Metal Coloring Processes

Although metals dull and tarnish in time, some of the patinas or surface effects so produced are considered to be visually pleasant, especially where patterns of clean and tarnished metal appear side by side due to constant handling. There is extensive literature on methods for the artificial production of other hardware. The erroneously termed oxidized finish on copper and silver articles is the best known in this class. The mauve, brown or black tarnish which appears in time on copper or silver articles exposed indoors is actually a supplied film. It can readily be produced artificially on the clean metal surface by immersing it in an aqueous solution of sodium or ammonium polysulfide, known in the trade as liver of sulfur. The simulate effect of constant handling of the coatings are then locally removed or ‘relieved’ by local rubbing with a cloth or brush loaded with pumice powder. All such tarnish film are neither stable nor wear resistant, so they are invariably further protected by a film or clear lacquer, usually or air drying nitrocellulose lacquer.

In the hardening of high carbon steel, it is first quenched from a high temperature into water or oil, and then tempered to improve the toughness by heating to a rather exact and moderate temperature, and thus the degree of temper, was judged by the interference color formed on the cleaned steel during tempering. When the desired color was attained, the parts were again quenched. Some of these colors are attractive in appearance, such as the peacock-blue color suitable for springs; they have moreover become associated with the properties of a particular grade of temper. The colors are caused by interference of light in oxide films of precise thickness determined by the temperature. These films are very thin, but when saturated with oil they have a slight protective value. They are therefore sometimes artificially produced by other chemical oxidation processes, such as heating in a melt of mixed sodium and potassium nitrates at the appropriate temperature. Such films are used, after oiling with e.g. linseed oil, in tool, watch and instrument manufacture.

The traditional attractive brownish black coating on firearms and weapons is also an oiled oxide film, but is much thicker and is a mixture of ferric and ferrous oxides. The traditional method of formation is lengthy and tedious; it involves repeated treatment with a complex oxidizing solution and successive heating. A simpler process of oxide blackening is widely used for sprongs, clips and similar small steel parts. They are immersed in a very concentrated solution of caustic soda containing sodium nitrate. Owing to the high concentration of caustic soda, this solution can be operated at 150oC. An oxide coating of 0.001 to 0.002 in thickness is obtained in 20 to 30 minutes. This is somewhat porous, but if keep oiled, it provides a fair protection from rusting.