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For most uses of gold the pure metal is too soft on its own and is therefore hardened by the addition of alloying elements, copper, silver, nickel, palladium and zinc. Gold is of course yellow and the various colours seen such as red, white and green are simply alloys of gold. The final colour is dependent on the ratios and type of alloys added.
Carat and Colour Pure 24ct Silver Copper Zinc Nickel 9ct Yellow gold 37.5% 12.1% 44.4% 6% NIL 9ct white gold 37.5% NIL 34.0% 10.8% 17.7% 14 ct Green gold 58.33% 32.5% 8.97% 0.2% NIL
Carat Silver Copper Colour Produced 9 ct 62% NIL White 9 ct 55.0 7.5 Pale yellow 9 ct 42.5 20.0 Yellow 9 ct 31.25 31.25 Rich yellow 9 ct 20.0 42.5 Pink 9 ct 7.5 55.0 Red
Gold is yellow, copper is reddish, and all other metals are a silvery grey in colour. There are however gold alloys, that is mixes of differing metals, that appear grey/white.
White gold (alloy) became popular around 1920 as a substitute for platinum, as platinum was quite expensive. Simply mixing a white and yellow metal together will not produce a colour in the same way that mixing paint does. The most common metal used to 'bleach' gold is nickel which is both inexpensive and provides a good platinum look-alike in 18ct alloys.
It has however been linked to dermatitis through allergic reactions with the skin. It has also been suggested that it may be slightly carcinogenic. A recent E.U directive has indicated that soon all jewellry will have to be nickel safe with only a few ppm being allowable.
A typical white gold alloy might be
Carat and Colour Pure 24ct Silver Copper Zinc Nickel Palladium 18ct White gold 75% NIL 5.5% 5% 14.5% NIL Nickel Free 18ct White gold 75% 4% NIL NIL NIL 17%
Palladium white golds
Palladium is the next choice for use in white gold alloys and, apart from its high price and high melting point (1552 degrees celsius), is an ideal alloy.
Additions of about 10 -12% palladium to gold impart a good white colour. But palladium is an expensive metal and it is also a heavy metal. Thus jewellery in such palladium white golds will be more expensive than identical pieces in nickel whites for 2 reasons: firstly, the cost of the palladium and secondly, the impact of density - palladium white golds are denser and so such jewellery will be heavier. It is also more difficult to process as the melting temperatures are substantially higher.
Many commercial palladium white golds only contain about 6-8% palladium plus silver, zinc and copper. Some may even contain some nickel [so a palladium white gold is not necessarily nickel-free]. These may also have less than a good white colour and so may also be rhodium plated.
Palladium white golds tend to be softer and more ductile compared to nickel whites and so generally will not wear as well. However, despite its higher base cost and workshop recovery costs, making a more expensive product, the advantages of a non nickel alloy now out weigh these factors.
Alternative white gold alloys
In the European Union especially, there is a demand for cheaper alternatives to white golds than the palladium whites which are 'nickel-free'. Many new alloys are coming to market, most of which rely on manganese additions as the main whitener. Some are palladium-free and others are low palladium alloys. Chromium and iron are also be used as whiteners. They tend to be hard and more difficult to process. Many of these alloys are not a good white colour, requiring rhodium plating, and many suffer cracking problems and tarnishing.
We recently tried to anneal a batch of white gold from Italy. It was so unmalliable we had to contact the manufacturer, and whilst the solution was simple, a high street working jeweller would find the new metals difficult, if not impossible to process.
Platinum, chromium, cobalt, zinc, tin and silver are all used as whiteners. Silver has excellent working properties but is poor as a whitener. Copper is also added to improve the ductility of most white alloys.
It is important to remember that even the best white gold alloy is actually grey, as is platinum. The white finish you are accustomed to is a rhodium plating, and whatever you are told, white golds are only available in caratages up to 21 carat. It is not possible to have a 22 ct white gold!
Resistance to Tarnish
18 carat and 22 carat gold alloys are almost completely resistant to chemical attack and 9ct alloys are much less resistant. Nine carat alloys can dull or even blacken from exposure to chemicals in the atmosphere, and might discolour in contact with household chemicals. 9ct chain is usually finished with a deeper yellow 18ct or 22ct coating by manufacturers, which is why the bright finish on a new 9ct chain gradually tones down to match other jewellery that you wear on a daily basis.
The original bright finish can usually be restored quite inexpensively by a working jeweller for little cost should your chain become chemically or mechanically damaged over the years.
As well as affecting physical properties, alloying gold generally increases the strength and hardness, with some reduction in malleability / ductility. The silver atom is slightly larger than that of gold, so alloying gold with silver gives a moderate improvement in strength and hardness. The copper atom is significantly smaller than that of gold and so it has a greater effect on strengthening gold than silver, as it distorts the gold crystal lattice more. Thus reducing caratage from 24 carats through 22 ct down to 18 carat gold results in stronger and harder alloys, but alloying beyond 18 ct down to 9 carat does not have much further effect.
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