Bronze disease

It is commonly present in all colors in this range due to the series of reactions that cause it and there may also be tiny, possibly microscopic, blue crystals.

Bronze disease typically affects isolated patches of the object in severe cases being a visibly and tactilely raised bloom of microscopic crystals as well as being associated with pitting.

These properties are all in comparison with verdigris, which is normally a duller shade, uniform across the whole of the affected object, and cannot be scratched off with wood or fingernails.

[2][3] Bronze disease is common or even ubiquitous on artefacts recovered from a marine environment due to the presence of chlorides in seawater.

[4] Chlorides may occur in or on the metal due to contamination from soil, water (especially seawater), the atmosphere, human sweat, or be present as impurities when the object was created.

In many cases chlorides may be present within the interior of the artefact; the disease may reoccur if not isolated from water and/or oxygen.

In practice this first involves physical cleaning (with a wooden or even metal pick) to remove the bulk of the chlorides and then chemical treatment.

Amateurs report that the patina may be stripped from the artefact but this is when the solution is boiled so that the carbonate rinse removes the chlorides in hours rather than the cool bath of long duration used by professional conservators.

[2][3] Soaking in sodium carbonate—which does not form a complex ion with copper and is unlikely to affect the patina but is slower than the sesquicarbonate—or benzotriazole aqueous solutions may also be used.

Once treated, the specimen should be held in a dry environment and periodically inspected for recurrence of bronze disease as no long-term treatment has been confirmed.

Storing the object in a completely dry or oxygen free environment will also prevent bronze disease as will isolation from contact with chlorides.

Formation of harmless unidentified sodium carbonate crystals on an ancient Roman bronze coin that has been treated in carbonate solution for bronze disease for four weeks and has been removed and rinsed several times. Video has been accelerated to ten times normal speed.
Unidentified sodium carbonate crystals (white) formed on the same Roman coin treated for bronze disease. The green masses on the coin's reverse are a copper carbonate which formed in the region where the bronze disease was most prevalent. It will be physically removed to ensure that the chlorides have also been removed prior to further conservation efforts. The photos were taken some 20 minutes after rinsing and surface drying.