Breaking Tradition

Black-Gray Gold

Unlike purple gold, which can be achieved by intermetallic compounds, black-gray gold can be obtained as a surface layer coloring only. The following approaches can be used:

Electroplating. A range of black surface effects can be achieved on karat gold and other precious metal jewelry by electroplating with so-called “black rhodium” or “black ruthenium.” These are special baths formulated with black additives to produce a hard black rhodium or ruthenium electroplated layer of up to 0.5 micron thickness on the surface. A post-plating treatment is necessary to give color stability and wear resistance. The colors range from gray to anthracite black. Hardness values typically range from 230 HV to 310 HV. Researchers claim that wear resistance is good.

Oxidation. While annealing of conventional karat golds in air will produce a black copper oxide layer, it is not physically stable or durable. Consequently, special gold alloys have been developed to produce good, stable black oxide layers. Many of these contain cobalt, which forms a dense black oxide when the gold alloy is subjected to an oxidation treatment. Such treatments are normally done on the finished jewelry piece, and the black oxide can be polished to produce a quality finish. Alloys of gold with nickel or iron additions are also claimed to give gray-black oxides.

Improved wear resistance of black gold can be obtained in 18k golds made from cobalt-containing alloys through chromium additions and the use of an electrolytic hardening cycle. The addition of chromium results in thinner oxide layers, comprising mostly chromium oxide, Cr2O3, which have an olive-green hue. In tests conducted by Mintek, the oxide on an 18k, 15 percent cobalt, 10 percent chromium alloy had better wear resistance than that on an 18k, 25 percent cobalt alloy.

Chemical Vapor Deposition (CVD). The third method of attaining a black-gold layer is through the deposition of hydrogenated amorphous carbon layers on gold by plasma-assisted chemical vapor deposition. (To obtain more information about CVD, see MJSA Journal Readers’ Resources, page 35). This process operates at 390°F to 450°F (200°C to 400°C) and produces a hard black coating that measures 1 to 1.5 microns thick. The coating, which resembles Chinese Lacquer, offers good resistance to wear and impact. The hardness of this layer is very high, about 1,800 HV to 2,000 HV. Wear tests showed this coating to be superior to electrodeposited black ruthenium.

Patination. This method is typically accomplished by the application of chemical treatments containing oxygen and sulfur compounds, which react with the copper in the alloy. The oldest technique consists of immersing the items in impure potassium sulfide salts, known as Liver of Sulfur. Other liquid sulfides and polysulfides are also used, preferably in diluted form to allow the films to build up more slowly. This makes them denser and more permanent. In his book, Jewelry Concepts and Technology, Oppi Untracht suggests immersing the karat gold hot to produce a black color, or using additions of “aqua ammonia” (ammonium hydroxide) in the solution at room temperature. Immersion in barium sulfide solutions will produce a bluish-black coloration.

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photo credit: Necklace by Yvel.