Plaster Residue. A hard gray-white substance found in the crevices of a numismatic item, which had once been cleaned or used as a pattern for a plaster cast. Evidence of such residue indicates the piece was mistreated by a previous owner. Two possibilities exist: (1) it was cleaned with a pumice-based cleanser, or (2) it was used as the pattern to make a plaster of Paris casting. It is difficult to identify which residue – the dried pumice or hardened plaster – is present. If the remainder of the piece is harshly cleaned it may be assumed to be the first possibility and then called

The second possibility indicates the plaster casting was not professionally made (a release agent was not used or if used was not done so properly). In most professional casting of numismatic items, the coin or medal never touches plaster – instead it is pressed into a clay or plasticine, which becomes the negative mold, and the plaster cast is made from that mold. The plaster never comes in contact with the coin or medal itself. Plaster residue can also be found in rubber or plastic molds, metal galvanos or patterns if not properly coated with a release agent or expertly handled.

Removal of either residue from the coin or medal is quite difficult, particularly the hardened plaster. The residue has great affinity for tiny crevices and becomes sealed in these. The first step should be ultrasonic cleaning. If this is unsuccessful the dried pumice or hardened plaster will have to be removed by chasing.