Dippal Manchanda, technical director of Assay Office Birmingham explains some of the technicalities of white gold alloys.
There is no such thing as white, red, pink or rose gold, but these colours can be created by using other elements alloyed with gold. Many white golds are produced by adding palladium or nickel as the primary whitener to lighten or ‘bleach’ gold’s natural colour, and then silver, zinc or sometimes even copper may be added too. The colour of the resulting white gold alloys varies from strong clean white to dull grey to brown yellow, according to the percentage of white metals used in the alloys. High nickel- and palladium-containing alloys tend to have a visibly strong white colour (nickel whites tend to be greyer than palladium whites), but for technical and economic reasons, many commercial alloys contain only small amounts of these metals, often with some copper added to enhance workability. The colour of such white gold alloys, therefore, becomes less white and more yellow-brownish; this can deteriorate further as some nickel whites can yellow over time due to metallurgical instabilities. In order to offer the consumer the uniformly bright white article they are looking for, it has been an accepted practice to plate white gold alloys with between 0.05 and 0.5 microns of rhodium. This makes the item very attractive but inevitably leads to problems. As the demand for the ‘white look’ has increased in recent years, so has the level of customer complaints as shiny white plating wears off white gold items to reveal dull grey/brown yellow alloys beneath. With the continuing popularity of platinum, and the advent of palladium as a fourth hallmarked metal, white alternatives are available, and it is increasingly important that white gold articles are crafted from an appropriate alloy and preferably one that is sufficiently white not to require plating. There is no legal definition for ‘white gold’ and so, in the last five years, the industry has established recognised standards to categorise white gold alloys into grades of whiteness. The classification system adopted, following work by a joint UK and American task force, uses the ASTM Yellowness Index D1925, originally created for paints and plastics. On this index the closer the value is to zero, the closer the colour is to white. The industry has accepted that only carat gold with a yellowness index (YI) of 32.0 or less can be called a ‘white gold’. White gold has been categorised into three grades of whiteness as shown below. Premium white does not require plating, standard white may require plating, and off-white definitely requires plating.
An analysis of the Yellowness Index ASTM D1925 value against the compositions of the alloy provides basic, non-definitive guidelines for alloy compositions to achieve specific colour grading as shown in the box below. Major suppliers will now provide sheet or grain for white gold, which is already categorised into one of the three grades, making life a lot simpler for manufacturers. Assay Office Birmingham is fully equipped to do the colour grading. A flat alloy sample of 12 millimetres diameter (or 12 millimetres by 12 millimetres square) is needed to measure the colour. As a precautionary note, when white gold is alloyed with nickel, nickel-sensitive consumers may react to the nickel in the alloy when the jewellery is worn. No limit has been set for the nickel content of items in direct and prolonged contact with the skin. The requirements in the regulations refer only to nickel release. There is no simple relationship between nickel content and nickel release – the amount of nickel released from a particular jewellery item may be influenced by a number of factors, so it is not possible to state which alloys will be compliant, based on their composition alone.
Note: The suitability of the resulting alloy for jewellery manufacture is also a crucial consideration.
Premium white category: 37.5 per cent gold + minimum 62 per cent silver. Standard white category: 37.5 per cent gold + minimum 45 per cent silver + 17.5 per cent other alloying elements.
If silver is to be replaced with other alloying elements then the following norms may be followed: Approximate effect of whiteners: one per cent silver is equivalent to one per cent zinc, 0.6 per cent nickel or 0.6 per cent palladium.
Premium white category: 58.5 per cent gold + minimum 26.5 per cent whiteners ie white coloured elements (of which 16.5 per cent are primary whiteners ie Ni or Pd or both) + 15 per cent other alloying elements. Standard white category: 58.5 per cent gold + minimum 22.5 per cent whiteners (of which 12 per cent are primary whiteners ie Ni or Pd or both) + 19 per cent other alloying elements.
Premium white category: 75.0 per cent gold + minimum 17.5 per cent whiteners (of which 13.5 per cent are primary whiteners ie Ni or Pd or both, combined with other alloying elements) + 7.5 per cent other alloying elements.
OR 75.0 per cent gold + minimum 24.5 per cent whiteners (of which 17 per cent are primary whiteners ie Ni or Pd or both) + 0.5 per cent other alloying elements. Standard white category: 75.0 per cent gold + minimum 19.5 per cent whiteners (of which 7.4 per cent are primary whiteners ie Ni or Pd or both, combined with other alloying elements) + 5.5 per cent other alloying elements.
There is less scope to create white 22ct gold due to the small amount of non-gold content which is permissible. However 91.7 per cent gold and 8.3 per cent palladium produce an ‘off-white’ (Grade III) gold alloy (Average YI:D1925 value: 29.336).
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