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Metals
Bronzes
When copper is alloyed with tin as the major additional component, different types of bronzes are formed. Bronzes can have significant differences in reactivity towards oxygen. The mechanical strength of the bronze normally increases with the addition of more tin, but the alloys become increasingly more brittle. Some of these alloys include:
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bell metal. With 20–25% tin, this is very strong, but very susceptible to cracking if struck with hard and sharp instruments;
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leaded bronzes: 80% copper/10% tin/10% lead. These are very robust and can be extensively cold-worked;
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statuary bronzes: 65–85% copper/10-30% zinc/2.5–5% tin. These are commonly used for casting; and
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china silver is an alloy of copper, tin, nickel and silver with 65% copper/20% tin/13% nickel/2% silver.
Bronze disease
Bronze disease is the name given to the type of corrosion of copper and its alloys in which light blue/green, pustular outgrowths form on the surface. It is due to the breakdown—by chloride ions—of the passivating layers of corrosion products which normally protect the surface of these metals.
The surface deposits tend to crumble and fall away when touched. As this loose debris is removed from the surface, a pitted surface is exposed directly under the corrosion mound.
Increased temperatures enhance this form of corrosion. A detailed description of the fundamental processes involved in the bronze disease cycle is given in an article by MacLeod, 1981.
This type of corrosion problem is most commonly found with bronzes—hence the name—less frequently in brasses, and occurs least frequently with copper itself.
Because of the relatively high background levels of chloride ions in much of the soil in arid Australia, the problems of bronze disease are encountered in objects that have been recovered from land sites, as well as those from a marine environment or coastal areas.
The corrosion problems are much worse when the materials are stored in high humidities. Under these conditions, a series of little dots and pustules will break out over the surface of an object. If the humidity is not lowered, or if some other form of treatment is not given to the object, then extensive corrosion and gross disfigurement of the object is inevitable.
One of the problems associated with bronze disease is that after the surface has been pitted, it is difficult to disguise that area, unless it is in-filled with coloured wax or some other suitable resinous material.
Treatment of bronze disease
The main aim of treating bronze disease is to remove all of the chlorides from the object.
This is done simply and effectively by immersing the object completely in a solution of 10g of sodium carbonate and 10g of sodium bicarbonate in 1 litre of distilled or deionised water.
A first wash of two to four months, followed by a second wash of four to six months in the solution is normally sufficient.
If the object originally had a bright metal surface, this treatment will produce a green-brown patina which is attractive and stable.
If a clean, metal surface is desired, the patina can be removed after treatment—using the citric acid stripping process or by polishing.
This method is effective for all cases of bronze disease, but the time required varies greatly from object to object.
Remove any protective coating on the object before treatment.
This treatment should be carried out by a conservator, or in close consultation with a conservator.
After removing the chlorides, apply a finishing treatment. Wax impregnation, a coating with benzotriazole or a coating with Incralac are commonly used.
Benzotriazole is one of a number of corrosion inhibitors for copper and its alloys. The reaction mechanisms are complex, but the primary protection is due to the formation of a very stable complex with copper, which protects the metal from further corrosion. Benzotriazole displaces chloride ions from cuprous chloride—this overcomes one of the fundamental causes of accelerated corrosion.
Microcrystalline/polyethylene wax
This is a good recipe:
Melt the waxes together and stir well to make sure they are mixed thoroughly. Quickly pour the molten mixture into the white spirits and stir it constantly while it cools. This makes a smooth, white paste. The sheen of the resultant wax film can be altered by varying either the grades or the proportions of the waxes used. When it is dry, the wax can be polished, for a shiny finish, or left untouched for a matt finish. If subsequent treatment is be necessary, this wax can be removed with white spirit.
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