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 chemical residue through 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 rinsed thoroughly in several changes of alternate boiling and cold de-ionized water to remove any residual electrolyte and chlorides. Boiling water rinses oxidize the surface of the metal to a flat, pleasing black color. Iron artifacts may rust in de-ionized water during prolonged periods of rinsing; this can be prevented by adding gluconic acid, sodium gluconate, or sodium glucoheptanate to the rinse water. The gluconates act as rust inhibitors during the rinsing process and continue to serve in this capacity during solvent dehydration, heat drying, or air drying. Pearson (1972a:13-14) prevented Captain Cook's cannons from rusting during the rinse process by rinsing with a potassium chromate solution (1000 ppm chromate) with a pH not lower than 8.5. The strict disposal requirements for chromate solutions, however, prevent their large-scale use. Neither the gluconates nor chromate solutions are used widely, and satisfactory results are achieved without them. Nonetheless, conservators should be aware of the protection that they can provide, when and if their use is required.
The artifact should 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 0.2 N silver nitrate for the presence of chloride. The silver nitrate test is suggested because it is quick, qualitative, and quite sensitive 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 many of the objects treated by water diffusion are, however, very fragile, they may not
be able to withstand the mechanical action of boiling water. The rinse water, if heated, should be kept below the
boiling point for such artifacts, and sodium glucoheptanate should be 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 de-ionized water for a few hours or overnight. A 10- to 20-ml sample of the solution is placed in a test tube and acidified with a few drops of dilute nitric acid (ca. 10 percent). The solution is mixed and five drops of 0.2 N silver nitrate (17 g 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 chlorides.
After rinsing, the moisture absorbed by the artifact must be removed before any sealant, with the exception of 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 water-miscible alcohol or acetone. After treating iron, the metal surfaces are in a reactive state and quickly rust on exposure to air. In order to prevent the rapid formation of superficial rust, contact with air should be minimized until tannic acid or the final sealant or insulating coating is applied to the surface of the artifact. Some exposure to air is inevitable, and it is particularly troublesome when drying by heat (ovens or infra-red lamps) or vacuum desiccation. Infra-red 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 water-miscible solvent, ethanol, methanol, isopropanol, or acetone. Isopropanol is recommended because it is non-toxic, has a high flash point, and does not have an obnoxious odor. Ethanol and acetone are as effective, or even more effective, as isopropanol but are toxic and have lower flash points. Each of these solvents prevents the problems of rusting when exposed to air and can be used on objects of any size. Drying in an oxygen-free environment, such as that provided by alcohols, is ideal for objects with little metal remaining, since it will 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 stains and undesirable features can be removed by brushing them while the objects are still in the alcohol. Artifacts also can be stored indefinitely in alcohol until it is convenient to process them.
When rinsing is complete, the artifact is removed while the
water is hot and wiped with rags. This allows most of the water to evaporate. The artifact is immersed in alcohol
that has been previously used for drying wet objects in order to remove the bulk of the remaining surface water.
It is then submerged in water-free isopropanol to dehydrate for a minimum of 24 hours. By taking these precautions,
the water content of the isopropanol bath will remain low and it can be thus used repeatedly. When the water content
of the isopropanol 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 material shortages and high prices.
SEALANT AND CONSOLIDATION
It is imperative that the surfaces of treated marine-recovered iron artifacts be covered with a protective coating to insulate the metal from the effects of moisture, chemically active vapors, and gases. The right sealant or coating should be chosen to provide a protective moisture barrier and prevent corrosion. In general, the sealant selected should be (1) impervious to water vapors and gases, (2) natural-looking, 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 of these sealants craze, peel, are irreversible, or have a high degree of permeability to water vapor. No single sealant is completely successful and all have some disadvantages, but microcrystalline waxes best satisfy the requirements of conservation. They have a high melting point and are relatively hard; in addition, they are the least permeable to water vapor of any of the sealants commonly used (Rudniewski and Tworek 1963:212). Besides sealing the surface of the artifact from the atmosphere and moisture, they provide considerable stability and strength to the object and are excellent for consolidating those which are fragile. In contrast, some laboratories (North 1987:230) use microcrystaline wax as the final coat only on cast-iron artifacts; its use on wrought iron that is to be stored and displayed indoors is also recommended.
Cosmoloid 80H is the most often-recommended microcrystalline wax mentioned in the conservation literature, but it is not available in the United States. Gulf 75 Micro-wax and Witco180M microcrystalline wax melt at approximately 180°F and make satisfactory substitutes. In some instances it may be advisable to dehydrate 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. Artifacts can be taken directly out of the rinse water and placed in a vat of wax that is heated to 175°C, well above the boiling point of water. The artifact must be kept in the wax long enough and at a high enough temperature to completely vaporize the water and until bubbles stop evolving from the artifact. This may require several days for large artifacts. After complete penetration, the wax is cooled to 93-107°C, the artifact is removed, and the excess wax is promptly wiped off with rags. Since the water boils out of the artifact, this technique should not be used on fragile objects or objects with a loose oxide layer; fragile specimens should be dried by one of the methods mentioned above. With this exception, combining the water removal and the sealant steps remains a very satisfactory approach. Time and expense are saved, and good results are achieved.
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 of the wax will run off the surface of the object. If any excess wax should remain after cooling, it can be removed with a torch, a hot air gun, or by scraping lightly with a knife. Scraping 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 other coatings must be used. There have been experiments with chromate paints, lacquers, clear epoxies, linseed oil, and polyurethane. In general, all but polyurethane are ineffective. Over a period of months they craze, peel, and become permeable to moisture. The opaque coatings hide 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 is too glossy and is irreversible, causing further damage to a few specimens which have to be re-treated.
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, fast-drying, tough, flexible coatings with excellent adhesion and are highly resistant to moisture, salt water, acids, alkalis, abrasions, 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.
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 (North and Pearson 1975:177; Hamilton 1976:55; North 1987:230). For example, a large 18-lb. Civil War cast-iron siege cannon recovered from Galveston Island on the Texas Gulf coast needed to be sealed after treatment, but the cannon was larger than any of the wax vats in the laboratory. The surfaces of the cannon were painted with a 20 percent tannin solution to form a corrosion-resistant ferric tannate film. The painted surfaces were then left to air oxidize for two days. 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.
Success has also been had with using Rustoleum, a fish-oil-based paint, but it has only a 10-year durability span, as opposed to 20 years for polyurethane (North 1987:230). For sealing wrought-iron artifacts that are to be displayed indoors, North (1987:231) recommends using clear- drying zinc phosphate-based anti-corrosion primer as the first coat, followed by up to six applications of high-durability, clear acrylic lacquer, and finished off with a final coat of Krylon Matte Spray Finish (North and Pearson 1975:177, North 1987:230). For large objects that are to be displayed outdoors, Townsend (1972a:253) suggests using a mixture of three parts zinc silicate powder to 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 many years.
With the exception of microcrystaline wax, which is easily removed by placing the artifact in a vat of boiling water, all other sealants present some problems in terms of reversibility. Polyurethane must be sandblasted off, and Rustoleum can only be removed with sodium hydroxide. For ease of application, resistance to water vapor, transparency, and the ability to strengthen the surface of the artifact, microcrystaline wax is the best sealant for both cast-iron and wrought-iron objects that are stored and displayed indoors. If an object is to be displayed outdoors, or if it cannot be treated with microcrystaline wax, polyurethane-based paints are recommended.
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 (1961:32-35) 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.
AND PERIODIC INSPECTION
The conservation of artifacts should produce objects that are chemically stable with an aesthetically acceptable appearance. Treatment should be reversible in the event that the object should require additional preservation. Re-treatment is generally prevented only if the artifact is stored or displayed under optimum conditions. Atmospheric pollutants, sulfur dioxide, hydrogen sulfide, sodium chloride, dust, and soot are detrimental, ubiquitous, and difficult to control even inside a reasonably sealed building. The relative humidity in which an artifact is stored is a particularly critical factor in its stability. The moisture level at which corrosion appreciably accelerates is called the critical humidity and is considered to be 60 percent for iron and steel (Cornet 1970:443). If iron still contains chlorides, a humidity as low as 50 percent may have to be maintained. Subsequent corrosion sometime in the future is inevitable with higher relative humidities. 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 inspections 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 ensure that an object will be preserved in perpetuity. At our present stage of knowledge, perhaps it is most realistic to say that the objective of artifact conservation is to delay eventual re-treatment for 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 the present state, there are a number of procedures that successfully contend with the majority of problems encountered.
The majority of iron artifacts recovered from marine sites is treated by electrolytic reduction. This treatment consistently produces stable artifacts with a minimum of inexpensive equipment and chemicals, as well as a minimum of 'hands-on' treatment time. The alkaline sulfite treatment is commonly used for artifacts with badly corroded surfaces that could possibly flake off during electrolysis. Less common, but still used by a few laboratories, is hydrogen reduction of iron, but the cost of the equipment prohibits it more general use; this also applies to hydrogen plasma reduction techniques. Although not a consistently reliable treatment, various forms of 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. Essential equipment includes various regulated DC power supplies, plastic and metal vats, anode material, wire, clips, fume hoods, pneumatic chisels, air compressors, a fork-lift truck if heavy artifacts are to be treated, a source for heating the rinses and wax, a pH meter, and an X-ray machine. All are easily secured at reasonable expense, and much of the equipment can be secured through federal surplus.
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Copyright 2000 by Donny L. Hamilton, Conservation Research Laboratory, Texas A&M University.
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