Gold Alloys

The most known gold alloys are made with silver. The proportion between gold and silver changes the alloy color. In this way, there are yellow and white alloys and the last ones present different metallurgical properties. Alloys are influenced by the presence of other minerals or metals. The next table shows the melting point of many elements and minerals that usually are in gold concentrates.

Melting Points

Metal/Mineral	Au	Ag	Cu	Hg	Zn	Pb	SiO2	FeS2	Al2O3
Melting point, oC	1063	960	1081	-39	419	327	1725	1170	2075

Basically, gold can form alloys with the metals mentioned in the table. There are other metals, but the alloys are no stable. For example, arsenic and antimony are decomposed by mercury. Only some metals such as zinc, silicon, iridium and cobalt can be used to prepared good alloys without modifying the gold properties too much.

Alloys based on the gold-silver-copper system can be employed for many applications. They can be rolled, flattened and stamped easily. It has been investigated the effect of different elements and compounds on gold bullion properties. For example, deoxidizers are added in order to minimize the formation of oxides (e.g. copper can be oxidized easily) when is necessary. The presence of zinc or silicon reduces the formation of oxidized copper compounds, but other oxides more stable are produced. Especially zinc oxides that are difficult to eliminate. Phosphorous and boron are good deoxidizers, but an excessive addition or presence is a problem because the metallic product can be brittle. So, we have to analyze the gold concentrate and the additives to be added in order to have a good alloy.

Elements have influence on the grain size. For example, large grains tend to create a peeling effect, this can produce losses when the bar is transported and handled. The presence of barium salts is good due to acts as a grain refiner. In general, some elements and compounds form fine disperse centers where the crystallization starts.

Silicon is not very soluble in gold and silver. In this way, a low melting eutectic point is produced which modify the melting point of the alloy. Normally the compound is formed on grain boundaries making the alloy a little brittle. If the gold concentrate has copper, silicon can form alloys with copper easily avoiding or minimizing the brittle effects. Other problem with silicon is the grain size, it tends to grow. When some corners are brittle, they were cause by silicon. Brittle points or located areas will produce gold losses. Then, it is important to know the material to be smelted and the fluxes or additives to be added.

Au-Ag-Cu Ternary diagram

Amalgams are other important group of alloys. Quicksilver (Mercury) reacts with gold at ordinary temperature, and forms alloys of variable composition. An amalgam containing 90% Hg is liquid, 87% Hg is pasty and 85% Hg is solid. Amalgams with smaller contents of mercury can be produced by heating the alloy at different temperatures. When amalgams are heated near of the boiling point of Mercury, the gold content is higher and reaches the maximum point when all the mercury was distillated.

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