Nickel Gold : Au Cu Ni : Green

Nickel-gold based white gold has long been the most commonly used white gold alloy for white gold jewellery production.

Rhodium Plating of Nickel-Gold White Gold Alloy

White gold nickel-gold alloys containing between 3.5wt% and 5wt% of nickel metal have excellent mechanical properties. Due to the off-white nature of these lower nickel concentration gold alloy there is a requirement to whiten the resultant white gold alloy with a final rhodium plating.

See Rhodium Plating :: Rhodium Plate by Humagaia.

One drawback is that these lower concentration white nickel-gold alloy show only low resistance to corrosion. This is the main reason for the rhodium plating process being employed.

There are many reasons why nickel metal has been the preferred metal in the production of white gold alloys. The main reasons for using such nickel-gold alloys are:

• the relatively low cost of nickel metal as the major constituent other than gold of the alloying elements required to make white gold alloy,
• the temperature at which nickel is completely liquid (liquidus) is lower than 960°C, The lower the liquidus is the less costly the manufacturing costs and the easier the alloying process becomes.
• nickel metal has good mechanical and casting qualities which lend themselves well to the jewellery manufacturing process required for white gold alloy.
• nickel has a strong bleaching effect on yellow gold, thus causing white gold alloy with a nickel-gold base to be advantageous in the creation of as close to true white gold alloy as relatively inexpensively as available.
• the white colour created by a nickel metal admix creating white gold is satisfactory, although somewhat like steel with a 'cool' overtone.

For any white gold alloy it is difficult to offset a good colour against good workability. To achieve both at the same time is the holy grail. White gold alloys containing between 9wt% and 12wt% nickel metal tend towards an almost white colour, whilst at the same time can be used mainly in lost-wax casting. Nickel-gold white gold alloy is viable for mechanical working where limited amount of deformation is acceptable.

The alloys of the gold-nickel group are based on the gold–copper-nickel triumvirate or ternary system with the addition of zinc metals.

Copper when added to white gold improves workability.The creation of the intermetallic phases AuCu and AuCu_3, which form at approx.400°C, cause a significant increase in hardness. Adding copper to a white gold-nickel alloy mix is a compromise of ductility over colour.

Zinc is used in gold-nickel white gold alloys as a secondary bleaching element. The zinc metal is added in order to compensate for the darkening colour effect of copper. An additional benefit of adding zinc to the gold-nickel-copper alloy is the increased bleaching effect it has with nickel. Zinc is a good deoxidiser in lost-wax casting and also increases the fluidity of the resultant alloy.

However, all is not roses. Zinc additions to the white gold nickel-gold alloy should be limited because zinc:

• reduces the strength of the nickel-gold alloy
• causes problems when white gold alloy is presented for refining using scrap white gold
• causes problems when the resultant alloy is vacuum melted and cast.

Nickel white gold alloys are utilised for all standard karatages of white gold from 8k to 22k. For the lower karatages there is a substantial requirement for copper and zinc in order to maintain good mechanical properties.

In 18k white gold alloys, hardness increases with increases in the percentage of nickel in the gold alloy mixture.

For 18k nickel-gold white gold alloys:

• Hardness is maintained at a high level even where the nickel content is lowered.
• Workability is compromised if the nickel concentration in the white gold alloy rises above 9wt%.
• Ductility is lessened when the zinc concentration rises above 4wt%, unless the nickel concentration in the white gold alloy mixture is less than 4wt%.
• Hardness is close to that of yellow gold alloys and is therefore not too high.
• Workability diminishes significantly when the nickel concentration rises above 13wt%.

The borders between white gold, yellow gold, red gold or yellow-green gold is blurred. This is a typical problem in the classification of gold alloys. There is a great reliance on the subjective perception by the observer. Not only this but there are also significant cultural influences on the perception of colour and how acceptable it is.

White gold for one may be off-white gold for another.

See Gold :: White Gold by Humagaia.

_______These pencils are for your personal use for comments_____.

__________________PLEASE DO NOT REMOVE_________________