Reply to Thread

Message:: [QUOTE="John Burgess, post: 7815781, member: 105098"]It is mostly the result of oxygen and/or sulfur reacting with the coin's metal.Silver does not readily react with elements in normal air (Nitrogen 79%, Oxygen 20%, remainder trace Carbon Dioxide, Argon, etc.), at normal temperatures, but the metal will oxidize at high temperatures and moist/humid environments.Most common is Silver Sulfide (Ag2S) as the type of toning that affects coin collections.  it is usually sulfur and halide containing compounds that make silver tone the various colors sometimes and seen as brown or black. Silver Sulfide as a compound is a dense black solid, it is the only sulfide of silver. It is useful as a photosensitizer in photography. It's the tarnish that forms over time on silver. there's also Silver halides, Silver Bromide (AgBr), Silver Chloride (AgCl),Silver Iodide (AgI), much less common to be the cause though. All of this happens on the molecular level, electrons from the contaminant, joining with the elemental silver, Silver is a metallic element that loses one electron to become a positively charged ion, Ag+1. The sulfur atom needs two electrons to fill its valence electron shell. A valence electron shell is the outermost part of an atom's electron cloud. When a sulfur atom gains two electrons, it becomes a negatively charged ion, S+2. Sulfur can get these two electrons from two silver atoms. After this transfer of electrons occurs, the two ions stick together forming an ionic compound, Ag2 SVisually, this process terminates as a coin with a flat black appearance that light cant' pass though the layer of Silver sulfide. As the layer of silver sulfide is formed on the surface of the coin, it gradually thickens.Different thicknesses produce the appearance of different colors. The colors are caused by "thin film interference".  rather than explain it, link to wikipedia:<a href="https://en.wikipedia.org/wiki/Thin-film_interference" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://en.wikipedia.org/wiki/Thin-film_interference" rel="nofollow">https://en.wikipedia.org/wiki/Thin-film_interference</a> the surface of the coin matters, because it is reflection of light that makes it all possible,  but as far as the colors that become visible due to the thickness of the silver sulfide film,  it's due to the film on the coin it's thickness and the optical path length difference for light reflected from the upper and lower boundaries of the thin film. similar to a rainbow in a soapbubble, or a rainbow in the sky, "thin film interference" with the light waves. The gist is that light reflects off the surface of coin at a certain angle, as the silver sulfide thickens, the light bends between the upper and lower layer of the film reflects off the surface, and bends again and produces a color for that wavelength of light. Now, color isn't "real", an apple isn't "red", the sky isn't "blue",  a color blind person isn't going to interpret colors the same way as a person that isn't color blind, or the the fine sighted, and the sight impaired might see color differently also. without light, there is no color, and color is our interpretation of what we see, the eyes (sensors) feed the data to our brain (computer) that translates the image.Colors don't really exist either, it's what out eyes pick up and our brains translate to color, and for the most part everyone's works the same, but there are malfunctions and defects, which in all truth, might just as well be the correct way the world looks, (the color blind) while the majority have this "color sight" as an advantage to identify hot from cold, or dangerous or not, or food from rock, ect. NOW, I am not a scientist, or physicist, or a chemist, just an interested hobbyist that reads a lot. So I've got a basic understanding, not a firm grasp on any of these concepts and I can be wrong on it in some areas, although I feel I've got a decent understanding of it. As far as the "rainbow toned morgan dollars" a whole lot of them were stored in vaults in canvas bags for long periods of time. There's a theory the canvas bags were treated with sulfur as I understand it ( and sulfur was used as a repellent and even a pesticide in the paper industry so I think this is plausible) , to deter rats from chewing the bags and dumping them all over the vault floors. In many cases, this did happen "overnight" as far as the people and the coin were concerned, they were put up and forgotten, and years later someone opened the vaults and the bags and the coins were colored up, whether over night, or over a decade or 20 or 30 years, who knows. it's a Schrödinger's cat kind of thing, who knows until you look and know, and then you know. it could have been an event over a short period of high heat or cold and humidity that settled in the vault, and then retreated, but that day or week or month of it, did the coloring up and then the humidity retreated. There's a lot of factors at play and it's all been explained if you care to read the science of it all. it could be that Morgan dollars weren't really used all that much when they existed, and they sat in canvas bags, and sat and sat. it could be the surface of the coin as struck at the time, it could be environmental factors, and it is likely a combination of it all at play here as to why "rainbow toned" Morgan dollars are much more common than peace dollars, or trade dollars flowing hair, draped bust, ect. toning can take centuries to develop and eventually turn flat black, and it could take seconds if you took a coin and stuck it in a container of sulfur. it depends on the conditions of the environment, temp, humidity, contaminant availability, ect. if you are familiar with Nitrofil for car tires, I am sure if you put a coin in a vessel and pumped it full of pure nitrogen, there would be no oxygen, or sulfur and so no forming of a layer of silver oxide or silver sulfide. as long as the vessel remained sealed and filled with nitrogen, the coin could sit as fresh as the day it was minted with no toning whatsoever for 1000 years or more.As far as coins that just tone dark, older coins, it's posible they don't have much surface reflection to begin with or have been worn down or cleaned and the surface damaged to where it's not reflecting light, in which case you won't get color, you'd just get blackening, silverware is a lot like this, it tarnishes dark, and requires polishing to make it bright again, but will again tarnish darkly. it doesn't have the right surface to maintain a luster that reflects light at the correct wavelength to produce colors from the silver sulfide film, if a coin has toned terminally, to a grey or black, dark color where light no longer passes through, enough surface of the coin silver has been sacrificed to silver sulfide that it may impair the luster of the coin forever, and it will never reflect a rainbow or colors as it once did.[/QUOTE]

Log in or Sign up

Reply to Thread

Useful Searches