Reproduced with permission from Glass on Metal 1986, vol 5, no. 6.
Gold is an ideal metal for enameling. It shows no appreciable solubility for hydrogen, oxygen, or nitrogen. Because of its medium high melting point, medium coefficient of expansion, and low modulus of elasticity it is easy to formulate enamels which are compatible and still possess the excellent properties the artist desires. It does not form oxides which affect transparent enamels, allowing them to be more brilliant and clear than over any other metal. Naturally the yellow color of the metal will have some effect on the color of transparents. The final product does not tarnish and the intrinsic value is unequaled. Opaque enamels work well over gold, but normally are used only for accent. Otherwise it appears wasteful since opaques look equally well on cheaper metals. The only complaint about gold, other than cost, is its softness. Uncoated areas scratch easily, therefore articles should be designed so all areas are protected from impact and abrasion.
Enamels which fire between 1300 and 1600 °F and having an expansion between about 250-350 are suitable. The purity of gold is expressed in Karats. The term “Karat” means a 24th part and is used to express the weight proportion of pure gold in an alloy. Pure gold is 24 kt., 18 kt. alloy contains 18/24 or 75% gold, 12 kt. alloy contains 12/24 or 50% gold, etc.
Gold is frequently alloyed with silver, copper, zinc, platinum, palladium, cadmium, nickel, etc. to decrease its cost, increase its hardness, or to change its color. Normally only the alloys with silver, copper, and occasionally platinum or nickel are used for enameling.
Each alloy will have a different solidus point, coefficient of expansion, modulus of elasticity, oxidation characteristics, etc. The color of transparent enamels applied over the alloys will be affected by the change in oxidation characteristics and the amount of oxides dissolved.
Both transparent and opaques will be affected by the change in expansion and modulus of elasticity of the metal. Additions of copper increase both the modulus of elasticity and expansion. Additions of silver reduces the modulus of elasticity and increases the expansion far more than copper additions. Thus as copper additions increase enamels with expansions in the lower ranges should be eliminated and total thickness of coating should be reduced. As silver additions increase it is wise to use higher expansion enamels, however the lower modulus of elasticity resulting from the silver additions makes the system a little more forgiving as to thickness of application and range of permissible expansions. Even though an enamel color works well on gold there is no reason to assume it will work on a gold alloy. And one which fits a gold-silver alloy may chip off a gold-copper alloy. The reason is not an alloy which is no good, but an enameler who selects enamels by color with no attention to the primary properties, firing temperature and coefficient of expansion.
Fig. 1 shows a triaxial diagram of gold-silver-copper alloys. Each apex represents 100% of the metal indicated. As the percentages of the three metals change the resulting alloys vary in color. These variations in color will affect the color and brilliancy of transparent enamels, especially paler shades. The areas labeled “Red” and “Reddish” contain more copper and thus more oxidation takes place. The combination of enamel dissolving this oxide and the reflec~nce from the red metal results in dark muddy transparent colors. The “Green Yellow” area imparts a brownish tint to reds and mauves. The “Greenish Yellow” area produces good transparent colors, however the expansions of alloys in the area are high. Enamels with expansions lower than about 300 should be carefully observed. The areas producing the best transparent colors are “Red Yellow”, “Yellow”, and “Yellowish”. Obviously these colors are obtained by using approximately equal parts of silver and copper along with the appropriate amount of gold.
The solidus temperature of alloys must be considered in selecting a proper enamel. Considerable trouble may be encountered if the solidus temperature of an alloy is assumed to be proportional to the melting points of the individual metals used to make the alloy. The melting points of gold, silver, and copper are 1948 °F, 1761 °F and 1982 °F respectively, however the solidus temperature of most alloys containing these three metals is below the melting point of silver.
Fig. 2 gives an indication of the solidus temperatures of gold-silver-copper alloys. Although constructed using data from a number of sources its accuracy is sufficient to illustrate the problem. 18 kt. alloys containing equal parts of silver and copper should be enameled with enamels which fire no higher than about 1500 °F. Lower karat alloys containing equal parts of silver and copper must be fired at lower temperatures. Thompsons Low Temperature High Expansion enamels work well when fired at 1050 °F on the lower alloys.