Black, silica-containing stone used in assaying to determine the purity of gold and silver. The metal to be assayed is rubbed on the touchstone, and then a sample of metal of known purity is rubbed on the stone right next to it. The streaks of metal on the stone are treated with nitric acid, which dissolves impurities, thus increasing the contrast between the two samples when compared. Because other metals, such as copper, can be alloyed to silver without changing its colour significantly, the touchstone method is not usually used now to assay silver, though it is still used to assay gold and provides a reasonably accurate guide to quality.
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By rubbing a metal on a touchstone a small amount of metal is ground off onto the stone, and forms 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 viscosity 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. Use of the touchstone in Ancient Greece and Anatolia dates to circa 500 BC. The fourth century philosopher Theophrastus in the tract de lapidibus (On Stones) described the testing of gold by fire or by the touchstone.
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 they were 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.
That such a test was not always successful however, is shown by the famous story about Archimedes and the golden crown, where the philosopher developed a much more accurate test involving the density of the suspect crown.
The character of Touchstone in Shakespeare's As You Like It is described as "a wise fool who acts as a kind of guide or point of reference throughout the play, putting everyone, including himself, to the comic test".
A touchstone can be a short passage from the great masters’ works of literature that is used in determining other poetry and artist's works of literature literary value or merit. This sense was first applied by Matthew Arnold, whose essay “The Study of Poetry” gives Hamlet’s dying words to Horatio as an example of a touchstone.