For these purposes it is alloyed with other metals to give it the necessary hardness. The gold content in alloys is expressed in carats. Coinage gold is composed of 90 parts gold to 10 parts silver. Gold is also used in the form of gold leaf in the arts of gilding and lettering. Purple of Cassius, a precipitate of finely divided gold and stannic hydroxide formed by the interaction of auric chloride and stannous chloride, is used in colouring ruby glass.
Chlorauric acid is used in photography for toning silver images.
Potassium gold cyanide is used in electro gilding. Gold is also used in dentistry. Radioisotopes of gold are used in biological research and in the treatment of cancer. The simplest process used for mining gold is panning, involving a circular dish often with a small pocket at the bottom. The prospector fills the dish with gold-bearing sand or gravel, holds it under a gentle stream of water, and rotates it.