Bronze disease

Copper alloys buried in salty soil take up chloride, and cuprous chloride in the corrosion crust reacts with moisture and oxygen to form powdery, pale-green basic copper chlorides and hydrochloric acid, which attacks fresh metal beneath. The result is a spot that grows, pits and eventually eats through a thin object — a fibula pin, a coin flan. It is the archaeological counterpart of pyrite decay and the same rule applies: it is chemistry, not contagion, and it needs humidity to run.

Tell it from a stable patina by touch and colour. Stable malachite and azurite are hard, glossy and sit as a continuous skin; bronze disease is matte, powdery, a lighter apple-green, and often at the bottom of a pit. Scrape a suspect spot with a wooden point: if it comes away as powder to bright or red metal, it is active. Check a new purchase under magnification before it joins the cabinet.

Stabilise by drying, then treating. Store below 35 % relative humidity with silica gel; that alone halts the reaction. A conservator will mechanically remove the powder, treat with benzotriazole (BTA) in ethanol to complex the copper, and coat with Incralac or Paraloid B-72. The old sodium sesquicarbonate soak works but takes months and can strip the patina, and any acid or electrolytic stripping destroys the surface the coin's value rests on — a stripped-and-repatinated coin is a disclosure, not a cleaning.

Record the treatment on the record's Preparation card: method, date, and what the surface looked like before. Stratum's Condition flag for active corrosion and its last-inspected date carry the reminder; inspect at-risk pieces twice a year, as for pyrite.