Why you should avoid lacquering your coins
- Published: Jan 13, 2017, 4 AM
Preserving Collectibles column from Jan. 30, 2017, Weekly issue of Coin World:
Here I finish discussing results of the coin cleaning, toning and coating survey that appeared in my Feb. 29, 2016, column. As my last column noted, a number of respondents asked for my feedback about techniques and products they were using. I would like to address the surface coatings some used. Readers reported using the following coatings: various lacquers; Blue Ribbon Professional Coin Conditioner and Preservative; Coin Care; Verdi-Care; olive oil; and a light application of machine oil.
By and large, I do not recommend that collectors apply lacquers to their numismatic collections.
Many numismatic museums lacquer coins that are going to be on extended display because it reduces maintenance time and costs by reducing cleaning frequency. Lacquers are also used to protect collections that are handled extensively.
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However, I do not advocate lacquering for two reasons. First, an uneven lacquer coating on the surface of a metal coin produces anodic areas or corrosion sites. In a corrosive environment, a poorly applied lacquer can cause a coin to corrode unevenly. Second, some lacquers are very difficult, if not impossible, to remove. Wherever possible, conservators strive to use reversible treatments that allow future removal of a coating without harming the object.
Lacquering is a job for a specialist. If you choose to apply a lacquer, choose wisely. Only stable, reversible lacquers should be used. You should be able to remove it easily with an appropriate solvent now and in the future.
Commercial products can be problematic. A manufacturer can change formulation at any time without informing the public. For this reason, conservators tend to formulate from scratch and time-tested materials any coatings, adhesives and solutions they use to treat an object.
Manufacturers do not disclose ingredients that are “trade secrets,” so it is hard to know what is in a commercial product to determine if it is acceptable. Blue Ribbon Professional Coin Conditioner and Preservative, Coin Care, and Verdi-Care are all commercial products. Unfortunately, I do not know enough about the main constituents of each to make an informed decision as to whether to recommend them or not.
I have several concerns with olive oil as a coating. First, the fatty acids in olive oil could cause corrosion of copper alloy coins. Second, it is, by its nature, greasy. Oil on a coin could make it slippery, hard to hold. It can also trap atmospheric dust. Finally, there is the question of removing the oil in the future. Collectors commonly use trisodium phosphate (TSP) to remove the oil. TSP is a degreasing agent most often used to prepare a surface for painting. At pH 12, TSP works because it is quite alkaline.
A strong alkali reacts with an oil or animal fat to create soap. The resulting soap can then be washed away with water. This reaction is called “saponification,” from the Latin “sapo” for soap. A borax solution (pH 9.5) has also been used to remove the oil. Alkaline solutions should not be used on coins struck from amphoteric metals such as aluminum, lead, tin and zinc, as the oxides of these metals can be dissolved in alkaline environment.
Like olive oil, the greasy aspect of machine oil is a concern, but removal of the coating is much simpler, as machine oil is soluble in mineral spirits.
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