Gold and gold alloys
are polycrystalline in nature; the crystals are also known as grains.
The size of the grains affects the properties such as strength, ductility and
deformability. Generally a fine (small) grain size is preferred, a large, coarse
grain giving rise to the well-known ‘orange peel’ surface. Grain refiners may
add other metals such as cobalt
or iridium, as master alloys, in
very small, controlled amounts to gold alloys to promote a fine grain size in
casting and/or
annealing.