Touchstone

A touchstone is a device for roughly ascertaining the purity of gold alloys. It consists of a smooth strip of hard black stone, on which a corner of the alloy is rubbed so as to leave a streak, which is then moistened with an acid composed of 78.4 per cent nitric acid, 1.6 per cent hydrochloric acid and 20 per cent water. By comparing the effect with that produced on streaks made with alloys of known compositions, an approximation to the gold content of the alloy is found.

See also: Gold.