IN THIS FILE:
Electrolytic Reduction Cleaning
Reduction in Formic Acid
Reduction in Sodium Hydroxide
Rinse and Sealant
Stabilization and Consolidation
Silver is a very noble metal and is often found in a native state combined with gold, tin, copper, and platinum. It is completely stable in aqueous solutions of any pH as long as oxidizing agents or complexing substances are not present. In addition, silver is not appreciably affected by dry or moist air that is free from ozone, halogens, ammonia, and sulfur compounds (Pourbaix 1966:393; Plenderleith and Werner 1971:239). Silver is particularly susceptible to the effects of the sulfide radical. This is best demonstrated by the formation of tarnish on silver objects that are exposed to sulfur in any form, particularly hydrogen sulfide and sulfur dioxide, which can convert to sulfuric acid.
In a marine environment, with its abundance of soluble sulfates and oxygen-consuming, decaying organic matter, sulfate-reducing bacteria utilizes available sulfates under anaerobic conditions to form hydrogen sulfides as a metabolic product. The hydrogen sulfide reacts with the silver to form silver sulfide. The overall reaction proceeds in the same process as described earlier for iron:
2Ag + H2S >> Ag2S + H2
In anaerobic marine environments, silver sulfide (Ag2S) is the most common mineral alteration compound of silver (North and MacLeod 1987:94). It is commonly reported from shipwrecks in the Caribbean and Australia and constitutes the most prevalent corrosion compound on silver objects from marine sites. Most marine-recovered silver artifacts have a thin sulfide surface layer, which has removed some surface detail, such as inscriptions, marks, and stamps. A large percentage of the artifacts, however, are completely converted to sulfide; others have only minimal metal remaining.
In aerobic seawater, the most commonly encountered corrosion product on silver and silver alloys is silver bromide (AgBr). Varying amounts of silver chloride (AgCl) and silver sulfide (Ag2S) may also be present (North and MacLeod 1987:94). Silver chloride is generally not extensive on silver recovered from salt water. Gettens (1964:563) notes that silver coins recovered from salt water are sometimes superficially altered to this mineral. In sites where the conditions vary between aerobic and anaerobic, combinations of all the major silver corrosion products are likely to be present (North and MacLeod 1987:94-95). In the case of relatively pure silver objects, silver sulfide (Ag2S) and silver chloride (AgCl) will predominate. In the case of base silver alloys with significant amounts of copper, the copper will corrode preferentially and form cuprous oxide, cupric carbonate, and cuprous chloride. In such cases, the silver alloy object should be treated as if it were copper.
Regardless of what silver corrosion products are formed, all are stable and do not take part in any further corrosive reaction with the remaining silver. In fact, the corrosion layers impart some degree of protection against further corrosion to the metal. They also often provide an aesthetically pleasing patina, which is often desirable and deliberately preserved. The only reason to treat silver is to remove disfiguring corrosion layers to reveal detail, for aesthetic reasons, to reduce mineral products back to a metallic state, and to remove the chlorides from the copper component part of base silver alloys.
Prior to conservation treatment, marine encrustation should be removed mechanically or, in some cases, by immersion in 10-30 percent formic acid solution. The conservation alternatives for cleaning silver and silver alloys are:
1. galvanic cleaning
2. electrolytic reduction
3. alkaline dithionite treatment
4. chemical cleaning
5. stabilization and consolidation.
Treating silver galvanically can be accomplished by using mossy zinc or aluminum in caustic soda, as described earlier for iron. Variations include using mossy zinc or aluminum granules with heated 30 percent formic acid (Plenderleith and Torraca 1968:241-246; Plenderleith and Werner 1971:197, 221). After treatment, the metal is rinsed thoroughly and then dehydrated in a water-miscible solvent and sealed with clear acrylic lacquer. Galvanic cleaning is effective, but there is no reason to recommend it over electrolytic reduction or alkaline dithionite treatments.
The electrolytic cleaning of silver takes advantage of the reduction action of electrolysis by removing the chloride and sulfide ions from silver chloride and silver sulfide. When a direct current is applied, the negatively charged chloride and sulfide ions migrate toward the positively charged anode. The chlorides may form as chlorine in the solution, and the sulfides oxidize to sulfates. Since the anions do not react with the inert anodes, they accumulate in the electrolyte and are discarded with it. During the process, the silver in the corrosion compounds is left in a metallic state.
Two methods of electrolytic reduction cleaning have been described in the conservation literature; the methods are referred to by Organ (1956) as normal reduction and consolidative reduction. Normal electrolytic reduction uses a fully rectified direct current (DC) power supply. Consolidative reduction employs a partially rectified (asymmetrical) alternating current (AC) power supply. Both techniques require that a metal core be present in the object. The Conservation Research Laboratory at Texas A&M University deals primarily with the normal reduction process in 5 percent formic acid, essentially as it is described in Plenderleith and Werner (1971:222). Both techniques are discussed below.
Two electrolytes, formic acid (HCOOH) and sodium hydroxide (NaOH), are used to clean silver. Although electrolyte concentrations of 5-30 percent HCOOH and 2-15 percent NaOH in de-ionized water have been proposed (Organ 1956:129; Plenderleith and Werner 1971:222; Pearson 1974:299), 5 percent HCOOH or 2 percent NaOH solutions are generally used as electrolytes for cleaning silver.
Silver is easily reduced in electrolysis, regardless of the voltage or setup. North (1987:240) has observed that good results have been obtained during silver electrolysis with a wide range of applied voltage, and that the voltage applied during electrolysis does not appear to be critical. Since the number and size of items being treated is variable, Pearson (1974:299) adjusts the current to produce a cell voltage of approximately three volts. Plenderleith and Werner (1971:198) state that the current density should not be allowed to fall below 0.02 amps/cm2 in order to prevent a film of salmon-pink copper from the corrosion materials, cathode screen, or copper leads from being deposited on the artifacts. In a series of experiments, Organ (1956:134) found that a low current density of 30-50 milliamperes/dm2 (0.3-0.5 amps/cm2) reduced more silver than higher current densities. Even the extremely low current density of 0.01 amp/cm2 recommended by Organ (1956:129) provides satisfactory results. In most cases, a very low current density in the range proposed by Organ is best for maximum metal reduction during the electrolytic cleaning of silver.
When treating silver, inert anodes, such as expanded platinized titanium and No. 316 stainless steel, are preferable. In some of the older conservation literature, carbon anodes are recommended, but they are no longer used, since they will invariably dissolve in the electrolyte. Platinized titanium can be used in both alkaline and acid electrolytes; it is especially recommended for use in acid electrolytes because it is almost totally inert and will not react with the electrolyte. The extremely high cost of platinized titanium, however, limits its widespread use. No. 316 stainless steel anodes are a good substitute for platinized titanium as long as formic acid is used as the electrolyte. Stainless steel anodes will oxidize after prolonged electrolysis in sodium hydroxide, resulting in the destruction of the anode and the deposition of iron on the silver. Mild steel anodes can be used in sodium hydroxide electrolytes; however, they should not be used in formic acid, as the mild steel will quickly break down and invariably result in iron deposition on the silver.
The electrolytic cell for silver cleaning can be set up using any of the methods described in the section on iron. As for iron, the setup in which artifacts are attached with clips to a cathode rod and sandwiched between two suspended anodes (Figure 10B.1D) is most commonly used by conservators. This electrolytic setup is useful for treating several artifacts at once and can be used for coins and other small pieces. When treating silver by electrolysis, however, the conservator may want to avoid attaching clips to small silver pieces in order to not scratch the surface. This is especially true for fragile coins and delicate pieces of jewelry. Direct, individual clip connections between the artifact and the cathode can be eliminated by using a cathode conductor screen made of copper mesh (see Figure 13.1). The specimen to be cleaned makes an electrical contact through the cathode screen, which is connected to the negative terminal. The areas of the screen not used for making contact between the cathode and the artifact should be covered with silicone rubber. The rubber keeps the objects separated and reduces the amount of exposed copper surface, which will minimize the problem of copper plating on the silver.
Figure 13.1. Electrolytic setup for cleaning silver coins or other small artifacts.
Silver artifacts are ready to be electrolytically cleaned after any encrustation has been removed with a small pneumatic chisel, and the artifacts have been thoroughly rinsed. Small specimens can be set up as shown in Figure 13.1. This setup is designed to clean coins, but it is applicable to any small silver or other non-ferrous metal objects. The setup uses a glass container, a copper mesh cathode conductor screen, a wooden support frame for the anode, and an expanded platinized titanium or stainless steel No. 316 anode attached to a mild steel rod. The rod is covered with silicone rubber to ensure that only the platinized titanium or stainless steel will act as the anode. After the artifacts are placed onto the cathode screen, the current is applied, and an electrolyte of 5 percent formic acid is added. In order to prevent any of the salts in the electrolyte from plating onto the surface of an artifact, the current should never be turned off while the artifacts are in the electrolyte. This will considerably reduce the problem of copper plating on the surface of the silver. While the current remains on, the objects should be periodically removed, brushed under de-ionized water, and dipped in a 0.2N solution of silver nitrate to remove any plated copper and superficial sulfides. The objects are then placed back upon the cathode screen with the opposite side facing up. Electrolysis is continued until each side of the artifact has a uniform appearance, and hydrogen is fully evolving from the surface. Small objects, such as coins, generally require only a few hours of electrolysis. Large silver objects or irregularly shaped pieces can be cleaned in the manner as described above, except that the object is connected to the negative terminal with a clip rather than via a cathode conductor screen.
IN FORMIC ACID
Organ (1956) conducted several detailed experiments on silver reduction techniques and alternatives. He recommends that standard electrolytic reduction be conducted in 30 percent aqueous formic acid because the electrolyte has no detrimental effect on silver and requires only minimum washing after electrolysis. (Tests performed at the Conservation Research Laboratory at Texas A&M University have indicated that a 5 percent formic acid electrolyte is adequate.) Organ observed that the reduced layer of silver ions and corrosion products external to the original surface delaminates and reveals the original surface when a formic acid electrolyte is used at a current density of 1 amp/cm2. For this reason, treatment in a formic acid electrolyte is often used for silver that has the original surface preserved in the corrosion layer. The treatment is effective as long as a substantive metallic silver core remains. During electrolysis, the reduced silver corrosion layers regenerated on the surface of the metal in formic acid are left in granular or particulated layers, which are physically weak and tend to separate from the metallic core. In order to preserve the detail of the specimen, clear acrylic lacquer should be applied to seal the corrosion layers in place on the surface of the artifact. Because the corrosion layers are particulated, silver that is reduced in formic acid tends to be dark, brittle, and rigid. However, the dark, reduced silver is stable, thoroughly cleansed of corrosion products, and 'antique' in appearance. If a brighter surface is desired, the silver can be lightly polished with a paste of sodium bicarbonate, a fine fiberglass brush, or a silver buffing cloth.
IN SODIUM HYDROXIDE
Reduction in a 3 to 15 percent aqueous solution of sodium hydroxide at a low current density (10-50 milliamps/dm2) will result in firm, hard, metallic silver capable of being polished (Organ 1956:135). The regenerated silver retains the detail and texture of the original laminated corrosion surface but is full of voids and is not ductile. More recent tests have shown that a NaOH electrolyte is more conducive than formic acid for the thorough reduction of silver.
Fully rectified direct currents have been used in metal conservation, electroplating and battery charging. It was discovered some years ago, however, that a small amount of reverse current (also called partially rectified or asymmetrical alternating current) produces smoother electroplated finishes, faster battery charging time, and increased battery life. The technique was first described in the conservation literature by Organ (1956) as consolidative reduction.
There are three possible kinds of induced electromotive currents: alternating current (AC), direct current (DC), and asymmetrical AC (Figure 13.2). In every cycle of AC (Figure 13.2A), there is an equal amount of forward current (current flow from negative to positive) and reverse current (current flow from positive to negative); therefore, an AC has a symmetrical sine wave form. If an artifact is that is undergoing electrolysis is hooked up to AC, metal and hydrogen are deposited, and metal is reduced from the corrosion compounds at the cathode during the forward half of the cycle. In the subsequent reverse half of the cycle, the metal and hydrogen deposited or reduced at the cathode are dissolved. No progress in reduction takes place.
Because DC flows only in a forward direction, only reduction and deposition reactions take place at the cathode (see Figure 13.2B). In normal reduction using DC, metal and hydrogen are reduced at the surface of the specimen being treated, but in the process, the cathode can become polarized by the accumulation of hydrogen gas bubbles formed and deposited at the cathode surface. The hydrogen gas can insulate the surface in some areas, while other areas are in direct contact with the electrolyte. Polarization will, therefore, result in uneven metal deposition and microscopic voids in the newly reduced metal.
In consolidative reduction, an asymmetrical AC of 10-20 percent reverse current and 80-90 percent forward current is generally used. During electrolysis, the net effect is a rapid succession of reduction and dissolution cycles (see Figure 13.2C). During the 90 percent forward half of the cycle, reduction of metal in the corrosion compounds and deposition of metal dissolved in the previous reverse current half cycle takes place. During the 10 percent reverse half cycle, there is a partial dissolution of the previously reduced or deposited metal; however, the 90 percent forward current places the emphasis upon reduction and deposition over dissolution as the current reverses 120 times a second. In the process, the polarization of the cathode is minimized.
Organ (1956) used asymmetrical AC in a sodium hydroxide electrolyte to regenerate the completely mineralized silver of the Ur lyre from silver chloride to massive metallic silver while preserving the surface details of the corrosion layers. The reduced silver was ductile and more homogeneous than silver reduced by normal electrolytic techniques using fully rectified DC. Organ used a 3 percent NaOH electrolyte, a carbon rod anode, and a very low current density of 10 milliamps/dm2 (1 milliamp/cm2) to reduce the silver and to prevent the rapid evolution of hydrogen that would possibly disturb the reduced silver.
For silver artifacts that are badly or completely corroded, more complete reduction is achieved if the cathode wire is laid against one side of the artifact and the exposed wire covered with wax or polymethacrylate. This ensures that the current passes through the corroded metal while flowing from the electrolyte to the cathode. The hydrogen discharges at the surface of the mineralized metal and reduces it. Organ (1956) used this technique in order to make an electrical contact with the nonmetallic, poorly conducting silver chloride on completely mineralized silver. This arrangement is beneficial even when only a thin core of metallic silver remains. During the process, which may take many weeks, the corrosion layers external to the original surface are reduced in situ, and all surface details are preserved. Since this technique preserves all of the outer corrosion surface, it should not be used on specimens with an original surface preserved within the corrosion crust. Following reduction, the artifact is rinsed in cold de-ionized water to remove all alkalis and then coated with a suitable sealant.
Additional details concerning the development and application
of consolidative reduction can be found in Organ (1956:137-144). Plenderleith and Werner (1971:223-226) provide
a useful summary of consolidative reduction techniques, and additional research is presented in Charalambous and
Oddy (1975); the description of the circuit for the partially rectified current is provided in both sources. Asymmetrical
alternating current appears to have some advantages over direct current, and may prove to be superior for treating
all metal artifacts, including iron. Electrolytic reduction techniques that use asymmetrical alternating current
have not been widely adopted by conservation laboratories, however, since efficient reduction of silver corrosion
products to metallic silver can be achieved with very low current densities using direct current and a sodium hydroxide
The alkaline dithionite treatment is similar to the alkaline sulfite treatment for iron. It is a relatively cheap, simple, and efficient method for the uniform reduction of silver corrosion products to metallic silver (MacLeod and North 1979). The steps involved in the alkaline dithionite treatment of silver are as follows:
1. Immerse the object in 10-12 percent hydrochloric acid to remove any surrounding encrustation that may consist of sand, shell, calcium carbonate, and copper and iron corrosion compounds. This step may take from 12 hours to a week, or until all cleaning action ceases, and no more gas bubbles evolve from the object. During this step, it is necessary also to make sure that the solution remains acidic. If necessary, concentrated hydrochloric acid should be added to the solution to maintain a working strength.
2. Rinse the object thoroughly in tap water to remove all residual encrustation and acid. A pneumatic chisel may be used to mechanically remove any resistant encrustation.
3. Prepare a solution of alkaline dithionite: dissolve 40 g of sodium hydroxide in a liter of water, add 60 g of sodium hydrosulfite (the amount of sodium hydrosulfite in solution is not critical and any amount within the 55- to 65-g range will be effective). Immerse the silver object quickly in the alkaline dithionite solution in order to eliminate oxidation of the solution in the container. The container should be completely full of solution and have an air-tight seal.
4. Agitate and turn the container daily to keep the solution mixed and to expose all surfaces of the object(s) to the solution.
5. After one week, remove and rinse the object(s) in water until the pH of the rinse water remains unchanged.
6. The corrosion products on the surface of the artifact will be reduced to a gray, metallic silver, which can be polished with a wet baking soda paste or a fiberglass brush.
In addition to being very effective for reducing silver corrosion products, the alkaline dithionite treatment has been used successfully on all cupreous artifacts, converting copper corrosion products back to metallic copper.
To dispose of the used alkaline dithionite solution, allow
it to air oxidize for several days in order to convert sulfites to sulfates. After oxidation, the solution should
be neutralized with hydrochloric acid. The solution can then be safely disposed down the drain; however, it is
possible to extract all the silver from the solution through electrolysis, which will plate the metal on the cathode.
The silver recovered from the cathode may come close to paying for the treatment.
RINSE AND SEALANT
Following electrolysis, the artifact should be rinsed with de-ionized water. If an alkaline electrolyte is used, the rinsing should be quite intensive in order to prevent the formation of a white precipitate on the object. After rinsing, the silver can be dried with hot air or dehydrated in acetone and then coated with a clear acrylic lacquer, such as Krylon 1301.
The majority of silver objects recovered from archaeological contexts require only limited treatment. In most instances, the various corrosion products can be removed with simple chemical solutions (Plenderleith and Werner 1971:227-229). Common tarnish caused by sulfur compounds can be eliminated easily with commercial silver cleaning solutions. Alternatively, a mild silver dip solution that consists of 5 percent thiourea and 1 percent non-ionic detergent in distilled water can be prepared. A solution of 15 percent ammonium thiosulfate in distilled water with a 1 percent non-ionic wetting agent is stronger than the silver dip and is effective for removing both tarnish and silver chloride. For base silver with copper corrosion compounds, concentrated ammonia effectively cleans all copper compounds from the silver. Care must be taken, however, because ammonia dissolves silver chloride and will substantially weaken badly corroded silver. A solution of 5-30 percent formic acid in de-ionized water is effective for dissolving copper compounds without affecting silver chlorides. Formic acid can also be used to brighten silver that has already been cleaned with another chemical or technique. Metallic copper films can be removed with a silver nitrate solution. In general, however, simple washing in soapy water or rubbing the silver object with a mild polishing abrasive is usually sufficient.
Since silver sulfide and silver chloride are stable compounds, corroded silver pieces do not need to be stabilized. Object consolidation, however, is often required. Many of the silver coins and other small silver pieces likely to be found within an encrustation may have been completely converted to silver sulfide. In some cases, all that remains of the silver is a wet, formless slush. In a few cases, an enlarged, deformed, or discontinuous crystalline structure remains, and all that can be done is to record any data contained as an impression of the coin in the surrounding encrustation.
When an artifact is nearly or completely converted to a compact,
cohesive silver sulfide, the form and all of the details of the original specimen are retained. Some 'coins' may
consist of a light silver sulfide wafer that can be crumbled to powder with slight pressure. If consolidative reduction
is not attempted, or is impossible, any cleaning treatment may dissolve the coin or at least destroy all the markings
and details that are preserved in the mineralized sulfide layer. In some instances, it may be possible to conserve
the artifact in the alkaline dithionite solution described above. In other instances, the only alternative is to
consolidate the sulfides. This is easily accomplished by first dehydrating the object in acetone. It should be
then placed in a dilute solution of polyvinyl acetate (PVA) and acetone. It is left in the solution until bubbles
cease to rise from its surfaces, whereupon it is removed and allowed to partially dry. The process should be repeated
two or three times followed by a thorough drying of the object. The process of repeated immersion and drying ensures
that a maximum amount of the acetate is absorbed by the object. The PVA will consolidate the sulfide layers, although
the aritfact will remain fragile and can be easily broken. If desired, any number of other consolidants, such as
butyl acetate, various polymethacrylates, or even wax, can be used in place of PVA.
Since the corrosion products of silver are stable, the treatment accorded silver artifacts is less critical than for other metal objects, especially iron. In some instances, however, when treating base silver with a significant amount of copper, it is the copper and its corrosion products that can create problems; in these cases, the artifact should be treated as copper. In many instances, silver may be treated exclusively by mechanical means or by various chemical treatments. Because of silver's susceptibility to corrosion in anaerobic conditions that are characteristic of marine environments, a treatment is often employed that will reduce the silver corrosion products back to a metallic state. If reduction is the objective, only electrolytic reduction and the alkaline dithionite treatments are effective treatments. It is for this reason that they are the treatments most often used to conserve silver artifacts recovered from a marine environment. Each is effective in its own way, and the decision to use either one should be based upon the particular resources of the laboratory and the number of artifacts to be treated.