"Touchstone" has many meanings and a long history ...
- A basis for comparison; a reference point against which other things can be evaluated (wordnet.princeton.edu/perl/webwn)
- By extension, the metaphorical use of touchstone means any physical or intellectual measure by which the validity of a concept can be tested (see also Acid Test, Litmus Test, Shibboleth).
- 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.
- A touchstone is a small tablet of dark stone such as fieldstone, slate or lydite, used for assaying precious metal alloys. It has a finely grained surface on which soft metals leave a visible trace. Because it aided in the identification of gold and silver, 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.
- Touchstone is the name of the court fool or jester character in Shakespeare's play As You Like It. The character of Touchstone 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.
- Touchstone (1831-1861) was a British Thoroughbred racehorse and champion sire. Owned and bred by Robert Grosvenor, 1st Marquess of Westminster, he was conditioned for racing by the preeminent trainer of the day, John Scott.
- Touchstone is a character from Garth Nix's Old Kingdom trilogy. He is a central character in the first book, Sabriel, and appears in all three parts of the trilogy.
- Touchstone is a 1980's rock band out of Vienna, Austria.
History of Probing with a Touchstone
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 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.