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Metals
Lead and pewter
Lead is a soft, grey metal, used mainly in combination with tin to form pewter. Because of toxicity problems associated with the use of pewter food vessels, the lead in pewter was replaced in the 19th century with antimony and some copper. Modern leadless pewters are usually alloyed tin and Britannia metal.
Corrosion of lead and pewter
The main corrosion product on lead and pewter is white-grey basic lead carbonate. This provides a deep, protective patina to the metal surface, which should not be removed.
If the pewter and lead have been in a low-oxygen environment and exposed to sulphide compounds, a rich, lustrous, grey-black patina of metal sulphides remains on the surface. These minerals are stable and should also not be removed.
Lead and pewter are particularly susceptible to attack by acids given out by wood. Acetic acid combines with lead and pewter to form lead acetates.
Both tin and lead are very soft and are susceptible to denting and scratching.
Cleaning lead and pewter
The stable patinas which form on lead and its alloys, the white-grey lead carbonate and the dark lead sulphide, should not be removed because they form protective layers which prevent further corrosion. Other corrosion products may require treatment.
Because it is difficult to obtain very mild abrasives, it is generally recommended that abrasives not be used to clean these soft metals. If, however, the white bloom on the surface of these metals is thin, the deposit can be removed from the surface using a one-micron-grade alumina powder mixed into a slurry polish.
If the white, bloom—an acetate layer—is pustular or thick, it is best removed by chemical or electrochemical reduction. These techniques are best left to conservators.
Thin layers of corrosion products can be removed by soaking the objects in a solution of 50g of disodium ethylenediamine tetraacetic acid in 1 litre of water. Avoid prolonged soaking because the dissolved oxygen can cause increased corrosion.
General cleaning can be carried out with warm water and a pure soap. Then rinse the object with fresh water, wipe it with methylated spirits, and polish it with a soft cloth.
A protective surface coating of microcrystalline wax may be applied after cleaning.
| For more information |
| For a recipe and for information on the application of microcrystalline-polyethylene wax, please see More About Metals, later in this chapter. |
| CAUTION: |
| Check your chemical safety data sheets, and take the appropriate precautions. |
Storing and displaying lead and pewter
Lead and pewter are prone to attack from acetic acid and other organic acid vapours, which are found in poor-quality papers and some fabrics, and given off by various woods.
Storing lead-based objects in cupboards containing these materials contributes significantly to the deterioration. This type of degradation usually shows up when white, powdery crystals form on these materials.
Because lead and pewter objects are particularly susceptible to attack by acids emanating from certain woods, it is very important to store and display them in enamelled metal cupboards.
The general handling, storage and display guidelines previously described earlier in this chapter should be followed.
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