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.
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
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.
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
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.