New Jersey Scuba Diving
Conservation of Nonferrous Metals
It is not uncommon to find non-ferrous metals, such as copper, silver, lead, tin, gold, and their alloys, in archaeological sites. These metals have been used in the manufacture of art objects, coins, jewelry, and various utilitarian items such as fasteners, navigational instruments, cooking vessels, and small tools. They are more noble than iron and survive adverse environments in better condition than do iron specimens. Nevertheless, the corrosion problems of each metal varies in different environments. Only those techniques applicable to the conservation of sea-recovered metals are considered here.
Sea-recovered materials are often encapsulated by encrustation. When encrustation is present on non-ferrous metals, it is generally much thinner than encrustation found on iron. Artifacts manufactured from these metals, however, are often found encapsulated in the encrustation that surrounds iron artifacts. Prior to any treatment of the metal artifacts, preliminary conservation steps must be completed. These include ( 1 ) initial documentation, ( 2 ) storage, ( 3 ) encrustation removal, and ( 4 ) artifact evaluation. The treatment of each metal group, i.e., cupreous metals; silver and its alloys; tin, lead, and their alloys; and gold and its alloys, is discussed in detail.
STORAGE OF NON-FERROUS METALS
A variety of metal artifacts made of different metals are often found encrusted together in marine sites. In these instances, it is necessary to store the material in such a way that the most susceptible metal is afforded protection, and little to no damage is done to the other metals and non-metals found association with it. Since iron artifacts are the most commonly found metal, the storage conditions discussed under iron are most often used. However, gold, silver, pewter, brass, bronze, copper, and lead artifacts, as well as ceramics, stone, glass, bone, cloth, seeds, and wood, are often all associated in various combinations within a single encrustation. In some cases, the best storage for the encrustation may be simply in fresh water. Once the different objects are removed from the encrustation, they are placed in the most appropriate storage environment for each material. While iron artifacts, as discussed earlier, should at a minimum be stored in an alkaline solution protected from light, this solution is not necessary or even recommended for artifacts made of other metals.
Copper is corroded by oxidizing solutions and strong alkaline solutions. In neutral or slightly alkaline solutions, the copper is passivated and corrosion is halted by the formation of an oxide film on the surface of the artifact. A 5 percent solution of sodium sesquicarbonate or sodium carbonate is recommended for storing copper.
Silver is stable in aqueous solutions of any pH value and in the atmosphere, as long as these environments are free from oxidizing substances. Since chlorides do not affect silver or lead, they do not need to be placed in an aqueous solution and can be stored dry once the encrustation has been removed. Prior to the removal of adhering encrustation, however, it is best to house such objects in an appropriate solution to keep the encrustation from becoming harder and difficult to remove. Silver objects can be stored safely along with iron artifacts in either a 5 percent sodium sesquicarbonate or sodium carbonate solution. When silver that is encrusted to an iron object is stored in a chromate solution, a film of brown Ag2O will form. This film can be removed during the conservation of the object; chromate solutions, however, are not recommended for the storage of singular silver artifacts.
LEAD, TIN, PEWTER
Lead, tin, and pewter are more easily stored. All are often stored dry; when the encrustation on metals is allowed to dry out, however, it becomes much harder to remove. For this reason, lead, tin, and pewter are stored in aqueous solutions. Lead is corroded by aqueous solutions free from passivating substances, especially soft water, de-ionized water, or distilled water, and should never be stored in them. However, since lead is corrosion-resistant in hard, passivating bicarbonate water, and both tin and pewter are passivated in slightly alkaline solutions, all can be stored in tap water, with the pH adjusted to 8-10 by the addition of sodium sesquicarbonate. Lead and pewter can also be stored in sodium carbonate with a pH of 11.5. Tin will resist corrosion in slightly alkaline solutions that are free from oxidizing agents but will react adversely to strongly alkaline solutions. Any alkaline solution with a pH of above 10 is potentially dangerous to tin. Lead, tin, and their alloys, such as pewter, should not be stored in a chromate solution because of the oxidizing effect of chromate, which will form an orange chromate film on artifact surfaces that is difficult to remove. In the absence of passivating substances, an oxidizing agent, such as chromate, can damage lead, tin, and pewter artifacts.
Conservation of Cupreous Metals
CUPREOUS METAL CORROSION
The term 'cupreous' is used to designate all metals that consist of copper or alloys that are predominantly copper, such as bronze ( an alloy of copper and tin ) and brass ( an alloy of copper, zinc, and often lead ). The term does not imply a valence state as does cupric-divalent copper or cuprous-monovalent copper. The cupreous metals are relatively noble metals that frequently survive adverse conditions, including long submersions in salt water that will often completely oxidize iron. Cupreous metals react with the environment to form similar alteration products, such as cuprous chloride ( CuCl ), cupric chloride ( CuCl2 ), cuprous oxide ( Cu2O ), and the aesthetically pleasing green- and blue-colored cupric carbonates, malachite [ Cu2(OH)2CO3], and azurite [ Cu3(OH)2(CO3)2 ] ( Gettens 1964:550-557 ). In a marine environment, the two most commonly encountered copper corrosion products are cuprous chloride and cuprous sulfide. The mineral alterations in copper alloys, however, can be more complex than those of pure copper.
The first step in the electrochemical corrosion of copper and copper alloys is the production of cuprous ions. These, in turn, combine with the chloride in the sea water to form cuprous chloride as a major component of the corrosion layer:
Cu -e >> Cu+
Cu+ + Cl- >> CuCl
Cuprous chlorides are very unstable mineral compounds. When cupreous objects that contain cuprous chlorides are recovered and exposed to air, they inevitably continue to corrode chemically by a process in which cuprous chlorides in the presence of moisture and oxygen are hydrolyzed to form hydrochloric acid and basic cupric chloride ( Oddy and Hughes 1970:188 ):
4 CuCl + 4 H2O + O2 >> CuCl2 3 Cu(OH)2 + 2 HCl
The hydrochloric acid in turn attacks the uncorroded metal to form more cuprous chloride:
2 Cu + 2 HCl >> 2 CuCl + H2
The reactions continue until no metal remains. This chemical corrosion process is commonly referred to as 'bronze disease.' Any conservation of chloride-contaminated cupreous objects requires that the chemical action of the chlorides be inhibited either by removing the cuprous chlorides or converting them to harmless cuprous oxide. If the chemical action of the chlorides is not inhibited, cupreous objects will self-destruct over time.
Copper objects in sea water are also converted to cuprous and cupric sulfide ( Cu2S and CuS ) by the action of sulfate-reducing bacteria ( Gettens 1964:555-556; North and MacLeod 1987:82 ). In anaerobic environments, the copper sulfide products are usually in the lowest oxidation state, as are the ferrous sulfides and silver sulfides. After recovery and exposure to oxygen, the cuprous sulfides undergo subsequent oxidation to a higher oxidation state, i.e., cupric sulfide. The whole chemical reaction generally proceeds along the same lines as those described earlier for iron.
Upon removal from a marine encrustation, copper and cupreous artifacts are inevitably covered with varying thicknesses of a black powdery layer of copper sulfide that imparts an unpleasing appearance. Occasionally, the corrosion process will pit the surface of the artifact, but this is more common on cupreous alloys where tin or zinc is corroded preferentially. The stable copper sulfide layer does not adversely affect the object after recovery from the sea as do copper chlorides; copper sulfides only discolor the copper, imparting an unnatural appearance to the metal, and are easily removed with commercial cleaning solvents, formic acid, or citric acid. ( See North and MacLeod  for a detailed discussion of the corrosion of copper, bronze, and brass in a marine environment. )
CONSERVATION OF CUPREOUS METALS
Copper and alloys in which copper predominates are all generally conserved by the same methods. Particular care needs to be taken only when there is a high percentage of lead or tin in an alloy; lead and tin are amphoteric metals and will dissolve in alkaline solutions. Although there are a considerable number of chemical treatments for the conservation of copper, bronze, and brass, most are not satisfactory for cupreous metals recovered from marine sites. Three effective chemical treatments are discussed below. Consult the bibliography for further information.
In some instances, it is necessary to mechanically remove gross encrustation and corrosion products from the artifact to reveal the preserved surface of the metal. This step is facilitated for sea-recovered cupreous objects because the marine encrustation will form a cleavage line between the original metallic surface and the encrustation. When the artifacts are removed from gross encrustation, superficial encrustation is often deliberately left adhering to the surface of the artifact due to the fragility of the artifact or to avoid marring its surface. Careful mechanical cleaning and rinsing in water may be all that is required to remove this remaining superficial encrustation. In other cases, all adhering encrustation can be removed by soaking the object in 5-10 percent citric acid with 1-4 percent thiourea added as an inhibitor to prevent metal etching ( Plenderleith and Torraca 1968:246; Pearson 1974:301; North 1987:233 ). Citric acid should be used cautiously, as it can dissolve cupric and cuprous compounds within the artifact. The artifact is completely submerged in the solution until the encrustation is removed. This may require an hour to several days, during which time the solution should be stirred to keep the acid concentration evenly distributed.
When a specimen is very thin, fragile, has fine detail, or is nearly or completely mineralized, any acid treatment may be too severe. In these cases, the artifact can be soaked in a 5-15 percent solution of sodium hexametaphosphate ( Plenderleith and Werner 1971:255 ) to convert the insoluble calcium and magnesium salts in the encrustation to soluble salts, which can be subsequently washed away.
Following any necessary preliminary treatment, the conservation of chloride-contaminated cupreous objects requires that the adverse chemical action of the chloride be prevented. This can be accomplished by:
- removing the cuprous chloride
- converting the cuprous chloride to harmless cuprous oxide
- sealing the cuprous chloride in the specimen from the atmosphere
The possible treatment alternatives include:
- galvanic cleaning
- electrolytic reduction cleaning
- alkaline dithionite treatment
- chemical cleaning
- sodium sesquicarbonate
- sodium carbonate
The first three alternatives can remove cuprous chlorides and reduce some of the corrosion products back to a metallic state; however, they are best used only on objects with a metallic core. If carefully applied, these treatments will stabilize the object and maintain a form approximating its original, uncorroded appearance. If misapplied, they can strip the corrosion layer down to the remaining metal core. Jedrzejewska ( 1963:135 ) draws attention to the fact that stripping, especially by electrolysis, may destroy significant archaeological data such as tool marks, engraved lines, and decorative elements, as well as alter the original shape of the object. For these reasons, the corrosion layers of any metal artifact should never be indiscriminately removed. The treatment should strive to preserve corrosion layers in situ through very controlled electrolytic reduction or alkaline dithionite treatment. The chemical techniques described do not strip the corrosion layer. Rinsing in a sodium sesquicarbonate solution removes the cuprous chlorides from the artifact, while benzotriazole and silver oxide seal the cuprous chlorides from the atmosphere. The chemical treatments are applicable to substantial objects as well as to completely mineralized pieces.
This procedure is carried out in exactly the same manner as described for iron. It is generally regarded as an obsolete technique, except under certain circumstances already mentioned in the section on the galvanic cleaning of iron.
ELECTROLYTIC REDUCTION CLEANING
Electrolytic reduction of cupreous metals is also carried out in the same manner as described for iron. Either 2 percent sodium hydroxide or 5 percent sodium carbonate can be used for the electrolyte. The latter is used most often, although acceptable results have also been achieved using 5 percent formic acid as the electrolyte. A mild steel anode can be used, but Type 316 stainless steel or platinized titanium is required if formic acid is used as the electrolyte. The same electrolytic setups described for iron or for silver ( below ) are used.
Precise data concerning optimum current densities for cupreous artifacts are not available. Plenderleith and Werner ( 1971:198 ) state that the current density should not be allowed to fall below 0.02 amp/cm2 in order to prevent the deposition of a salmon-pink film of copper on the objects. Keel ( 1963:24 ) states that a current density above 0.01 amp/cm2 will damage cupreous objects. Along these same lines, Pearson ( 1974:301-302 ) correctly observes that care must be taken when electrolytically cleaning marine-recovered mineralized bronze in order to prevent damage to the artifact surface by the evolution of hydrogen gas. Current densities, both within and in excess of the given ranges above, are commonly applied to different cupreous objects. North ( 1987:238 ) recommends using the hydrogen evolution voltage techniques described for the treatment of iron. In general, the same procedures regarding current density that are described for the treatment of iron apply to the treatment of cupreous artifacts. The main variations in treatment involve the fact that the duration of electrolysis for chloride- contaminated cupreous objects is significantly shorter than that for comparable iron objects. Small cupreous artifacts, such as coins, require only a couple of hours in electrolysis, while larger cupreous specimens, such as cannons, may require several months.
This treatment was developed for consolidating mineralized silver. Since then, it has also been found to be effective on cupreous objects. A complete description of the treatment can be found in the file on silver. Alkaline dithionite treatment will destroy any patina on the surface of the cupreous object, but it effectively removes the bulk of the chlorides in the shortest period of time; further, it reduces some of the copper corrosion products back to metal.
Many cupreous artifacts with chloride contamination, such as well-patinated bronzes with bronze disease, extensively mineralized bronzes with or without cuprous chlorides, bronzes without a substantial metallic core, and bronzes with mineralized decorative features, cannot be treated by either of the reduction techniques. There are three different chemical treatments available that are used to stabilize the artifacts while leaving the corrosion layers intact: treatment with sodium sesquicarbonate, with sodium carbonate, or with benzotriazole.
The cuprous chloride components of copper and its alloys are insoluble and cannot be removed by washing in water alone. When bronzes or other alloys of copper are placed in a 5 percent solution of sodium sesquicarbonate, the hydroxyl ions of the alkaline solution react chemically with the insoluble cuprous chlorides to form cuprous oxide and neutralize any hydrochloric acid by-product formed by hydrolysis to produce soluble sodium chlorides ( Organ 1963b:100; Oddy and Hughes 1970; Plenderleith and Werner 1971:252-253 ). The chlorides are removed each time the solution is changed. Successive rinses continue until the chlorides are removed. The object is then rinsed in several baths of de-ionized water until the pH of the last bath is neutral.
In practice, the superficial corrosion products are mechanically removed from the metal objects prior to putting objects in successive baths of 5 percent sodium sesquicarbonate. For the initial baths, the sodium sesquicarbonate is mixed with tap water; de-ionized water is used for subsequent baths. If the chloride contamination is extensive, baths prepared with tap water can be used until the chloride level in the solution approximates the chloride level of standard tap water. De-ionized water is then substituted. This procedure is very economical when processing objects that require months of treatment.
Initially, the baths are changed weekly; as the duration of treatment progresses, the interval between bath changes is extended. Monitoring the chloride level by the quantitative mercuric nitrate test enables the conservator to determine precisely how often to change the solution. In lieu of a quantitative chloride test, the qualitative silver nitrate test can be used to determine when the solution is free of chlorides. The cleaning process is slow and may require months and, in some cases, even years.
The sodium sesquicarbonate treatment is often used by conservators because, unlike other cleaning treatments, it does not remove the green patina on the surface of cupreous objects. This treatment may encourage the formation of blue-green malachite deposits on the surface of the objects, which will intensify the color of the patina. If malachite deposit formations occur during treatment, the object should be removed from the solution and the deposit brushed off. On some bronze pieces, this treatment will result in a blackening of the surface, which obscures the original green patina and is difficult to remove. This blackening is attributed to the formation of black copper oxide and appears to be an inherent characteristic in some cupreous alloys.
Sodium Carbonate Rinses
The sodium sesquicarbonate treatment outlined above has been the standard treatment for fragile cupreous artifacts with chloride contamination and for artifacts that have a patina that is desirable to preserve. In practice, however, conservators find that the treatment often enhances the patina, making it much bluer in appearance. In other examples, it has considerably darkened or blackened the patina.
With regard to the sodium sesquicarbonate treatment, Weisser ( 1987:106 ) states:
Although initially the sodium sesquicarbonate treatment seems to be ideal, since you do not need to remove the outer corrosion layers while the cuprous chloride is removed, it has been found to have a number of disadvantages. First, the treatment may require well over a year before all the cuprous chloride has been converted. This fact makes other drawbacks more serious. It has been shown that sodium sesquicarbonate ( a double carbonate ) forms a complex ion with copper and therefore preferentially removes copper from the remaining metal ( Weisser 1975 ). This can be potentially structurally damaging over a prolonged period. It has also been shown that a mixture of carbonates, including chalconatronite, a blue-green hydrated sodium copper carbonate forms over the patina and also seems to replace other copper salts within the patina ( Horie and Vint 1982 ) This creates a color change from malachite green to blue-green, which in many cases is undesirable. In the objects the author has examined the blue-green color can be found in cross section from the outer corrosion crust extending down to the metal substratum.
Weisser ( 1987:108 ) concludes:
The stabilization of actively corroding archaeological bronzes remains a difficult problem for conservators. At the present time no known treatment can be called ideal. A sodium carbonate pre-treatment in conjunction with a standard treatment with benzotriazole offers one more option to the conservator who is faced with difficulties in stabilizing bronzes. Although successful stabilization has been achieved with this treatment where others have failed, it should be used with caution until the problems observed have been more thoroughly investigated. Bronzes which cannot be stabilized by this treatment should be stored or displayed in a low relative humidity environment. In fact it is recommended that all bronzes be kept in a low relative humidity environment if possible, since the long-term effectiveness of 'bronze disease' treatments has not been proven.
Weisser suggests that if previous treatments with BTA have not been successful, the objects can be treated with 5 percent sodium carbonate in distilled water. The sodium carbonate removes the cuprous chlorides and neutralizes the hydrochloric acid in the pits. Sodium carbonate, unlike sodium sesquicarbonate, which is a double carbonate and acts as a complexing agent with copper, reacts relatively slowly with copper metal. Still, in come cases, slight alterations in the color of the patina can occur.
The use of benzotriazole ( BTA ) has become a standard element in the conservation of cupreous metals. BTA follows any stabilization process and precedes any final sealant. In some cases, it can be a single treatment unto itself. When marine cupreous objects are conserved, however, BTA is usually used in addition to some other treatment, such as electrolytic reduction or alkaline rinses, which remove the bulk of the chlorides. For artifacts from a fresh water site, it may be the only treatment required.
Treatment with BTA does not remove the cuprous chloride from the artifact; rather, it forms a barrier between the cuprous chloride and moisture of the atmosphere. In this method of cleaning ( Madsen 1967; Plenderleith and Werner 1971:254 ), the benzotriazole forms an insoluble, complex compound with cupric ions. The precipitation of this insoluble complex over the cuprous chloride forms a barrier against any moisture that could activate the cuprous chloride and cause bronze disease. Tests at the British Museum ( Plenderleith and Werner 1971:254 ) indicate that if active bronze disease is present, all attempts to stabilize the object with BTA may fail due to the widespread distribution of cuprous chloride in the corrosion layers.
The treatment consists of immersing an object in a solution of 1-3 percent BTA dissolved in ethanol or water. In general, the best results are achieved if the specimen is impregnated with the solution under a vacuum for 24 hours. If the artifact is left in the solution for at least 24 hours, 1 percent BTA mixed with de-ionized water works as well as more concentrated solutions. For shorter treatments, 3 percent BTA mixed in either water or ethanol in recommended. In some cases, ethanol is preferred when the BTA treatment is of short duration. The main advantage of using ethanol in the solution is that it penetrates cracks and crevices better than does water. After the artifact is removed from the solution, it should be wiped off with a rag saturated in ethanol to remove excess BTA. The artifact then is exposed to the air. If any fresh corrosion appears, the process is repeated until no adverse reaction occurs. ( See Green 1975; Hamilton 1976; Sease 1978; Walker 1979; Merk 1981 for additional information. )
It must be emphasized that the BTA treatment does not remove the cuprous chloride from the artifact. It merely forms a barrier between the cuprous chloride and the moisture in the atmosphere. Therefore, for artifacts heavily contaminated with chloride, such as marine-recovered cupreous objects, BTA treatment should follow the sodium sesquicarbonate or sodium carbonate treatment to ensure long-term artifact stability. BTA is a suspected carcinogen, and contact with the skin should be avoided, and the powder should not be inhaled.
FINAL TREATMENT AND SEALANT
Following electrolytic or chemical cleaning, the objects are put through a series of hot rinses in de-ionized water until the pH of the last rinse bath is neutral. Because copper tarnishes in water, Pearson ( 1974:302 ) recommends washing the objects in several baths of denatured ethanol. If a water rinse is used, any tarnish can be removed with 5 percent formic acid or by polishing the area with a wet paste of sodium bicarbonate.
After rinsing, copper objects should polished to any degree desired and treated with BTA. The object is then dehydrated in acetone or a water-miscible alcohol and coated with clear acrylic lacquer or microcrystalline wax. The commercially available KrylonClear Acrylic Spray No. 1301 is recommended for ease of application, durability, and availability. For increased corrosion protection, Pearson ( 1974:302 ) recommends that 3 percent BTA can be added to the drying alcohol, as well as to the lacquer. Microcrystalline wax can be used, but in most cases, has no special advantage over acrylics.
All the treatments discussed here are effective for the treatment of all artifacts from marine sites that contain cupreous metals. Of the conservation alternatives considered in this file, electrolytic reduction, alkaline dithionite treatment, and alkaline rinses are the only ones which actually remove the cuprous chlorides. For this reason, they promise the most enduring protection.
Electrolytic reduction cleaning of copper-alloyed objects, such as brass and bronze, is often avoided because it removes any aesthetically pleasing patina and may change the color by plating copper from the reduced corrosion compounds onto the surface of the alloyed metal. In the case of cupreous metal recovered from marine environments, however, the chemical stability provided by electrolysis often takes precedence over aesthetics. The history of success in applying electrolytic reduction techniques to cupreous artifacts clearly demonstrates that electrolysis is the quickest, most effective, and enduring means of processing copper, brass, or bronze objects from a salt water environment. This statement is especially true for larger objects, such as cannons.
The extremely long time required for sodium carbonate and sodium sesquicarbonate treatments discourages their use. Preliminary treatment of artifacts with sodium carbonate followed by benzotriazole treatment may provide satisfactory results, but more experiments are needed before a final judgment can be made. Alkaline dithionite treatments have also proven effective for conserving cupreous alloys.
Regardless of the preliminary treatment, an application of BTA should be an inherent step in the conservation of all cupreous metal artifacts. In most cases, if the artifact is effectively treated with any of the treatments discussed above, as well as with BTA, and then sealed and stored in the proper environment, it will remain stable.
Conservation of Silver & Gold
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:
2 Ag + 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:
- galvanic cleaning
- electrolytic reduction
- alkaline dithionite treatment
- chemical cleaning
- 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.
ELECTROLYTIC REDUCTION CLEANING
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.
REDUCTION 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.
REDUCTION 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 electrolyte.
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:
- 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.
- 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.
- 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.
- Agitate and turn the container daily to keep the solution mixed and to expose all surfaces of the object( s ) to the solution.
- After one week, remove and rinse the object( s ) in water until the pH of the rinse water remains unchanged.
- 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.
STABILIZATION AND CONSOLIDATION
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 artifact 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.
GOLD CORROSION & CONSERVATION
Gold is a relatively inert metal and thus undergoes minimum corrosion. It is the copper- and/or silver-based gold alloys that easily corrode, resulting in silver or copper corrosion compounds that leave an enriched and possibly weakened gold surface.
Pure gold and high gold alloys do not require any conservation treatment. Gold objects from shipwreck sites appear to look the same when recovered as the day they went down with the ship. The copper and silver in low- alloy gold do corrode. When present, the copper and/or silver corrosion compounds of low-alloy gold are treated by the processes described for these two metals ( see Files 12 and 13 ). Silver corrosion products can be removed with ammonia; copper compounds with formic acid, citric acid, or alkaline sequestering agents, such as Rochelle salts or alkaline glycerol. All the pertinent comments applicable to silver and copper conservation are made under those headings.
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.
Conservation of Lead, Lead Alloys, & Tin
LEAD, LEAD ALLOYS, AND TIN CORROSION
Articles of tin are seldom encountered in archaeological sites. This metal is found more often in various alloys, particularly in combination with copper for bronze and/or tin pewter. Gettens ( 1964:560 ) notes that tin seldom survives in archaeological sites because of the transformation of tin to a mix of stannous and stannic oxide by direct intercrystalline oxidation ( SnO and SnO2 ) or to a loose powdery gray tin, commonly referred to as 'tin pest, ' by allotropic modification the alteration compounds of tin in a marine environment have not been adequately studied; it is known, however, that sodium chloride stimulates the corrosion of tin. Ingots of tin that were completely oxidized to tin oxide were recovered from a Bronze Age shipwreck off the coast of Turkey ( Bass 1961 ). Although not often mentioned in literature, tin sulfide can also be expected to be found where sulfate-reducing bacteria are active in anaerobic environments.
Lead is commonly found in shipwrecks; it was used on ships for weights, cannonballs, sheeting, and stripping. Lead is a stable metal in neutral or alkaline solutions that are free from oxidizing agents, especially if carbonates are present in the water ( Pourbaix 1966:488-489 ). Basic lead carbonate ( 2 PbCO3 Pb(OH)2 ) and lead oxides ( PbO and PbO2 ) are formed under most archaeological conditions where there is prolonged atmospheric exposure. The gray lead carbonate and lead oxide generally form a protective layer on the artifact that prevents further oxidation. Both these corrosion compounds are found on lead from a marine environment, but lead chloride ( PbCl2 ), and especially lead sulfide ( PbS ) and lead sulfate ( PbSO4 ), are also common.
Gettens ( 1964:558 ) noted that few occurrences of lead sulfide have been reported on archaeological objects, but more recent research ( North and MacLeod 1987:89 ) shows that the primary lead corrosion product in anaerobic marine environments is lead sulfide, while lead sulfate is commonly found on objects recovered from aerobic marine environments. It is not unusual in shipwreck excavations to find the remains of lead straps that have been completely converted to a black slush. The bulk of this corrosion is most likely lead sulfide which results from the action of sulphate-reducing bacteria. Some intermediate forms of lead oxides ( PbO and PbO2 ) may be formed, and oxysulfides are also present. Lead often exhibits extensive corrosion attack when it is in contact with wood. Lead strips that were nailed onto a ship's keel have been observed in a state of severe deterioration. The oxygen-consuming, decaying wood and the marine encrustation that forms over the lead apparently creates the anaerobic conditions conducive for the metabolism of the sulfate-reducing bacteria; in addition, the decaying wood provides nourishment for the bacteria.
Lead alloys, such as old pewter, which is an alloy of tin and lead, oxidize to the same compounds as the two parent metals. The condition of different pewter pieces varies widely both between and within archaeological sites, primarily because of different local conditions and varying percentages of tin to lead in each individual object. In general, leaded pewter always survives in better condition in marine environments than does lead-free pewter; this is most likely due to the formation of lead sulfate ( PbSO4 ) that protects the surface of the artifact. Lead-free pewter suffers extensive corrosive attack in aerobic sea water and is often completely mineralized as stannic oxide ( SnO2 ) and lead sulfide ( PbS ), and various very brittle, mineralized antimony and tin ( SbSn ) compounds are formed. In contrast, in anaerobic environments, both leaded and lead-free pewter survive in good condition through the protective formation of lead and tin sulfide films ( North and MacLeod 1989:90-91 ). In fact, the only corrosion present on pewter recovered from anaerobic marine environments may be a thin sulfide film on the surface of well-preserved metal. Various combinations of lead carbonate, lead oxide, lead sulfide, lead chloride, and tin oxide are possible. Pewter objects often have wart-like blisters on the surface of the metal, which possibly result from localized contaminations of salts ( Plenderleith and Werner 1971:278 ). These should not be removed, for under most of them there are either holes or pits in the metal.
CONSERVATION OF LEAD, TIN, AND PEWTER
Once recovered from the sea, the corrosion products of objects of lead, tin and their alloy, pewter, are stable. The corrosion products may be unsightly or even disfiguring, but they do not take part in chemical reactions that attack the remaining metal. The objects should be cleaned only for aesthetic reasons and to reveal surface details under the corrosion layers. Old pewter, an alloy of lead and tin, must be treated as tin, which is the more anodic and chemically sensitive metal. Therefore, no acids, or sodium hydroxide should be used, unless, in the case of electrolysis, the metal is given cathodic protection.
CHEMICAL TREATMENT OF LEAD
Because of the ease of treatment and the availability of the chemicals, the most widely used conservation treatment for lead from any archaeological environment is the acid treatment described by Caley ( 1955 ). The lead is immersed in 10 percent hydrochloric acid, which will remove any adhering marine encrustation, along with lead carbonates, lead monoxide, lead sulfide, calcium carbonate, and ferric oxide. This treatment is good for lightly corroded specimens, and it gives lead surfaces a pleasing appearance. The surface detail that is preserved by this treatment varies with the degree of corrosion when recovered. For more diagnostic lead artifacts, Caley's method has been superseded by electrolytic reduction, which has the ability to convert mineral products back to a metallic state. For the general cleaning of lead without a lot of hands-on labor, however, Caley's method remains an acceptable and much-used technique, provided that the object is thoroughly rinsed after treatment in order to remove all remaining hydrochloric acid residue. This will prevent contamination of any chloride-sensitive material with which the treated lead may be stored.
If lead dioxide is present, it can be removed by soaking the object in 10 percent ammonium acetate. The ammonium acetate will also act as a buffer to protect the lead from the action of any hydrochloric acid that may remain. If treated with ammonium acetate, lead should be left in the solution only as long as necessary, as the solution can etch the metal. For most lead objects, however, the ammonium acetate step is not required.
If the objective is to completely remove all of the lead corrosion products from a lead object, a 5 percent solution of ethylenediaminetetraacacetic acid ( EDTA ) disodium salt is most effective. After complete immersion in the EDTA solution for two to three hours ( up to 24 hours for large objects ), the object is rinsed in tap water.
After treating lead by Caley's method, the conservator still has the option to use electrolytic reduction to reduce any corrosion layer that are still in place back to a metallic state.
GALVANIC CLEANING OF LEAD
Any solid object of tin can be cleaned galvanically or by electrolytic reduction in the same way as described for iron and the other metals. Normally, in galvanic cleaning, the vat with the electrolyte, anodic metal, and specimen is heated to speed the reaction; however, since tin is an allotropic metal that is slightly soluble in sodium hydroxide, heating should be avoided and the treatment time kept to a minimum. Tin coins respond well to cold electrochemical reduction, using zinc, aluminum, or magnesium powder in caustic soda ( Plenderleith and Werner 1971:275 ). Magnesium is often substituted for zinc, since zinc sometimes discolors the tin ( Plenderleith and Organ 1953 ). However, if electrolytic reduction equipment is available, there is little reason to use galvanic cleaning for any object of lead, tin, or their alloys.
The only conservation alternative for badly oxidized tin objects is to consolidate them in microcrystalline wax or embed them in a plastic material. Slow, extended diffusion of chlorides in an alkaline solution is not an option due to the solvent action of the solution on tin objects.
ELECTROLYTIC REDUCTION CLEANING
The ability to control the speed of the electrolytic reaction through current controls makes electrolytic reduction especially useful for lead coins and medals or, indeed, any specimen where surface detail is important or reduction and/or consolidation of the corrosive layers is the objective. Two electrolytic reduction techniques, normal reduction ( Plenderleith and Werner 1971:267-268 ) and consolidative reduction ( Organ 1963a:131; Plenderleith and Werner 1971:268-270 ), are used for treating lead.
Lead artifacts with substantial metal remaining can be cleaned by the normal electrolytic reduction process using 5 percent sodium hydroxide, anodes of mild steel or stainless steel, and a current density of 2-5 amps/dm2. Very satisfactory results are achieved by this technique. However, since lead is susceptible to solvent action by the electrolyte, when it is not cathodically protected, the current must be flowing before putting the specimen in the electrolytic tank and must not be cut off while the specimen is immersed in the tank. A good electrical contact, as indicated by evolution of hydrogen from the object, must be made with the lead, and the contact should be sufficiently supported to ensure that the electrical contact is maintained.
Since lead, tin, and pewter are susceptible to attack by strong alkalies, a sodium carbonate electrolyte is safer for use in electrolysis than a sodium hydroxide electrolyte. If the electricity were to go off during electrolysis while the lead or tin object or alloy was immersed in NaOH, the object would be attacked by the alkaline solution. If sodium carbonate was being used as the electrolyte, however, a passivating film of carbonate would form on the object, and the alkaline attack would stop. The attack on tin and tin alloys by sodium hydroxide solution is particularly aggressive. Since sodium carbonate does a reasonably good job on artifacts made of these metals, the use of sodium hydroxide electrolytes should be reserved for consolidative reduction on special artifacts where there is some reason to attempt to achieve the absolute maximum reduction of corrosion products back to metal. For example, when there are inscriptions or marks that are preserved in the corrosion layer of an object, sodium hydroxide should be used as the electrolyte.
This technique was developed by Organ ( 1963a:131 ) to consolidate the inscriptions contained in a fragile corrosion layer of basic lead carbonate on a group of lead seals. The removal of the corrosion layer would have obliterated the inscription. Consolidative reduction converts the basic lead carbonate and other lead corrosion products to a compact mass of lead. The object is tightly compressed between two polyurethane foam pads in order to support and put pressure on the corrosion layers while they are cathodically reduced at a current density of 100 to 200 milliamps/dm2.
In consolidative reduction, which employs very low current densities, mild steel anodes cannot be used because the current flow is so low that there is no way to keep the anodes passivated against anodic dissolution; therefore, stainless steel anodes and a 5 percent sodium hydroxide electrolyte are recommended. The procedure described by Plenderleith and Werner ( 1971:268-269 ), who use a 10 percent solution of sulfuric acid with a lead anode, is not common because of the difficulties of handling sulfuric acid and the deposition of lead from the anodes onto the artifacts being treated. In addition, more recent research has shown that the most thorough reduction is achieved when NaOH is used as the electrolyte.
Plenderleith and Werner ( 1971:269 ) suggest using a partially rectified alternating current source, which provides a 'bumping' effect, for better results. As discussed in the section on silver, however, the use of an asymmetrical alternate current is not widely used since low current density electrolysis using straight direct current effectively reduces lead corrosion products back to metallic lead, especially when sodium hydroxide is used as the electrolyte. The use of an asymmetrical alternate current does not appear to increase the degree of reduction ( Lane 1975; 1979 ). The most important thing for the conservator to keep in mind during any electrolytic cleaning process is the importance of maintaining a constant flow of electrons to the lead or tin metal that is being treated to ensure cathodic protection.
Rinsing Procedure Following Electrolytic Reduction
Sodium hydroxide electrolyte residues cannot be removed completely from lead through simple water rinsing; a more complex procedure must by followed ( Plenderleith and Werner 1971:269-270 ). The object should be submerged in a dilute solution of sulfuric acid ( 15 drops of concentrated H2SO4 per liter of tap water ) with a pH of 3 to 3.5 to neutralize the alkalinity of the electrolyte and to form a protective coat of lead sulfate on the surface of the object. The artifact is then taken through a succession of H2SO4 baths until the pH ceases to rise due to the diffusion of alkali from the lead. After removal from the sulfuric acid bath, the residual acidity present on the surface of the lead is removed through immersion of the object in successive baths of cold distilled water with a pH of about 6, until the pH of the water does not drop.
Following the rinsing, the reduced object is dried with hot air or dehydrated in a water-miscible solvent. The fragile reduced metal is then strengthened and protected from atmospheric corrosion by submersion in molten microcrystalline wax.
Lead is particularly susceptible to organic acids, such as acetic acid, humic acid, and tannic acid. Lead artifacts, therefore, should not be stored in oak cabinets or drawers. If so, even small concentrations of vapors of these acids can initiate corrosion, which progresses rapidly. To be safe, lead should by stored in sealed containers or polyethylene bags.
Donny L. Hamilton
1998. Methods of Conserving Underwater Archaeological Material Culture. Conservation Files: ANTH 605, Conservation of Cultural Resources I. Nautical Archaeology Program, Texas A&M University, World Wide Web, http://nautarch.tamu.edu/class/ANTH605/.
Copyright 2000 Donny L. Hamilton, Conservation Research Laboratory, Texas A&M University
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