FILE 12: CONSERVATION OF CUPREOUS METALS
(COPPER, BRONZE, BRASS)
IN THIS FILE:
Cupreous Metal Corrosion
Conservation of Cupreous Metals
Electrolytic Reduction Cleaning
Sodium Carbonate Rinses
Final Treatment and Sealant
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):
4CuCl + 4H2O + O2 >> CuCl2 0 3Cu(OH)2 + 2HCl
The hydrochloric acid in turn attacks the uncorroded metal to form more cuprous chloride:
2Cu + 2HCl >> 2CuCl + 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:
1. removing the cuprous chloride
2. converting the cuprous chloride to harmless cuprous oxide
3. sealing the cuprous chloride in the specimen from the atmosphere
The possible treatment alternatives include:
1. galvanic cleaning
2. electrolytic reduction cleaning
3. alkaline dithionite treatment
4. chemical cleaning
a. sodium sesquicarbonate
b. 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
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 percemt 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 artefacts 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.
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 judgement 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.
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Copyright 2000 by Donny L. Hamilton, Conservation Research Laboratory, Texas A&M University.
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