Warning: we do not, and never will ever, ever, ever recommend cleaning coins. Doing so can easily damage a coin and remove 10's, 100's, or even 1000's of dollars of value in just a few short seconds. Please consider this before continuing to read this article.
An industrial chemist who frequents the Coin Community Forum noticed a recurring problem among collectors of copper coins. Namely how to remove the verdigris that so plagues copper and bronze coins that haven't been stored properly. Taking this as a challenge he spent several years developing a safe verdigris removal fluid that supposedly can safely help to remove verdigris and debris from coins as well as providing a level of surface protection that will help preserve the coin into the future. The product, known as Verdi-Care isn't available in Australia but we've managed to get hold of some and decided to run a short experiment on a couple of coins we had laying about to see how this product performed. We should note that Verdi-Care styles itself as an "all metal" conservation fluid but at this time we've decided to stick with some bronze coins to see how it performs on those.
The Coins
We decided to trial the product on a 1966 2c and a 1966 1c that had just been broken out of a 1966 Carded Mint Set. The coins both had visible green discolouration to the rims and clearly had to be removed to save them. The coins can be seen below.
Australia 1966 2c - Before Treatment with Verdicare
Australia 1966 1c - Before Treatment with Acetone
The discolouration to the rims was actually quite green (it looks blackish in the images) and certainly would usually consign a low value coin to the scrap bucket. However, given these coins were nice UNC examples otherwise we decided to try to remove the green from each coin. To put a 'control' aspect into the experiment we decided to treat the 1c with acetone, an organic solvent that is considered to be quite safe to use on coins to remove organic substances like skin oils and debris. The 2c we decided to treat with Verdi-Care.
Treatment Method
1. Placed both coins in a clean glass container.
2. Add enough of the treatment medium (acetone or Verdi-Care) to just cover the coin.
3. Allow to rest for 5 minutes in the treatment medium.
4. Remove coin from medium and work green stain around gently with a wooden tooth pick.
5. Put back in treatment fluid for a further 2 minutes.
4. Remove each coin from medium and rinse with distilled water.
5. Gently pat each coin dry with soft paper towel.
Of course we took care to use safety precautions with both treatment fluids, gloves were worn and were were careful not to inhale fumes or splash the fluids about.
Observations
Australia 1966 2c - After Treatment with Verdicare
Australia 1966 1c - After Treatment with Acetone
You can see images of the treated coins above. The 2c shows some discolouration in the region where the green stain was originally on the coin. Generally lustre of the coin has improved slightly. The 1c shows no observable discolouration where the green stain was, the lustre and overall colour of the coin appears largely unchanged.
Conclusions
In this case we'd say that acetone was the superior treatment. This may be because the green stain was not actually verdigris but rather some sort of "green slime" that was easily removed with a solvent. Perhaps the carded mint set these coins came from was stored in close proximity to PVC for a period of time and some of the PVC softeners leached through the very thin plastic protecting the coins in the mint set. Perhaps this was an unfair test, next time we'll put Verdi-Care up against acetone on a coin that has real, hard, encrusted verdigris.
Posted by mnemtsas at February 19, 2011 3:05 PM
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