Touchstone
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- A separate article is about Touchstone Pictures, a film production division of The Walt Disney Company.
For the Shakespearean role, see As You Like It.
A touchstone is a small tablet of dark stone such as fieldstone or slate, used for probing of precious metal alloys. It has a finely grained surface on which soft metals leave a visible trace.
By rubbing the metal on a touchstone a small amount of metal would be ground off onto the stone, and form a colored stripe. This is typically compared to a stripe ground from an alloy of known high-quality composition next to the sample (this is called "priming" the stone).
The simplest, but crude, method of probing is comparing the color of rubbed stripes, which varies depending on the amount of the base metal in the alloy. However, the standard probing method involves subjecting the stripes to graded solutions of acid which dissolve the alloys with lower content of precious metal but don't affect the alloys with higher content of it. Modern touchstone kits include the touchstone tablet proper, flasks of acids graded for standard alloys — e.g. 18K (karat) (75%) gold, 14K (58%), etc. — and priming pencils made from the standard alloys.
Use of the touchstone revolutionized the concept of money. Prior to its introduction gold and silver were common currencies, but these could easily be alloyed with a less expensive metal (tin and lead were common). These were less valuable, but it was difficult to test for. The invention of touchstone made it possible to test for such forgeries quickly and efficiently, and also to determine the relative value of different alloys. That paved the road for gold and silver to become standard equivalents of value, and eventually to government-issued currency which began as coins of pre-probed alloys and weights guaranteed by the mint.