But gold is yellow?

Pure gold is a deep yellow colour and conventional carat gold jewellery 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 jewellery that exhibits unusual colours 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 colours but they do have some disadvanges over normal carat gold alloys.

1. Intermetallic compound colours

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

When gold and aluminium 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 aluminium. This compound has an attractive purple colour, 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, according to P. Gainsbury (Aurum, no 20, p.40, 1984).

 

Figure: Purple Gold pendant

 

 

 

Melting gold and aluminium 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 mould. The compound has a melting point of about 1060°C, higher than that of both gold and aluminium, which is indicative of the compound's high stability. The purple colour can be retained at aluminium contents as low as 15%, but such alloys will be 2 phase, comprising the purple compound and some aluminium-rich solid solution. These non-stoichiometric alloys will tend to be less brittle in their mechanical properties, but the colour 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 jewellery, as seen in the figure (right).

An alternative approach to making jewellery with purple gold decoration is to physically vapour deposit (PVD) the two metals, gold and aluminium, in the correct ratio onto a carat gold substrate. Such processing can be done by a number of PVD techniques such as sputtering. Jewellery 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-aluminium powder, which is pressed and sintered (Japanese patent JP62240729, 1987). It is claimed that such alloys are of good purple colour and have satisfactory workability. Similar alloys are also claimed in a patent, WO 00/46413, granted in 2000 to Singapore Polytechnic.

In a new patent (Japanese patent JP 2003183710), ornamental purple gold alloys containing 70-85% gold, rest aluminium, are claimed which are made by vacuum melting an ingot, atomising it centrifugally and the powders packed in a mould 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 (Japanese patent JP4176829, 1992) by bundling gold-plated aluminium and aluminium-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 jewellery by depositing a layer of aluminium onto the surface and doing a thermal diffusion treatment to form the purple compound.

Thermal spraying of gold and aluminium powders onto a substrate can also achieve a purple coating, according to W.S.Rapson (Gold Usage, publ. Academic press, 1978).

A more detailed explanation of purple gold can be found in Gold Technology No 30, 2000. [LINK to GT 30, article by Cretu & v.d.Lingen] The colour co-ordinates (CIELab) and reflectivity curves have been published by Agarwal & Raykhtsaum in Proceedings of the Santa Fe Symposium, 1988, p229, publ. by Met-Chem Research Inc and by Saeger & Rodies in Gold Bulletin, vol 77 (1), 1977, p10 respectively.

b] Blue gold

The intermetallic compound formed between gold and indium, AuIn2, gives rise to a clear blue colour and that between gold and gallium, AuGa2, to a bluish hue. The reflectivity curves for these 2 intermetallic compounds are also published in the paper by Saeger & Rodies in Gold Bulletin (see above for full reference) and the Cielab colour co-ordinates for AuIn2 by Agarwal & Raykhtsaum in Proc. Santa Fe Symposium, 1988.

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 Colours by surface coatings and patinas

c] Black gold (grey - black & brown)

There are several ways of obtaining a black colouration on carat golds. Faccenda has described some of these recently in Proceedings of the Santa Fe Symposium 2002, p 227 , publ. by Met-Chem Research Inc.

He lists 3 techniques:
- Electrodeposition of, for example, 'black' rhodium or ruthenium
- Plasma Assisted Chemical Vapour Deposition (PA-CVD) of amorphous carbon
- Controlled oxidation of carat golds containing cobalt or chromium.

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. Faccenda reports typical plating conditions. Coatings range in colour from grey to 'anthracite' black. The blacker the colour, 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.

Figure: Black gold coatings: left - hydrogenated amorphous carbon (Blacktop®), right - black ruthenium electroplate

A black oxide coating or patina can be produced by controlled oxidation of carat golds containing cobalt, iron or chromium additions. For example, a gold 75% - cobalt 15% - chromium 10% alloy is oxidised 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 colour 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 potasium 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. For details, see, for example, the book by Oppi Untracht, 'Metal Techniques for Craftsmen'.

d] Blue gold

A blue patina can be produced on gold alloys by oxidation treatments. In one case (see Cretu & van der Lingen in Gold Technology no 30, p38), 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 (US patent 5,059,2055, 1991), an 18 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 colour is produced.

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

e] General comment

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

Read a conference paper on this topic ( 461kb)

CWC June03