Why do some uncirculated coins have toning (tarnish, patina) and others, while still totally original (i.e. never dipped), do not? Why is the toning on some very dark while on others light enough for the sparkle of the coin’s lustre to clearly show through? Why are some coins toned in a single (monochromatic) color compared to others that have a multitude of rainbow colors? The toning on some coins can be very delicately balanced whereas on others it might be quite lopsided. Why are silver coins more likely to tone than nickel or gold coins? Questions like these are regularly asked by collectors. Sometimes the answers given aren’t very complete or accurate.
One of the best explanations of toning on silver coins and its causes was that by Wayne Miller in his book The Morgan and Peace Dollar Textbook (Adam Smith publisher). He describes the toning on silver coins as being the result of two factors. The first is the chemical interaction of the silver metal with certain ambient elements which include oxygen and chemicals such as sulphur. These reactions take place over long periods of time, usually decades. The result is the creation of a very thin, transparent film covering the entire surface of the coin. Once formed this film is responsible for the “colour” we actually see through an optical phenomenon known as thin film light interference. This film also performs a valuable protective function for the long-term storage of the coin. It restricts the ongoing interaction of the silver with outside elements. In those cases where the coin has been “dipped”, the film is removed and the surfaces are exposed to increased chemical activity. This may cause greater damage to the coin than if it had never been “dipped” in the first place.
Formation of the Thin Film: In most cases the toning we see on silver coins is a result of a sulphur- based chemical reaction. Other chemicals may produce a similar result. A possible source of that sulphur might have been in the cloth of the bags that were used when the coins were originally shipped from the mint to the banks. Those brown paper coin wrappers you use to put coins in “rolls” could also contain some sulphur. Left undisturbed for many years the silver and nearby sulphur combined to form that transparent film. In those areas where the coin’s surface was closest to the sulphur source, the film was thickest. The actual speed of the film buildup is a function of several factors. These include temperature, relative humidity of the surrounding air and time of contact. Moderate, dry climates are unlikely to produce as thick a film as hot, humid climates might over the same time frame. Similarly, the clean unpolluted country air will probably be of less influence than the polluted atmosphere of a heavy industry area. Few modern date coins show much toning because there simply has not been enough of a time lapse for a film to form.
Film Thickness: When a silver coin has generally been in even contact with a sulphur source for some time, that coin displays an even toning of a single (monochromatic) colour. Often this monochromatic colour is a yellowish-gold if the sulphur contact has been for short time frames. Longer exposure will result in an even blue colour. Over very long periods the colour will be jet black. In each of these cases the thin film coating is evenly distributed over the coin’s surface. The yellow-gold results from a film that is not as thick as that for the blue colour and neither is as thick as that for the black impression. Some coin’s actually have multicoloured toning which can include all of the colours red, yellow, green and blue. The thin film coating on such coins actually varies in thickness from point to point across the coin’s surface. When the thickness of these films is in the order of one to two 100-thousandths of an inch it is possible for the phenomenon of thin film light interference to occur. If the film is thicker than this the phenomenon doesn’t happen and the result is a jet-black appearance.
Thin Film Light Interference: First it must be stated that colour is perceived as a difference in the length of light waves which radiate from a light source or are reflected from a shiny object. Short wave lengths are perceived by our eyes as blues and violets. Medium length waves appear as greens and the longest as reds. These wave lengths range from one to two 100- thousandths of an inch. That happens to be the same measure required of the thickness of the transparent film in order for thin film light interference to work.
The way the interference works is that light which shines on a coin and its film is reflected from two surfaces so that our eyes see two reflected beams of light combined into one. The reason there are two reflected beams is because incoming light bounces off both the surface of the thin film and the surface of the coin (because the film is transparent and some of the light gets through to the coin). Depending on the film’s thickness at various points over the coin’s surface these two beams may interfere at several different wave lengths of light. At one point the shorter wave lengths (blue-green, purple) may interfere (i.e., be cancelled) and the eye will perceive only the longer wave lengths associated with colours like red, orange and yellow. The situation may be different at another point on the coin because the film is thicker or thinner. If the film is too thick then the light interference won’t work and the result is black. A common everyday example of this same phenomenon occurs when you observe the many colours seen in thin films of oil on water or in soap bubbles.
Toning on Non-Silver Coins: While the above explains toning on silver coins, there are variations to this for copper coins. Copper will tarnish under the influence of hydrogen sulphide in much the same manner as silver but at much faster rates. Also, copper is a less reflective metal than silver and it is unlikely that thin film light interference is responsible for many of the colour variations in toned copper. Rather, the different colours of green, red and brown are the result of three different chemical reactions and the resultant copper compounds left us a coating on the coin’s surface. The green patina, which is leafy green to bluish-green, can occur in the form of basic copper sulphate. Red patina, which is red to dark red to violet, consists of cuprous oxide produced by decomposition (reduction) of the copper oxide initially formed when copper is oxidized. The brown patina is either a very thin film of light brown to red and black copper oxide formed through the action of oxygen as cupric oxide, or it may be a thick mixture of green and red patina. Pure nickel and gold do not oxidize except for an extremely thin invisible film. Apparently thin film light interference doesn’t occur. With gold-based coins that have a 10% copper alloy there is often a hint of green and reddish-gold present. These colours are a result of the copper content not that of the gold itself.
Much of the above may not be truly understandable unless you’ve studied some physics and chemistry but it does give an explanation as to why some coins tone differently than others. Countless numbers of people have tried to apply their chemistry knowledge to artificially tone coins hoping the results will be as good as that produced over long periods by nature. There still remains an element of mystery because the results aren’t nearly as attractive.
By Brian Cornwell, Canadian Coin News: Volume 24, Number 16 December 9, 1986