VOCABULARY TERM: OXIDATION

Oxidation. A reaction of a metal surface to certain chemicals, naturally from the environment or artificially induced, which tones or darkens the surface. Also called blackening. The term oxidation comes from the chemical change of iron, where iron oxidizes in the presence of moist air (containing oxygen) to form iron oxide or rust. For bronze, silver and other coin and medal compositions, the term is a misnomer; the actual process is one of sulphatization, where the metal reacts – not with oxygen as does iron – but with sulphur in any of several forms. The process turns the metal a darker color, as dark brown or black. The sulphur can come from any source, as a rubber band (made with sulphur) laid across a silver coin will, in time, form a dark streak of oxidation. Such chemical action is a form of toning. See tone, toning.

Artificial oxidation. French medalists in the 1880s developed a process of controlled darkening – using chemicals to purposely darken the surface of a medallic item for a more attractive appearance. As a part of finishing they also developed the process of oxidizing and relieving metal surfaces. Bronze and silver medals are oxidized, but in reality it should be noted, a surface of copper sulphide is formed on bronze, or silver sulphide is formed on silver medallic items.

A portion of this darkened surface is then removed – by RELIEVING – on the high points and field while the sulphide coating remains in the crevices and low points of the medallic item. This provides the two-toned effect highlighting the design. This finish is called antique, oxidized finish or French finish – all three terms mean the same. The process is called oxidizing or antiquing.

The process is as follows. Completely struck up and trimmed edgelettered medallic pieces (all the steps of the pressroom completed) are brought to the finishing department. Here they are abrasive blasted on both sides first of all. Such abrasive action prepares the surface by roughing it up (actually forming a matte surface) which creates a tooth of microscopic cups where the chemical next applied can react.

The medals – of bronze or silver – are then doused in a sulphide solution (as ammonium sulphide) which is the source of sulphur for the chemical reaction. This turns the entire piece black; the surface quickly becomes coated with the copper sulphide or silver sulphide. Immersion time is critical, the longer the immersion, the greater the sulphide coating and the blacker or darker the surface. Even so, medals are usually immersed for less than ten seconds.

After rinsing in clear water to stop the chemical action each medal is then relieved on a buffing wheel with a pumice slurry. The slurry is flooded over the blackened surface of the medal – each side at a time – and buffed under a wet wheel. The buffing removes the sulphide coating on the high points and in the field of the medal where the buffing wheel came in contact with the surface. For the crevices and low points – where the wheel cannot reach – the coating remains intact. Thus the two-toned highlighting becomes apparent and the oxidized surface becomes attractive to the human eye.

The buffing accomplishes one other thing as well as removing a part of the sulphide coating, it also polishes a portion of the surface. Where the surface is buffed the matte surface becomes smooth, the microscopic cups (having served their purpose) have been flattened. The sharp edges (as on lettering) become slightly rounded off. Large flat areas, as the field, become smooth again and somewhat reflective. All this tends to give a softer appearance to the piece and much more pleasing effect – along with the two-toned highlighting – to the overall appearance.

The medals are then rinsed again, dried (even baked to remove all moisture), then lacquered. The lacquer protects the coloration, the patina finish, and somewhat the metal surface. However, no matter how perfect the relieving process and how hard and airtight the lacquer coating, there remains some residual sulphide beneath the lacquer. In time (several years) the medals may slightly darken or tone, but this is natural and can be expected. This residual toning should not be unattractive (it is generally uniform over the entire medallic surface); however, if it is not uniform or the toning is unattractive the piece can be refinished if so desired.

See FINISH AND FINISHING.

To read the complete entry on the Newman Numismatic Portal, see:
Oxidation (https://nnp.wustl.edu/library/dictionarydetail/516451)

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