Before this, have you read: "What is alloying? Surely, pure gold is best?"
Hallmarks of Gold
It is wrong to think of Hallmarks as a measure of purity. They are a measure of fineness rather than the weight fraction that is a particular precious metal. Alloys of different Karat have different properties; other elements are deliberately added to improve consumer wear and manufacturing properties.
The main hallmarks of gold are 24 Karat, 22 Karat, 18 Karat, 14 Karat, and 10 Karat; their basic details are outlined below:
Gold fraction by weight / % (fineness)
Hardness / HV
The basic alloy system
Almost all karat gold alloys are based on the Gold-Copper-Silver ternary alloy system. While Gold and Silver form a single solid solution, Gold and copper form several ordered intermetallic phases at low temperatures. The binary phase diagrams are shown below:
The ordered Gold-Copper intermetallics are one of the reasons why we water quench 18K and lower karat golds that contain high copper contents after annealing. This prevents these less ductile phases from forming and keeps the whole system in the single-phase ductile condition, which we wanted to form by annealing.
Type I, II, and III Classification
Alongside deciding the karat and color (see below), the composition determines the proportion of the second phase. This determines how hard and difficult to work an alloy is. Alloys are sometimes classified into Type I, II, and III. Type I are single-phase and easy to work with, while type III alloys have second phases, making them difficult to work with.
Beyond silver and copper
Typical further gold alloying additions include Zinc, Silicon, and Iridium.
A deoxidizer to clean melt (<1%), improve melt fluidity and gold filling (2%), and improve workability (10%)
Zinc is volatile and so can evaporate from the melt. The effect varies with the amount. Zinc also whitens gold.
Used as a deoxidizer to give a cleaner, brighter casting
Only small quantities can be added to avoid embrittlement and cracking.
Iridium, Ruthenium & Cobalt
Refines grain size by promoting nucleation of crystals
Too much can lead to hard particles at the surface and "comet tailing" in polishing.
Colour of Gold Alloys
Pure gold has a characteristic "golden" yellow appearance. But by alloying gold, even with just copper and silver, a wide range of colors can be achieved across the different karats of gold. This can be illustrated on a color triangle or ternary diagram:
Pure gold is at the top corner of the triangle, copper on the bottom right, and silver on the bottom left. Horizontal lines illustrate the gold content for 18, 14, and 10 karats on this triangle. At each karatage, all possible alloy compositions can be plotted. On the left side, we can see that if we come down to 9K, a gold-silver alloy is actually white. On the right side, a 10K gold-copper alloy is very red, but we cannot get a good red at high karats, such as 22K.
Alloys of gold containing both copper and silver lie between the two extremes on the appropriate lines at any karatage. The amount of each depends on the position of the line and is in proportion to the relative lengths of the lines on either side of the point.
White gold alloys are typically based on Gold-Nickel and Gold-Palladium compositions. A detailed discussion of white golds can be found here.
The key points include the following:
Gold is primarily alloyed with copper and silver
The color of gold can be varied from white to yellow and red by tuning the copper and silver content.
Copper forms ordered intermetallic phases with gold that must be avoided on cooling to maintain ductility for working. 18K and lower karat gold alloys are then age hardened.
Various alloying other additions are made to clean melts and ensure bright castings as well as to finely tune the microstructure (grain refinement).