Copper Alloy Conservation
Copper alloys is a term used to describe a metal where copper is mixed with another type of metal to form “copper alloy.” Two very common types of copper alloys are: brass (a combination of copper and zinc) and bronze (a combination of copper and tin).
Copper alloys are more resistant to corrosion than ferrous alloys. When brass decays in the ground, it is subject to selective corrosion, and the zinc component usually leaches out of the brass leaving behind a spongy brass matrix. Copper alloys are generally found discolored, obscured by corrosion products and brittle upon excavation. Unlike other metals which can easily be identified by their corrosion products, copper alloy corrosion can vary greatly between sites, features and contexts and can be found to be a wide range of green colors with purple, red and brown mixed in.
Some more common examples of copper corrosion found on archaeological sites such as those at Historic St. Mary’s City are listed below. Factors which cause copper alloys to degrade during burial include water, oxygen, pollutants, micro-organisms and soluble and non-soluble salts found in the soil. Most of the corrosion products can be identified by a trained archaeologist or conservator or by someone who is familiar with archaeological objects. It is only through practice that you begin to recognize the features and types of corrosion that can form on objects. Due to the complex nature of many of these corrosion products, they can only be correctly identified after completing scientific analysis. It is better to describe what you see, and not to guess!
| TYPES OF COPPER ALLOY CORROSION |
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Oxides |
Red- cuprite – forms on damp, aerated sites
Black tennorite- usually found in waterlogged conditions
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Carbonates |
Malachite/azurite –- green/blue –- can form in a variety of burial environments and may appear waxy/ smooth or grainy/ crystalline. Mixes of blue and green are often found together
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Bronze Disease |
A term that is often used to describe very bright green and blue corrosion on bronzes. This is a typical phenomenon of outdoor sculpture, church steeples and architectural elements that turn “green” over time. In order for copper alloy corrosion to be correctly identified as bronze disease, the presence of cupric chlorides — often in contact with moisture — must be present. The term is used more widely these days, but should only be used to describe copper chloride corrosion. Bronze disease got its name from scientific studies which have shown that it can actually spread to nearby artifacts — so if your collections are infected, you may want to segregate them from nearby copper alloys. The very active corrosion, often seen in storage post-excavation, seems to literally “spread” to artifacts nearby causing a chain reaction! |