Learn about Special Gold Colors

Special colors of Gold: Blue, Black and Purple!

But gold is yellow?

Pure gold is a deep yellow color and conventional carat gold Jewelry alloys can range from red through yellow to pale yellow/green and even white by varying the alloying metals. But it is possible to make gold Jewelry that exhibits unusual colors such as purple and blue and black. How is this possible? Well, this can be accomplished by one of two techniques: formation of special gold metal compounds (intermetallic compounds) or by a surface coating or patination. Both approaches can yield attractive colors but they do have some disadvantages over normal carat gold alloys.

1. Intermetallic compound colors

a] Purple gold (also known as amethyst or violet gold)

When gold and aluminum are alloyed in a certain fixed ratio, they form a gold intermetallic compound with the chemical formula AuAl2. That is one atom of gold to two atoms of aluminum. This compound has an attractive purple color, as the pendant illustrates. In terms of composition, this compound is about 79% gold by weight and hence is hallmarkable as 18 carat gold.

All intermetallic compounds, and purple gold is no exception, tend to be very brittle. They cannot be easily worked by conventional metal working processes. If one attempted to roll or hammer a piece of purple gold, it would shatter into pieces! It also tends to tarnish easily.

Melting gold and aluminum together to make purple gold is not easy and requires vacuum melting equipment. However, it is possible to melt and cast pieces of purple gold into a mold. The compound has a melting point of about 1060°C, higher than that of both gold and aluminum, which is indicative of the compound's high stability. The purple color can be retained at aluminum contents as low as 15%, but such alloys will be 2 phase, comprising the purple compound and some aluminum-rich solid solution. These non-stoichiometric alloys will tend to be less brittle in their mechanical properties, but the color will be diluted.

Cast pieces can be machined or faceted by grinding or milling to form pseudo 'gem stones' which can be set in conventional gold Jewelry.

An alternative approach to making Jewelry with purple gold decoration is to physically vapor deposit (PVD) the two metals, gold and aluminum, in the correct ratio onto a carat gold substrate. Such processing can be done by a number of PVD techniques such as sputtering. Jewelry made by this approach is commercially available.

A powder metallurgy approach is also possible, with additions of 7-30% cobalt, nickel or palladium powders added to the gold-aluminum powder, which is pressed and sintered. It is claimed that such alloys are of good purple color and have satisfactory workability.

In a new process, ornamental purple gold alloys containing 70-85% gold, the rest aluminum, are claimed which are made by vacuum melting an ingot, atomizing it centrifugally and the powders packed in a mold and electrical discharge sintered. Partial surfaces may be strengthened by diffusion bonding with pure gold, silver or platinum or alloys thereof.

Purple gold wires can be made made by bundling gold-plated aluminum and aluminum-plated gold wires together and drawing them down to produce a composite wire, which is then subjected to a thermal diffusion treatment at 450-700°C in a reducing atmosphere. This way, a wire with a fibrous structure of purple gold (with some gold in a 2 phase structure) is claimed that is tough and flexible. Such a diffusion process can also be used to provide a purple gold effect on gold Jewelry by depositing a layer of aluminum onto the surface and doing a thermal diffusion treatment to form the purple compound. Thermal spraying of gold and aluminum powders onto a substrate can also achieve a purple coating.

b] Blue gold

The intermetallic compound formed between gold and indium, AuIn2, gives rise to a clear blue color and that between gold and gallium, AuGa2, to a bluish hue.

AuIn2 (46% gold) and AuGa2 (58.5% gold) have melting points of 540.7°C and 491.3°C respectively. Off-stoichiometric compositions, like purple gold, will be 2 phase and so can be expected to have some measure of workability and toughness. Manufacturing techniques will also be similar to those for purple gold.

2 colors by surface coatings and patinas

c] Black gold (grey - black & brown)

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.

d] Blue gold

A blue patina can be produced on gold alloys by oxidation treatments. In one case a 20 -23 carat gold alloy that turns to a rich sapphire blue is alloyed with ruthenium, rhodium and 3 other metals. It yields a blue surface layer 3 -6 mm thick. In another case an18 ct gold with 24.4% iron and 0.6% maximum nickel forms a blue oxide layer when heated at 450- 600°C. At a higher, 83% gold content, a blue-green color is produced.

Oxidation of gold alloys containing 25% iron or arsenic is also reported in the literature to yield a bluish color.

Note - Many of these coatings will be vulnerable to rubbing or abrasion and so should be protected
where possible.

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