I have an uncirculated WWII era zinc coin that I would like to conserve. It has an extremely light zinc oxide film along the edge, and on a small part of the rim on the reverse. The rest of the coin is darkly toned with pristine surfaces. About as nice an example as I've seen in zinc. I am very familiar with conservation techniques for other metals, but I am hesitant to attempt conservation on zinc. Does anyone have any tips for conserving the coin? I'd like to remove and arrest the corrosion safely without affecting the rest of the surfaces, especially the dark patina.
I would be interested to see what others recommend. I saw a site once that recommended sulfuric acid. I have tried hydrochloric acid (muriatic acid) but for either, use very low strength since what they are doing is dissolving the surface.
I've worked with a lot of zinc coins. I've run into a lot of higher grade ones with a spot or two of that white/gray light corrosion. First I apply some acetone with a q tip. Usually more is needed to be done though, depending on the level of corrosion. So I'd next use verdicare. This stuff works wonders on zinc coins. I apply it over the obverse for 5-10 minutes, lightly swab it in the trouble area with a q tip, pat it dry, then do the same on the other side. Then I wait a day. The next day I pat excess product off the coin and holder it. The white/gray corrosion spot disappears, the large majority of times.
The zinc oxide is that coin's version of toning. Removing it is cleaning, not conserving. No different than dipping a copper coin to remove carbon spots, etc.
I would lean toward leaving the patina alone, rather than exposing fresh metal for the oxidation process to begin upon again. Especially with something as reactive as zinc. That said, I believe thermal reduction using carbon will work for zinc oxide.
That's exactly what I'm looking to do. It's the fuzzy white/gray light corrosion I want to spot treat. I can tell that underneath the corrosion spots is the same dark patina that is on the rest of the coin. I've used verdicare before, but not on zinc. I'm assuming it's not going to touch the rest of the dark patina in this case (which is what I want).
I wouldn't just spot treat though, I'd do the whole coin with verdicare. It is supposed to protect it so it won't hurt any part of the coin or it's color. It works. It actually leaves the coin looking better overall. Especially with zinc coins, in my opinion. In my experience, verdicare works best with zinc, copper, cu-ni, brass, and then last silver. With silver coins i haven't had much luck for some reason
There's been a few occasions where the white corrosion penetrated too deeply and still showed up after treatment (although mitigated). But most cases I was very happy
