Preserving Collectibles column from Oct. 31, Weekly issue of Coin World:

This month I continue reviewing the results of the coin cleaning, toning and coating survey that appeared in my Feb. 29 column.

Readers will remember that this is the third time I have asked readers these questions. The first survey appeared in 1992, the second in 2006. This year I received 38 responses, considerably fewer than the past two times the survey ran.

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Remember, I am reporting what I heard and am not giving percentages or other summary statistics. Not everyone answered all of the questions, and, for those who did, multiple answers were often given. 

More responses

The second question in the survey asked if readers toned or repatinated their collection. I received many strong “no” answers to this question and very few “yes” answers. Of those who did tone their coins, the predominant reason was to reinstate a patina that was lost through a cleaning treatment. One reader toned his coins to hide flaws.

Toning techniques listed were: Deller’s Darkener (a commercial patinating solution); a dilute bleach solution (used to cover light hairlines); sulfur and Vaseline mixture; and wrapping the coin in kraft paper (e.g., a brown paper bag) or placing it in a high sulfur paper envelope. The kraft paper technique was often speeded up by placing the coin in a sunny windowsill under clear plastic to create a warm, humid, environment. The reader admitted that this was a slow process requiring patience.

I also asked if there was a type of coin that a collector would never tone. Only one reader answered this question. He would never tone Indian Head or Lincoln cents.

The last survey question asked about applying surface coatings. Like the toning question, I got a number of strong “no” answers and only a few “yes” replies. In the yes camp, five respondents applied surface coatings to prevent corrosion and two to reduce the damage from inappropriate handling. 

A range of coating materials were reported including: a light application of machine oil; Blue Ribbon Professional Coin Conditioner and Preservative; various lacquers; Care; olive oil; and Verdi-Care.

Discussion

A number of respondents asked for my feedback regarding the techniques that they were using. Commercial silver dips were one of the most popular responses to my “how do you clean your coins” question. Regular readers will remember that I have several concerns with silver dips.

Although coin dip recipes vary from product to product, the two principal components are a strong acid and a sequestering agent. Sulfuric, formic, hydrochloric and phosphoric acids have all been used in silver dips. The concentration of the acid is quite low. Thiourea is the sequestering agent. When a silver coin is placed in the dip the tarnish is dissolved by the acid and removed from the surface of the coin.

The thiourea sequesters or seizes the silver ions and holds them in suspension allowing the acid to remove more tarnish.

Thiourea’s chemical formula contains sulfur. Research has shown that a silver dip leaves an adsorbed stable film of aqueous thiourea on the object and that the bond between the sulfur atom of the thiourea and the silver is so strong that it is impossible to break.

Cleaning with a silver dip leaves behind sulfur that will, in turn, cause the silver to recorrode. No amount of rinsing will remove the adsorbed thiourea. 

Researchers also found that the acid in the coin dips leaches copper out of silver alloys, resulting in the surface of the metal being slightly etched, making it even more reactive to the sulphur and, so, causing it to recorrode faster. 

E*Z*EstRust Remover was listed by a number of readers. Sulfuric acid and thiourea are listed on the manufacturer’s Material Data Safety Sheet (MSDS) indicating that this product is a silver dip.