfree environment, such as provided by alcohols, is necessary to prevent the remaining metal from rusting and ferrous compounds from oxidizing to a ferric state. Both reactions will cause artifacts to expand and slough off the oxide layers. Alcohols also have the advantage of enhancing the removal of any remaining soluble chlorides and water in the specimens. In addition, all stainsand undesirable features can be removed by brushing while the objects are still in the alcohol. Artifacts also can be held indefinitely or stored in alcohol until it is convenient to process them.

Upon completion of the waterrinsing, an artifact is removed while the water is hot and wiped with rags. This allows most of the water to evaporate before giving it a preliminary rinse in alcohol that has been previously used for drying wet objects to remove the bulk of the remaining surface water. It is then submerged in the water free isopropanol to dehydrate for a minimum of 24 hours. By taking these precautions the water content of the alcohol bath is kept low and it can be used for long periods. When the water content becomes sufficiently high it is used for the preliminary rinse and fresh alcohol is used for the dehydration bath. This efficiency procedure is important during periods of shortages and high prices.

Sealant and Consolidation

After treatment of searecovered iron objects it is imperative that their surfaces be covered with a protective coating to insulate the metal from the effects of moisture, chemically active vapors, and gases. It is highly important to choose the right sealant or coating to provide a protective moisture barrier and prevent corrosion. In general, the sealant selected should be: 1) impervious to water vapors and gases, 2) naturallooking so that it does not detract from the appearance of the artifact, 3) reversible, and 4) transparent or translucent so any corrosion of the metal surface can be quickly detected.

Various monomers, acrylates, acetates, epoxies, paints, oils, lacquers, and other sealants have been used in the past, but few have withstood the test of time. Many craze, peel, are irreversible, or have a high degree of permeability to water vapor. No one sealant is completely successful and all have some disadvantages, but microcrystalline waxes best satisfy the requirements of conservation. Most importantly, they are the least permeable to water vapor of any of the sealants commonly used (Rudniewski and Tworek 1963:212). They have a high melting point and are relatively hard waxes. Besides sealing the surface of the artifact from the atmosphere and moisture, they provide considerable stability and strength to artifacts and are excellent for consolidating fragile objects. I recommend and have been using microcrystaline wax on both cast iron and wrought iron that is to be stored and displayed indoors. In contrast, some laboratories (North 1987:230) use microcrystaline wax as the final coat only on cast iron artifacts.

Cosmoloid 80H is the most oftenrecommended microcrystalline wax seen in much of the conservation literature, but it is not available in the United States. At various time over the past 20 years I have used Gulf 75 Microwax, but for the last 15 years I have relied exclusively on Witco 180M microcrystalline wax. Both melt at approximately 180ø F. and have been satisfactory, although Gulf 75 is a little harder and more brittle. There are other satisfactory substitutes both here in the United States and other countries.

In some instances it may be advisable to dewater, dry or dehydrated the artifact in alcohol before it is placed in a vat of microcrystalline wax; however, when microcrystalline wax is used as the final sealant it is possible to eliminate the drying process for a great many iron artifacts. The wax isheated to 350ø F. (176ø C.) to 400ø F., well above the boiling point of water. Therefore, it is possible to take the artifact directly out of the water rinse and place it in melted wax. As the temperature of the wax rises, any water in the artifact is vaporized. The object must be kept in the wax long enough and at a high enough temperature to completely vaporize the water. Since the water boils out of the artifact, this alternative should not be used on fragile objects or objects with a loose oxide layer. These fragile specimens should be dried by one of the methods mentioned above. With this exception, combining the water removal and the sealant steps remain a very satisfactory approach. Time and expense are saved and good results are achieved.

If the object is in fragile condition, all the alcohol must be evaporated; otherwise, the vaporization of the alcohol in the corrosion layers by the hot wax can damage the specimen. The object is left in the wax until all bubbles stop evolving from the artifact. This may require several days for large artifacts. After complete penetration the wax is cooled to 200ø to 225ø F. (93ø to 107ø C.), the artifact is removed and the excess wax is promptly wiped off with rags.

The temperature at which the artifact is removed determines the thickness of the wax coating. Too low a temperature results in an obvious layer of wax, while at too high a temperature all the wax runs off. After cooling, if any excess wax should remain, it can be removed with a torch, a hot air gun, or by scraping lightly with a knife. The lastnamed is the simplest method and leaves the least obvious scars on the wax film. Additional wax or wax with graphite added to it as a pigment can be used to cover\ surface defects in the metal and enhance the appearance of badly corroded objects.

In many laboratories facilities are not available to impregnate large objects such as cannons and anchors with microcrystalline wax, so various other coatings must be used. There have been experiments with chromate paints, lacquers, clear epoxies, linseed oil, and polyurethane. In general, all but polyurethane were found to be ineffective. Over a period of months they crazed, peeled, and became permeable to moisture. The opaque coatings hid the surface of the artifact from view, preventing one from observing the corrosion occurring under the coating. In addition, the surface finish of the epoxy was too glossy and was irreversible, causing further damage to a few specimens which had to be retreated.

Polyurethane based paints or coatings are thermoplastic polymers that have many favorable attributes for serving as a protective coating on treated iron objects. They form clear, fastdrying, hard, flexible coatings with excellent adhesion that are highly resistant to moisture, salt water, acids, alkalis, abrasion, impact, and weathering. The coatings can be removed with aromatic and chlorinated solvents such as toluene or ethylene dichloride. Polyurethane comes in gloss and stain finishes. The gloss finish has more resin and is therefore more durable. It is recommended for outdoor use. The satin finish has less resin and has silica added to give a more acceptable flat finish, but it is less durable than the gloss finish and is generally recommended for interior use. By using an undercoat of gloss and a second coat of satin finish and by adding graphite to one or both coats, a very acceptable, translucent finish can be obtained that does not detract from the underlying surface color of the specimen being coated. A number of years back, I processed a large 18pound Civil War cast iron siege cannon recovered from Galveston Island on the Gulf Coast. After cleaning, a sealant had to be applied, but the cannon was larger than any of the wax vats in the laboratory. It wasdecided to paint the surface of the cannon with a 20% tannin solution to form a corrosionresistant ferric tannate film on the surface of the cannon. The painted surface was allowed to air oxidize for two days, changing the gray surface color of the cannon to a more pleasing blueblack color. The cannon was then painted with a coat of clear gloss polyurethane, allowed to dry, and then painted with a coat of satin polyurethane. Graphite was added to both the gloss and the satin polyurethane to completely dull any glossiness to the surface finish. The results were very satisfactory. The use of polyurethane coatings in the manner described above or by themselves can be recommended for maximum protection of large iron artifacts to be displayed outside or in areas of high humidity and salt vapor in the air (Hamilton 1976::55; North and Pearson 1975:177; North 1987:230)

Success has also been had with using Rustolium, a fish oil based paint, but it has a only 10 years, as opposed to 20 years for polyurethane (North 1987:230).

For wrought iron artifacts to be displayed indoors, North (1987:231 recommends, after drying with commercial dewatering fluids , using clear drying zinc phosphate-based anti-corrosion primer as the first coat, followed by up to 6 sprays of high durability, clear, acrylic lacquer, and finished off with a final coat of Krylon Mat Spray Finish (North and Pearson 1975:177, North 1987:230).

With the exception of microcrystaline wax, which is easily removed by placing the artifact in a vat of boiling water, all the others present some problems. Polyurethane has to be sandblasted off, and the Rustolium has to be place in sodium hydroxide to remove the paint. For ease of application, resistance to water vapor, the presence of chloride, transparency, ability to strengthen the surface of the artifact, microcrystaline wax stands out for use on both cast iron and wrought iron that are stored and displayed indoors. In an object is to be displayed outdoors, or if it can not be treated with microcrystaline wax, then polyurethane based paints is recommended. .

For wrought iron artifacts to be displayed indoors, North (1987:231 recommends, after drying with commercial dewatering fluids , using clear drying zinc phosphate-based anti-corrosion primer as the first coat, followed by up to 6 sprays of high durability, clear, acrylic lacquer, and finished off with a final coat of Krylon Mat Spray Finish (North and Pearson 1975:177, North 1987:230).

With the exception of microcrystaline wax, which is easily removed by placing the artifact in a vat of boiling water, all the others present some problems. Polyurethane has to be sandblasted off, and the Rustolium has to be place in sodium hydroxide to remove the paint. For ease of application, resistance to water vapor, the presence of chloride, transparency, ability to strengthen the surface of the artifact, microcrystaline wax stands out for use on both cast iron and wrought iron that are stored and displayed indoors. In an object is to be displayed outdoors, or if it can not be treated with microcrystaline wax, then polyurethane based paints is recommended.

For large objects to be displayed outdoors Townsend (1972a:253) suggests using a coating made up to three parts zinc silicate powder mixed with two parts water. This mixture forms a light beige paint that oxidizes to a light nautical gray. The coating, being anodic, provides cathodic protection of the iron object and is said to be highly resistant to salt spray, rain, sunshine, and temperature fluctuations. Large anchors and other implements painted with zinc silicate have been displayed outdoors in North Carolina without damage for more than three years.

Artifacts that are so badly corroded that they cannot be treated, and compound objects with metal and organic parts requiring treatment but which cannot be separated, can be embedded in clear plastic blocks. Smith and Ellis (1961:3235) describe the process of embedding a wrought iron swivel gun and a Spanish battle sword in Selectron 5000 Resins. This technique is drastic, with no hope of ever extracting the artifacts from the blocks, but it remains a possibility for very select, problem artifacts.

Storage and Periodic Inspection

The preservation of antiquities should produce objects that are chemically stable with an aesthetically acceptable appearance. Treatment should be reversible in the event the object should require additional preservation; after an artifact has been completely processed it can deteriorate. Only if stored or displayed under optimum conditions can this be prevented. Atmospheric pollutants, sulfur dioxide, hydrogen sulfide, sodium chloride, dust, and soot, to name the more common, are detrimental, ubiquitous, and difficult to control even inside a reasonably tight building. Even more critical is the relative humidity in which an artifact is stored. The moisture level at which corrosion appreciably accelerates is called the critical humidity and is considered to be 60% for iron and steel (Cornet 1970:443). If iron still contains chlorides (theoretically it remains doubtful that all can be removed), a humidity as low as 50% may have to be maintained. Below this critical humidity, subsequent corrosion sometime in the future is inevitable. All the potential corrosive factors should be taken into consideration when storage facilities are being planned.

Since metal artifacts can eventually become chemically unstable from a myriad of causes and may need additional treatment, periodic inspection, and evaluations of the artifacts are necessary. A conserved artifact of iron from a marine site, remains a piece of metal, just as susceptible, and in fact more susceptible, to continued corrosion as any other piece of iron. Proper conservation does not assure one of an object preserved in perpetuity. At our present stage of knowledge, perhaps it is most realistic to say that the objective of antiquities conservation is to delay by proper storage reprocessing as long as possible and to make any necessary treatment simple and brief. There remains a lot of room for improvements in the conservation of iron. Still, at our present state, there are a number of procedures that successfully contend with the majority of problems encountered when conserving iron.

SUMMARY

Based on my own experience, and the experiences of others as reflected in the published literature, the majority of the iron recovered from marine sites is treated by electrolytic reduction. This treatment consistently produces stable artifacts with a minimum of equipment, less costly equipment, chemicals and less, hand on treatment time. For a number of problem artifacts, the alkaline sulfite treatment is commonly used. Less common, but still utilized by a few laboratories is hydrogen reduction of iron, but cost of the equipment prohibits it more general use, and the same applies to hydrogen plasma reduction. Although not a consistently reliable treatment, various formsof intensive rinsing are sometimes employed on problem artifacts, and usually in conjunction with other treatments.

To carry out the above treatments, adequate space and equipment are required. Equipment includes: various regulated D.C. power supplies, plastic vats, metal vats, anode material, wire, clips, fume hoods, tap water supply, D.I. water, sodium hydroxide, sodium carbonate, tannic acid, sodium sulfite, pneumatic chisels, air compressor, microcrystaline wax, polyurethane, fork-lift if heavy artifacts are to be treated, source for heating the rinses and wax, mercuric nitrate, silver nitrate, nitric acid, sulfuric acid, expanded scale, pH meter, x-ray machine, epoxies and materials for casting are the more essential items. All are easily secured at reasonable expense, and much of the equipment can be secured through federal surplus.

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