Touchstone (assaying tool)

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Touchstone set

A touchstone is a small tablet of dark stone such as

lydite, used for assaying precious metal alloys. It has a finely grained surface on which soft metals leave a visible trace.[1]

History

The touchstone was used during the

Harappa period of the Indus Valley civilization ca. 2600–1900 BC for testing the purity of soft metals.[2] It was also used in Ancient Greece.[3]

The touchstone allowed anyone to easily and quickly determine the purity of a metal sample. This, in turn, led to the widespread adoption of gold as a standard of exchange. Although mixing gold with less expensive materials was common in coinage, using a touchstone one could easily determine the quantity of gold in the coin, and thereby calculate its intrinsic worth.

Operation

Drawing a line with gold on a touchstone will leave a visible trace. Because different alloys of gold have different colors (see

karat
gold is not affected but 14 karat gold will show chemical activity.

See also

References

  1. ^ "Touchstone". Oxford English Dictionary. Retrieved 10 March 2013.
  2. .
  3. ^ Bisht, R. S. (1982). "Excavations at Banawali: 1974–77". In Possehl, Gregory L. (ed.). Harappan Civilization: A Contemporary Perspective. New Delhi: Oxford and IBH Publishing Co. pp. 113–124.