Why Is Old Bronze Green? A Surface That Protects What Is Under It

Copper and its alloys react with oxygen, water, carbon dioxide and whatever else is in the air, producing a complex of compounds on the surface. The first stage is a dark reddish oxide, which develops within weeks. Over years that converts to green and blue compounds formed with carbonates, sulphates and chlorides depending on what the atmosphere supplies, which is why the colour differs between a coastal site, an industrial city and a rural one. The resulting layer is adherent and dense, so it acts as a barrier between the metal and the air, which slows further corrosion to a very low rate and is why bronze objects survive for millennia.

The compounds formed depend on the surroundings:

• •Carbonates from ordinary air, giving the familiar blue green
• •Sulphates where industrial pollution supplies sulphur, giving darker greens
• •Chlorides near the sea, which produce a different and more damaging result
• •Sulphides, which blacken silver and darken copper alloys
• •Burial conditions, which produce quite different colours from exposure
• •The alloy itself, since tin, zinc and lead content all change the result

Not every such layer is protective, and the exception matters enormously in conservation. Where chlorides are present, typically from burial in salty soil or from a marine environment, a corrosion process can establish itself that is self-sustaining rather than self-limiting. It produces a pale powdery green material, progressively converts sound metal beneath, and continues indefinitely as long as moisture and oxygen are available, eventually reducing an object to a shell. It is recognised by the powdery texture and by pitting beneath, and treatment involves removing chlorides chemically and then storing the object in very dry conditions. Museums monitor affected objects continuously and the condition has consumed important pieces.

The term is used of surfaces well beyond metal and the meanings differ enough to cause confusion. On furniture it describes the accumulated effect of centuries of polishing, handling, light and wear, which produces a depth of surface that cannot be imitated convincingly and which is destroyed permanently by stripping and refinishing. On stone it describes weathering and the growth of lichen and algae. On leather it describes darkening and softening with use. On archaeological flint it describes a gloss produced by use on particular materials, which is studied microscopically to determine what a tool cut. In each case the value attached is the same, namely evidence of genuine age that a faker must reproduce and generally cannot.

Sculptors and metalworkers have induced these layers artificially for as long as they have cast bronze, treating the surface as something to be coloured rather than left. Chemicals applied to heated metal produce blues, greens, browns and blacks within minutes rather than decades, and the recipes are a recognised part of the craft with substantial regional traditions. Japanese metalworking developed an elaborate set of alloys and treatments producing colours unavailable otherwise. Modern foundries finish sculpture this way as a matter of course, and a new bronze arrives with the colour its maker chose. Whether a given surface is natural or applied is frequently difficult to determine, which matters for dating and authentication.

Copper alloys react with the air to form an adherent layer of oxides, carbonates and sulphates that seals the surface and slows further corrosion, which is why bronze survives for millennia. Colour depends on what the atmosphere supplies. Chloride-driven corrosion is the exception, being self-sustaining rather than protective and capable of consuming an object entirely. Sculptors have induced the colouring deliberately for as long as they have cast bronze.