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White gold alloys have an extremely wide composition range. They are essentially 3 types:

the nickel whites, the palladium whites and the mixed (nickel and palladium containing) white golds. More recently, another class of nickel-free ‘alternative’ white golds have been developed, based on use of metals such as manganese and chromium as the primary whitener. In each class, a large variety of alloys are possible. In general, high concentrations of nickel or palladium (circa 12%+) are required for a good white colour. Many commercial alloys are thrifted in nickel (less hard) or palladium (less expensive), often with copper additions, and are not a good colour, so requiring rhodium plating.

In Table 16 and Table 17, some examples of 18 and 14 carat ‘classical’ alloys are given. Many other combinations are possible.

Nickel white gold

Nickel-containing white gold alloys tend to be hard and less ductile. Table 18 illustrates how the hardness of 18 ct gold increases with increasing content of non-precious metals. These values are for the soft annealed state.

The mechanical properties of investment cast nickel white gold are not predictable. At a low cooling rate, a nickel-rich phase segregates, causing embrittlement. Fast quenching can cause cracking. As it is almost impossible to cool the tree in a flask under defined conditions, the properties are not fully predictable.

This disadvantage is not only true for mechanical properties, but also for corrosion resistance and, consequently, for nickel release. The corrosion resistance decreases if segregation of nickel-rich phases occurs. Alloys with such segregation will release more nickel than homogeneous alloys.

A relatively high zinc content is necessary to avoid undue brittleness. However, zinc-containing alloys should not be melted in a vacuum due to excessive evaporation of zinc. Because of this, frequent re-melting of scraps will cause an unwanted change in composition, again resulting in increased brittleness.

Concentration, ‰ Temperature, °C*

Au Ag Pd Cu Zn Ni Solidus Liquidus

Nickel white gold

750 0 0 55 50 145 895 945

750 0 0 10 75 165 888 902

Palladium white gold

750 100 150 1240 1300

750 150 100 1180 1225

751 118 130 1180 1235

751 80 170 1300 1315

750 40 170 40 1200 1290

750 60 130 58 2 1090 1185

Table 16 Typical 18 carat white gold alloys

Concentration, ‰ Temperature, °C*

Au Ag Pd Cu Zn Ni Solidus Liquidus

Nickel white gold

585 270 50,0 95 920 990

585 185 75,0 155 915 1020

Palladium white gold

585 215 150 50 1080 1165

Mixed white gold

585 180 140 65 10,0 20 1010 1080

585 180 140 45 50 995 1090

Au – gold, Ag – silver, Pd – palladium, Cu – copper, Zn – zinc, Ni – nickel

* approximate values

Table 17 Typical 14 carat white gold alloys

Au+Ag+Pd Cu+Ni+Zn HV

Table 18 – Influence of composition on hardness of 18 ct white gold

Whereas melting of clean alloys in graphite crucibles is possible, the use of gypsum-bonded investment is problematic. An increased reaction with the investment is likely. This has a detrimental effect on surface quality and can increase gas porosity. This effect depends strongly on the mass of the cast item and on the flask and melt temperatures.

A further disadvantage of nickel-containing alloys is the pronounced affinity of nickel for sulphur, present in the gypsum. Nickel sulphide can be formed which segregates on grain boundaries and causes embrittlement. For this reason, re-melting of scraps and pieces is especially critical, due to adhering old (gypsum-bonded) investment. Sulphate (gypsum) will be reduced to sulphide on melting in a graphite crucible. Nickel sulphide segregation results.

Additionally, silica (the main component of investment) can form silicide compounds, with a similar embrittling effect as sulphide, if melting unclean scrap under reducing conditions.

Nickel white gold can cause allergic skin reactions in nickel-sensitised people.

Therefore, the European Community has promulgated a Directive, EN 1811, to protect the consumer. This Directive forbids the use of nickel only in the jewellery used for piercing or in a healing wound. In all other cases where jewellery is in direct and prolonged contact with the skin, the use of nickel is not forbidden, but a maximum nickel release rate has been defined. This is determined with a specific test in an artificial sweat solution. Alloys releasing nickel exceeding the limit are not allowed.

Palladium white gold

The most important properties of palladium white gold of relevance to the caster are:

• High melting temperature, which requires suitable casting equipment.

• High casting temperature, which might exceed the thermal stability of the investment.

The wide use of induction melting techniques reduces the problem of high melting temperatures. However, measuring the temperature with nickel/nickel-chromium thermocouples is not possible; platinum/platinum-rhodium thermocouples are necessary. The question of the use of graphite crucibles is discussed frequently.

Palladium in pure or low-alloyed form reacts with carbon (solubility of carbon in palladium). However, the small concentration of palladium in gold alloys does not give problems if the alloy is melted in graphite crucibles.

The great amount of heat, which is introduced into the mould by the melt, can decompose gypsum-bonded investment, which leads to surface defects and gas porosity. The danger of decomposition depends strongly on the mass of items. Thin walled items may be cast using gypsum-bonded investment without too much problem. On the other hand, heavy items can produce problems. In this situation, the only solution is the use of phosphate-bonded investment.

Another disadvantage is the softness of the alloys for many applications. Some addition of nickel (i.e. ‘mixed alloys’) will improve hardness and strength.

Alternative alloys

As mentioned earlier, ‘alternative’ white gold alloys have been developed as a nickel-free substitute for the expensive palladium white alloys. Additions with a bleaching effect used in such alloys are manganese and chromium.

Alloys with small additions of manganese have been known for many years. Higher concentrations of manganese are necessary for a sufficient bleaching effect if nickel and palladium are to be substituted completely. Such alloys have proved brittle and susceptible to corrosion.

Chromium-containing alloys with a narrow specified composition show a good colour and a good workability. However, casting and annealing of such alloys is difficult due to the high reactivity of chromium. Chromium reacts not only with oxygen but also with nitrogen and carbon. Investment casting must be performed in a very clean argon atmosphere and in a special ceramic crucible. Phosphate-bonded investment should be used. At present, these requirements cannot be fulfilled in standard casting shops.