Working with White Gold

The Birmingham Assay Office was founded in 1773 to provide a hallmarking facility to the rapidly expanding local silver trade. Over 235 years it has become established as the largest UK Assay Office. During the past decade the Assay Office has expanded its services further, far beyond its statutory Assaying and Hallmarking duties and offers independent expert opinion on every aspect of the precious metal, jewellery and gemstone trade.

White gold is a phenomenon unique to the jewellery industry. Everyone else knows gold is yellow. Think Goldilocks. Think yellow! Dippal Manchanda, Technical Director of The Birmingham Assay Office 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 D: 1925, 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 have 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 categorized into 3 grades of whiteness as shown below. Premium White does not require plating, Standard White  may require plating and Off White definitely requires plating.

This approach relies on whiteness being measured using a spectrophotometer. Samples need to have  a flat, specially prepared minimum surface area of 12 sq. mm.

High palladium content “White Golds” are usually “Premium White”.   High nickel content alloys tend to range within the Standard Grade and many commercial white golds with low nickel or palladium content  are classified as “Off White”. 
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 table below.

All copy in blue to go into a table in a separate panel

BASIC GUIDELINES FOR ALLOY COMPOSITIONS TO ACHIEVE SPECIFIC COLOUR GRADING
NB The suitability of the resulting alloy for jewellery manufacture is also a crucial consideration

9ct Alloys
Premium White Category: 37.5 % Gold + Minimum 62% Silver
Standard White Category: 37.5 % Gold + Minimum 45% Silver + 17.5% other alloying elements.

If silver is to be replaced with other alloying elements then the following norms may be followed:

Approximate effect of whiteners: 1% Silver is equivalent to 1% Zinc, 0.6% Nickel or 0.6% Palladium.

14ct Alloys
Premium White Category:
58.5 % Gold + Minimum 26.5% whiteners i.e. white coloured elements (of which 16.5% are primary whiteners i.e. Ni or Pd or both) + 15% other alloying elements

Standard White Category
58.5 % Gold + Minimum 22.5% whiteners (of which 12% are primary whiteners i.e. Ni or Pd or both) + 19% other alloying elements

18ct Alloys
Premium
75.0 % Gold + Minimum 17.5% whiteners (of which 13.5% are primary whiteners i.e. Ni or Pd or both, combined with other alloying elements) + 7.5% other alloying elements
OR
75.0 % Gold + Minimum 24.5% whiteners (of which 17% are primary whiteners i.e. Ni or Pd or both) + 0.5% other alloying elements

Standard
75.0 % Gold + Minimum 19.5% whiteners (of which 7.4% are primary whiteners i.e. Ni or Pd or both, combined with other alloying elements) + 5.5% other alloying elements

22ct Alloys
There is less scope to create White 22ct gold due to the small amount of non gold content which is permissible. However 91.7% gold and 8.3% Palladium produce an ‘off-white’ (Grade III) gold alloy (Average YI:D1925 value:- 29.336).

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 maufacturers.

The Birmingham Assay Office is fully equipped to do the colour grading. A flat alloy sample of 12 mm diameter (or 12mm X 12mm 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. “

FOCUS ON THE EXPERT
Dippal Manchanda, MSc CSci CChem FRSC, Technical Director

Dippal Manchanda is the Chief Assayer & Technical Director at The Birmingham Assay Office, responsible for maintaining high analytical standards and providing scientific and technical expertise to all divisions of the business.
Dippal holds a Masters degree (MSc) in inorganic chemistry and over 20 years experience in assaying and the examination of precious metals and alloys. He is also a Fellow of the Royal Society of Chemistry and has attained the level of membership of "Chartered Chemist". The UK Science Council has awarded him the status of "Chartered Scientist", a recognition awarded to those scientists who demonstrate the application to stay up-to date in their field. During his varied career Dippal has been involved in several prestigious precious metal projects including setting up a state of the art Gold/Silver Medallion Manufacturing facility for a Public Sector Undertaking in India.

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