There are several electroplating solutions on the market for the deposition of black coatings, but the most popular are those based on rhodium or ruthenium with special blackening additions.
April 21, 2006 (Press Release) --
There are several ways of obtaining a black coloration on carat gold's.
There are several electroplating solutions on the market for the deposition of black coatings, but the most popular are those based on rhodium or ruthenium with special blackening additions. The ruthenium bath gives slightly harder coatings than rhodium. Coatings range in color from grey to 'anthracite' black. The blacker the color, the less wear resistant is the coating. Hardness of the coating ranges from HV 230 to 310 and is inversely proportional to the level of blackening agent. Overall, wear resistance is not high and so rubbing or abrading conditions should be avoided.
PA-CVD has been developed for the watch industry and enables 1 - 1.5 mm thickness coatings of hard, amorphous hydrogenated carbon to be deposited at 200-400°C. The coating has an appearance of Chinese lacquer and can be gloss or matte depending on the substrate surface condition. It is very hard (HV 1800-2000), wears well and is biocompatible.
A black oxide coating or patina can be produced by controlled oxidation of carat gold's containing cobalt, iron or chromium additions. For example, a gold 75% - cobalt 15% - chromium 10% alloy is oxidized in a furnace at 700-950°C (1292-1742°F).
This promotes black oxides of cobalt and chromium, which are wear resistant. However, such alloys are not suited for working and lost wax casting, so are not suitable for mass manufacture of black gold items.
A grey color can be obtained by oxidation of a gold alloy containing 15-20% iron.
Brown to black patinas can also be obtained in copper-containing carat gold alloys at 18 ct or less by treatment with Liver of Sulphur (impure potassium sulphide) or other sulphides to produce a sulphide layer on the surface. They are used dilute and the patina is built up slowly to produce more permanent, denser coatings.
Source: http://www.jewelrybynet.com
There are several electroplating solutions on the market for the deposition of black coatings, but the most popular are those based on rhodium or ruthenium with special blackening additions. The ruthenium bath gives slightly harder coatings than rhodium. Coatings range in color from grey to 'anthracite' black. The blacker the color, the less wear resistant is the coating. Hardness of the coating ranges from HV 230 to 310 and is inversely proportional to the level of blackening agent. Overall, wear resistance is not high and so rubbing or abrading conditions should be avoided.
PA-CVD has been developed for the watch industry and enables 1 - 1.5 mm thickness coatings of hard, amorphous hydrogenated carbon to be deposited at 200-400°C. The coating has an appearance of Chinese lacquer and can be gloss or matte depending on the substrate surface condition. It is very hard (HV 1800-2000), wears well and is biocompatible.
A black oxide coating or patina can be produced by controlled oxidation of carat gold's containing cobalt, iron or chromium additions. For example, a gold 75% - cobalt 15% - chromium 10% alloy is oxidized in a furnace at 700-950°C (1292-1742°F).
This promotes black oxides of cobalt and chromium, which are wear resistant. However, such alloys are not suited for working and lost wax casting, so are not suitable for mass manufacture of black gold items.
A grey color can be obtained by oxidation of a gold alloy containing 15-20% iron.
Brown to black patinas can also be obtained in copper-containing carat gold alloys at 18 ct or less by treatment with Liver of Sulphur (impure potassium sulphide) or other sulphides to produce a sulphide layer on the surface. They are used dilute and the patina is built up slowly to produce more permanent, denser coatings.
Source: http://www.jewelrybynet.com
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