Description of the metal colouring process
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Metal colouring
It is possible to obtain a wide range of shades and colours over different metals by heat treatment, chemical dip or electrolytic treatment. These processes are used for brass, copper and steel parts. The most commonly used system is chemical dipping. The results obtained will depend more on the practice of the operation and process parameters than on the formula used. Originally, sulphide and polysulphide solutions in alkaline media using sodium, ammonium or barium salts from room to high temperatures were used. It is now more usual to use solutions containing metallic ions (copper, selenium, molybdenum, etc.) in an acid medium and at room temperature.
Small articles may be coloured in bulk, then proud surface areas relieved by tumbling with an abrasive media. All sulphide treated workpieces require wet or dry scratch-brushing and must be protected by a topcoat of clear lacquer.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
