Tone, Toning. A slight surface coloration from normal exposure to atmosphere and chemicals in the environment; or such darkening artificially produced. Metal which is aged – allowed to oxidize naturally – changes color or shade; copper tones most dramatically from bright yellow-red to brown, pewter tones from white-gray to dark gray, other metals tone with a slight darkening. Baser metals tone more often than rarer metals; silver tones in small degree (from gray to black), gold does not tone at all.Natural toning occurs slowly, sometimes as slowly as six months to two years or more. Toning is due to the metal composition exposed to chemicals in the air or environment. Bronze and silver – the two most popular medallic compositions – are most affected by air containing sulfur or hydrogen sulfide. (Needless to say, direct contact of the metal with sulfur compounds in liquid or solid speed up toning.) Copper or bronze also tone by exposure to moist air containing carbon dioxide – what humans breath out – forming copper carbonate on a copper surface.Toning is a very light chemical change affecting only the outermost exterior of a metal surface. On a medallic or numismatic item the first casualty to toning is luster if there is any; the minute ridges or striations forming the shiny reflectiveness of a coined surface fill and darken causing the loss of the brilliance of such a surface. The color of coined bronze changes from mint luster, what collectors call red, to brown. This is gradual at first, but continued exposure turns the entire surface brown.There is little harm done to the surface by toning, other than the change of surface color. However, should the exposure be intense (severe in degree or time), then tarnish could occur. (Tarnish is a far more severe condition of the surface, which may include a physical attack to the metal, as with rust, corrosion, bronze disease, tin pest or such.)On most metals toning discoloration is called toned, it is called darkening on white metal alloys, and clouding on proof surfaces. Often toning is not displeasing to the aesthetic eye, it is looked upon as a natural coloring that comes with age. There are some toning anomalies but these can be easily dealt with.Toning of coins. Unlike medals, which normally have some protection of a lacquered surface, coins are fully exposed, naked to every environment. Coins are subject to every form of corrosive exposure – salt air and water, acids, oils, chemicals – hot moist climates to sub-freezing cold. Even finger perspiration from simply handling coins attract contaminants – all tend to reduce luster, expose the pieces to adverse chemicals and hasten toning.Toning of medals. Those medals that have been oxidized and relieved have undergone the most severe toning – oxidation. Here the entire piece has been instantly and completely darkened by immersion in a chemical like ammonium sulphide (in which "oxidation" – actually sulphidation since the metal combines with sulphur – occurs in seconds). Such "oxidized" medals are then relieved of the darkened surface on the high points and fields by buffing on a wet wheel – it leaves some of the sulphur-darkening chemical permanently in the crevices and low points – to cause the two-toned artistic effect. The medals are then dried and lacquered for permanent protection. However, that existing oxidation chemical may form a residual toning under the lacquer, by slightly darkening the entire medallic surface uniformly.Medals that have not been oxidized and relieved are said to have a coin finish – no finish – just like a coin. Medals that have been oxidized and relieved – but not given the additional protective coating of lacquer – are subject to toning. Both of these situations are highly susceptible to toning more so and quicker than if the pieces were protected by the lacquering.If a piece has been cleaned, bright dipped or inexpertly treated, it may have developed an activated surface – a chemical change which causes a rapid toning – or it may have developed a haze – a chemical residue (like benzine does) leaving a cloudy surface. These drastically affect toning.Any metal cleaner which brightens metal generally creates an unstable surface. Often this is an unsightly bright exterior. Artificial toning with the use of a commercial toner or other oxidation agent will cure this surface condition. (It is not a professional finish but can be applied by most anyone who can read a label.) See metal cleaning.Toning anomalies. Unsatisfactory methods of applying the oxidizing chemical in the finishing of medals – or the washing, relieving, or drying after this application – can create a toning anomaly. Most notable of these are fishtail or wipe marks on bronze or silver medals, streaks or blotches of darkened toning, or moisture spots. These can easily be removed by refinishing to a nearly new pristine condition, in effect removing the existing toning down to base metal and replacing with a new refinished surface. See refinishing.Also white metal has a peculiar toning anomaly – an area of great stress leaves a bright area, a negative shadow, on struck pieces. See white metal.Toning is not a patina, even though both are somewhat self sealing. As metal tones, it prevents further toning (or further oxidation) from occurring. If a medal has toned before it has been completely finished, as a delay in the later steps of manufacture say, it can easily be finished with the normal process through the finishing department.Toning is natural. It can be postponed, prevented, removed, replaced or allowed to remain. It is not a serious detriment to any numismatic or medallic item. It always provides the color of the piece and is the surface that is seen by the human eye.
excerpted with permission from
An Encyclopedia of Coin and Medal Technology
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor