Please note that I have an earned PhD in chemistry and biology and have been a practicing scientist for a number of years. This article is not meant to "convert" anyone who does not like toned coinage; rather, it is here to educate those who might like to learn more about the science behind the toning. In my personal experience, I have found that most collectors and dealers who do not like toned coinage, and who prefer white or blast white coinage, are quite knowledgeable and passionate about numismatics. Unfortunately, there is a vocal minority who appear to have little training in science and who would write badly, and incorrectly, about nearly all toning. If the reader disagrees with the ideas put on these pages, can communicate the reasons why and can provide sound scientific evidence or theory as to the fault, I would be happy to receive an email describing the problem. Thank you for your time.
This section will start this with working definitions for what dipping does to a coin, what the term rust typically means and then what toning is. Dipping simply removes the top few atomic layers of the coin's metal, also known as the "skin", "patina" or "toning" from a coin's surface. On a silver coin what is removed is the outermost layer of silver, typically in the form of the thin layer of silver sulfide or silver oxide that forms on the surface of the coin over time. These silver compounds are produced by oxidation. This is true whether the coin is white or toned. It is also in these outermost atomic layers of the coin's metal where flow lines will be most obviously present. These are the raised lines that are on the surface of the coin after minting. These lines are from the flowing of the metal during the minting (striking) process. It is the scatter of light off of these lines that gives a coin its flash and that causes the cartwheel luster seen on mint state coins. By removing this surface and the flow lines present in this area, the coin is more smooth and light does not bounce off the same way as previously; these coins may appear dull or lifeless. Hence, coins that are improperly or repeatedly dipped lose this upper layer of metal and lose their luster-they looked "dipped out".
As generally used, rust is the oxidation product of iron in metal alloys whereas toning is the product of thin film interference caused by the oxidation of the silver alloy present in a coin. Therefore, they (toning and rust) are both oxidation products but they are not equivalent to one another. The term rust most specifically relates to the oxidation of ferrous iron to ferric iron. That is, conversion of iron in a +2 oxidation state to that in a +3 oxidation state. This requires oxygen and most rapidly occurs in moist air indicating the catalytic activity of water. In its most broad sense of the term, rust may mean “any film or coating on metal caused by oxidation” this is the definition from The Random House Dictionary. In its broader usage, and specifically in scientific usage, rust is associated with the oxidation of metallic iron only, although, as the dictionary points out, one may use it much more liberally. Think of it this way-if one says the word “rust” then one immediately and most likely only thinks of iron, and does not think of other metals.
Non-bullion silver coinage is not made from 0.999 fine silver. In the United States it has historically been an alloy of 0.900 fine silver and 0.100 copper. Copper is a strong anti-corrosive agent and is commonly used to inhibit the corrosion of a variety of metals in many applications. Other corrosion resistant metals are nickel, chromium and zinc and it might be noted that some of these metals are common in circulating coinage. In this alloyed case the corrosion of silver is kept to a minimum while the oxidation of silver is allowed. A subtle yet profound difference between toning and corrosion is that true corrosion is the end product of the process of destruction of the metal involved. With toning we do not see the destruction of the metal. Instead, the oxidation forms a skin or patina on the metal that acts to protect the underlying surface and actually inhibits further chemical reaction. If one choses to use the terms rather expansively then one may interchange rust, toning, oxidation and corrosion. However, I believe in a more strict and proper sense these are not the same entities.
The oxidation product of silver is a physical entity that has mass. Toning, however, is not a physical entity that has mass. This may be explained as follows. The surface of the coin will develop a very thin layer of silver sulfide on it over time. This layer is so thin that a beam of light aimed at the coin, from Point A, in the figure below, splits into two parts, or phases, at Point B. One part goes through the layer of silver sulfide and strikes the surface of the coin at Point E. This is then reflected back to the viewer. The other part (phase) of the light beam reflects off of the silver sulfide layer and never penetrates it. This light is also reflected back to the viewer. However, although these two parts (phases) of this single light beam travel at the same speed and were generated at the same time (from a light source like a lamp) one part of the beam of light travels a longer distance than the other part. The part of the beam that traveled through the silver sulfide to strike the coin and was then reflected back to the viewer has actually traveled twice through the silver sulfide-once from the lamp to the surface of the silver sulfide and then to the surface of the coin and out to the viewer (A-B-E-F-G) while the other beam reflects off the surface of the silver sulfide and to the viewer (A-B-C). This total distance through the silver sulfide is on the order of nanometers (nm). The difference in the total distances that these two parts of the light beam travel is, obviously, incredibly small. Oddly enough, it is precisely because they are so small that we see toning. Toning is the result of the refraction of the light beam through the silver sulfide and the resulting interference with the other portion of the beam that did not penetrate the silver sulfide-this is called thin film interference and is the exact same phenomena that causes oil on water to look like a rainbow. Because the silver sulfide is not of uniform thickness we see different colors on the coin. Once the layer of sulfide becomes a certain thickness the light no longer penetrates through to the coin's surface and the toning becomes dark or brown/black.
