The Probert Encyclopaedia of Science & Technology

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% nitric acid, 1.6% hydrochloric acid and 20% 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.
Research Touchstone