Hi all, I am relatively new to the hobby and the
FORVM. I collect
Byzantine, so commenting here. Thanks so much for all of your
help. I do chemistry and physics professionally currently, and I collect minerals and study minerology. I find most discussion on
patina with coins to be mostly "unscientific." I get that there is no substitute for practical experience and many people do not want to resurrect high school chemistry for fun, but I find surprisingly little that is technical on this subject.
For example, I read often that silver does not develop
patina, but tones instead. I find that confusing as silver is oxidized (loses electrons) by sulfur, chlorine, and more. Not sure why we don't call it
patina. I believe horn silver is a term used in coin collecting that seems different from the AgCl that is found in nature and is very very soft. Some of the pictures I see don't look like chlorargyrite at all. I'm not sure we talk about the same thing. Nobody talks about ruby silver (?). The most obvious and by far most common oxidation of silver is by sulfur, acanthine in nature, and it happens very quickly if the silver is anywhere near a source. Sulfur is in the air and concentrates in some substances like eggs and asparagus - it is hard to avoid at times. God
help you if you live on the Big Island or in Taipei. That being said, the black mineral is generally protective of the underlying silver. I agree it can look pleasing as "
toning" but can also look really bad at times and is easily cleaned with various methods. I have used
electrolysis on silver
jewelry in selective spots with great results, improving the
toning to emphasize high relief. Ag2S is also fairly soft at 2.0 on the Mohs
scale.
BTW, putting your coin in an aluminum can with baking soda and water is
electrolysis. Use distilled water if you do! No battery or power source is required unless you want to do it quickly. Heating baking soda in the oven releases carbon dioxide and makes a better electrolyte - washing soda. there are a million tricks related to this topic that I won;t go into. And yes, the process can be used to ADD
patina, you just
reverse the current and be selective about which ions you want to bond with. If you get battery polarity wrong, you will discover this in seconds probably.
Copper is very complicated. One of the reasons it is less precious as a metal is that it is far more reactive. Cu2O is a beautiful
red. But I hardly every find in on mine dumps because it turns into black-as-coal tenorite (CuO) fairly quickly. Copper oxidation is usually a combination of oxides, hydroxides, phosphates, carbonates, chlorates, acetates, sulfides, etc. Just like the oxides, carbonates can be green, blue, green-blue. Some of this mineralization is quite hard and forms very specific crystal structures. Soak a bronze coin in vinegar and you will get big beautiful blue-green copper acetate crystals in no time (quite cool actually).
Other topics are accelerants of oxidation such as heat, very thin oxide films that create
iridescence through physics (and this is also how you "tune" the colors of bismuth to purple, green, blue, gold, etc. - by changing the rate of cooling of the easily the melted metal, it changes how thick the oxide film is).
Why are hammered coins possible? Metals form metallic bonds and freely move their electrons around when squished. This makes them malleable. It also makes them conductive and ductile and many other things too.
I could go on and on.
Bronze disease is a fascinating thing! Chlorine is just evil.
Are any of these topics interesting? Can anyone point me to a thorough scientific approach to
patina, etc.?
Thanks!
Paul
