This is a lengthy post about a coin conservation experiment. I wasn't planning to learn as much as I did so I didn't take any pictures. Sorry about that. You'll have to trust my descriptions. Most of the results I report were relatively obvious so you should be able to reproduce them if you try. For what it's worth, I'm not a chemist but I worked as a technician in a chemical lab during college so I have some idea what I'm doing. In this context, I'm completely unbiased. I'm just someone with an interest in conserving coins who is reporting objectively.
The CoinI selected a
Walking Liberty half that was a complete basket case. It had obvious grime and grit, a large blob of adhered organic red stuff, probably paint or nail polish, and at least three different colors of crusty inorganic corrosion.
Acetone FirstI wanted to start with acetone to remove as much of the grime and grit as possible before focusing on more challenging issues that might involve more than just liquid contacting the coin. The biggest issue with acetone is purity. I found a great source for MSDS-verified 100% pure acetone (which I assume means >= 99.5% pure). Dupli-Color brand acetone is made in America by Sherwin Williams. I found it on the shelf at Autozone. Even the packaging was superior to the other products I saw at other stores.
When the coin was placed in the acetone, the acetone quickly dissolved the large red blob and much of the other organic gunk stuck to the coin. As far as I could tell, it did not oxidize the coin. The net result was a big improvement, with no detectable damage, and low cost. In other words, it was an unqualified success. It's not really news that acetone dissolves organic materials without hurting coins but I was pleased to demonstrate it.
KOINSOLV SucksI had a container of KOINSOLV sitting around so I thought I should try it as well. I didn't expect it to remove anything that the acetone didn't and that is what I found. However, after about a minute in KOINSOLV, I noticed that the coin began to turn yellow, which is the first color expected as a thin layer of oxidation forms. Since it says "Harmless to coins!" on the front of the container in large letters, I was shocked. After a little longer in the liquid, it turned darker. I ran a side test with a shiny 1964 dime. It was grotesquely oxidized in 10 minutes. Shame on the manufacturer for advertising that this product won't hurt coins - that's nonsense. For what it's worth, the instructions say to only leave the coin in KOINSOLV for a few seconds but that's only because they know your coin will be destroyed a few seconds later. A product either damages coins or it doesn't and this product severely damages coins. Coin stores that stock KOINSOLV should stop selling it and demand their money back from the manufacturer. Seriously, stop selling it. For those who seek a "re-toning" product (i.e., a coin damaging, irreversible oxidizing product for use after a thiourea dip), KOINSOLV may be a candidate.
VerdiCARE RocksDeveloped by the forum's BadThad, VerdiCARE is the only product I'm aware of that attempts to selectively remove undesirable inorganic cruft, which we loosely refer to as "verdigris", without adding or removing the undesirable-to-add and undesirable-to-remove inorganic cruft we loosely refer to as "patina", "toning", or "oxidation". A quick application of a few drops of VerdiCARE did a fabulous job dissolving several types of verdigris, particularly a very dense reddish brown patch. It didn't completely dissolve some yellowish green cruft but it noticeably reduced it. I saw no signs of additional oxidation. This was my first use of VerdiCARE and I was very impressed. My hat is off to BadThad! I was able to remove a little more of the verdigris by gently rubbing it with a VerdiCARE moistened Q-Tip. I wouldn't try that with an uncirculated coin because it's possible to scratch a coin with a Q-Tip but I was very gentle and detected no related damage. VerdiCARE is expensive by volume but, since it's applied in drops, not much is required. It's very convenient that you don't need to fill a container to use it.
As great as VerdiCARE is, it isn't perfect. My biggest complaint about VerdiCARE is that a significant amount of the product remains on the coin. Among other things, VerdiCARE contains a chemical named ReAcT2 intended to protect the coin from further oxidation. That's a noble goal. However, the material that remains is way too shiny and wet looking for my tastes. I also have a secondary concern about material that dissolved from the coin surface, didn't evaporate, and became suspended in the dry but wet looking material that remains.
Acetone AgainI suspected that I could remove the residue left by VerdiCARE using acetone so I gave it a shot. It worked beautifully, completely removing the wet shiny appearance. Of course, it also makes the coin vulnerable to oxidation in the future but proper storage can address that. (BadThad, if you're listening, I'd be interested in a ReAcT2-only product if it wouldn't alter the appearance of the coin, to be used as the last step before storage. Who wouldn't want an invisible barrier to oxidation.)
Quite surprisingly, and importantly, the acetone also knocked off more of the yellowish green cruft that remained. I really didn't expect that result. I don't know why that happened but one theory is that the undesirable materials were present in geometrical arrangements that couldn't all be dissolved without alternating applications of different chemicals. To me, this result suggests that alternating acetone and VerdiCARE dips could be even more effective.
The final product of my experiment was a
Walking Liberty half that wasn't a basket case at all. It was more toned than I would prefer (KOINSOLV's fault), and it had some uneven toning (because some toning occurred after the red gunk stuck to the coin), but it would still be a decent addition to many collections.
Conclusions1) Alternating applications of pure acetone and VerdiCARE, always using acetone first and last, appears to be essentially harmless to circulated silver coins, extremely effective at conserving them, and reasonably cost effective.
2) Unless you're trying to tone your coins, avoid KOINSOLV.
Future TestsThere are a few follow up experiments that I'd like to see done. I doubt I'll get to them any time soon so have at it if you'd like.
The first test is a very long Dupli-Color acetone exposure test to assess whether any oxidation occurs. Essentially, this test would assess the purity of the acetone and the undesirability of its impurities. It would require a completely repeatable camera and lighting setup for before and after pictures. The procedure would involve a quick acetone dip, a before photo, a very long acetone dip, and an after photo.
The second test is a very long VerdiCARE exposure test to assess whether any oxidation occurs. The procedure would involve a quick acetone dip, a before photo, a very long VerdiCARE dip, another quick acetone dip, and an after photo.
Of course, similar tests with coins of other materials would also be helpful.
Posted 10/05/2012 4:01 pm
