Notable for it’s exceptional DENSITY, resistance to TARNISH and ability to resist attack from strong acids and alkalies; gold has played a central role in art and visual culture from pre-Mesopotamian civilisations up to the present day.
Gold is alloyed with complimentary elements to produce material suitable for either casting or fabrication. The unique system used for identifying the FINENESS of a gold alloy is the KARAT (k). The karat value indicates the purity of gold as a portion of 24 parts, therefore 24/24 or 24k gold is of the highest (100%) purity. 18/24 or 18k gold on the other hand is 75% pure, with alloying elements comprising the remaining 25% of the material. Most cast and wrought gold alloys used to create design work range between 14k (58.3% gold) and 22k (91.7% gold), the optimum metal fineness for a particular work varies according to the mode of construction and intended use of the finished article.
Although the karat value indicates the quantity of pure gold within a each alloy, it does not identify which individual elements have been added to the PARENT METAL, or their relative proportions. Alloying elements for both cast and wrought alloys typically include silver, platinum, nickel, zinc and copper (fine gold/silver alloy is known as ELECTRUM). Contamination by LEAD or MERCURY seriously affects the metgallurgical quality of gold – although gold has historically been distributed in mercury to form an AMALGAM for FIRE GILDING. Gold and mercury may also have been minor constituent elements within the ancient Japanese SHAKUDO casting alloy, which was otherwise a basic copper/tin bronze [ref].
A typical 18k gold cast alloy contains 75% gold, and up to 16% added silver. Combined, these two metals form an alloy slightly harder than pure gold. Copper is also present in 18k gold (to about 9%), this alloy addition both significantly HARDENS the alloy and modifies it’s METALLURGICAL structure to permit post-cast HEAT TREATMENTS (if desired). Zinc is present in most casting alloys up to a maximum of 2%, functioning to some degree as a deoxidising agent during the melt. Zinc also significantly lowers the melting point of gold alloys (the melting point of pure gold is 1064.4°C, the melting ranges for 18k alloys described above are typically between 850-900°C [1560-1650°F]).
As well as affecting the mechanical characteristics of a gold product, alloying elements can also be added to vary the visual appearance of the metal. Silver added to gold can change the intensity and depth of the yellow lustre in the metal from a green-gold, through to a silver-gold appearance – variable according to the proportion of silver added. Gold alloys with a red lustre (referred to as RED GOLDS) are usually copper rich. The addition of platinum also improves the metal’s colour and palladium lightens the alloy. Nickel will whiten gold, though silver and palladium are primarily responsible for the group of alloys known as WHITE GOLDS.
Golds of various descriptions are also available in wrought sheets and other pre-formed cross sections. Most of this stock is specifically designed for use within the jewellery industry, and consists of wires, strips, sheet, bar, tube and so forth. These and other various shapes are especially well suited for ring, chain and other decorative constructions. To manipulate this wrought stock into specific items, small scale versions of standard fabrication workshop tools, including rollers, presses and lathes, are available from specialist machinery suppliers.
Gold alloys work harden and will require ANNEALING if extensively COLD WORKED, this heat induced treatment must be carried out at regular intervals to prevent the design from cracking. Joining gold sections is usually done with a hard SOLDERING technique, using a filler wire of some 16k fineness, with small amounts of tin added to improve flow. The use of an appropriate flux prevents oxide contamination of the joint. Gold work is almost always finished to a high state of polish and is not considered suitable for patination.
< BACK / NEXT >