FILE 14: LEAD, TIN, AND LEAD ALLOYS
IN THIS FILE:
Tin, Lead, and
Lead Alloy Corrosion
Conservation of Lead, Tin, and Pewter
Chemical Treatment of Lead
Galvanic Cleaning of Lead
Electrolytic Reduction Cleaning
Rinsing Procedure Following Electrolytic Reduction
TIN, LEAD, AND LEAD ALLOY 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 (2PbCO3 0 Pb[OH]2) and lead oxides (PbO and Pb O2) 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.
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
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.
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 (4 drops of concentrated (15-18%) H2SO4 per liter of tap water) with a pH of 3 to 3.5 neutralizes the alkalinity of the electrolyte and forms 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.
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