Metal Cleaning. Any of various methods of removing dirt, some corrosion and minute surface defects from metal; these include: abrasive blasting, acid dip, blanching, heat treating, sandblasting, shot peening, barrel tumbling, and water hone. Blanks are often metal cleaned by pickling or by one of these abrasive methods at the same time they are annealed – softened by heat treating – before striking as a customary step of metalworking. Any of these steps will produce an activated surface on the blank. This is not a problem if the striking is done within a reasonable time (up to three months) otherwise the blanks will tone before striking.

Coin blanks are annealing and cleaned at most mints prior to being upset and struck. United States mints clean and anneal all denominations and compositions except cents (which are annealed only). At U.S. mints the cleaning process is called “whitening” and where this occurs is called the “whitening room.” Use of this term is obvious, as the blanks emerge from the processing considerably lighter then beforehand.

Art medals are cleaned between multiple striking (as part of annealing) during work in process. The first step of finishing also includes some form of metal cleaning, usually abrasive blasting. Since some additional finishing will take place the activated surface caused by the abrasive blasting will be eliminated by the permanent finish on the medals.

Recent developments in metal cleaning include a vibratory bowl (using a detergent) and ultrasonic cleaning (using a solution). Both of these are nonabrasive methods, in contrast to the abrasive methods listed above. Blanks or struck pieces – at any step of the process – can be cleaned with nonabrasive methods. In this book metal cleaning is used for blank cleaning only. For the cleaning of existing coins and medals see cleaned, cleaning. See also repair and restoration.

References:

C66 {1988} Cooper pp 190-93, 240.