Definizione

• Lead Disease.  A condition of lead in which impure lead oxidizes to a pale gray color, and if left untreated the object forms a white powdery surface. Pure lead is, of course, inert and very stable; medals made of pure lead in the Renaissance are still intact today. But objects made of impure lead – or, more likely, exposed to adverse metal contacts – may have caused an electrochemical interaction with dissimilar metals, as zinc, tin, copper, others, causing the lead to discolor, granulate and disintegrate. The extreme white powdery surface is actually lead carbonate (PbCO3). Once the lead disease advances to such a state the original surface is lost forever.
  Removing lead disease.  To remove the stains on lead objects in the earliest
  stages of lead disease, use rottenstone on an oily rag and rinse with denatured alcohol. Once the advanced white powdery surface forms, wipe off as much of the granules as possible, then immerse the object in a bath of mild vinegar and water solution, then follow immediately with a bath of mild solution of calcium carbonate and water to neutralize the acid (from the vinegar); and finally wash several times in clean water. If this fails then the corrosion must be scrapped off – in effect chasing the object. Not only has the original surface been lost but the chaser may have difficulty in returning the surface to anywhere near the original relief.

  excerpted with permission from

  An Encyclopedia of Coin and Medal Technology

  For Artists, Makers, Collectors and Curators

  COMPILED AND WRITTEN BY D. WAYNE JOHNSON

  Roger W. Burdette, Editor

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