touchstone

touchstone
/tuch"stohn'/, n.
1. a test or criterion for the qualities of a thing.
2. a black siliceous stone formerly used to test the purity of gold and silver by the color of the streak produced on it by rubbing it with either metal.
[1475-85; TOUCH + STONE]
Syn. 1. standard, measure, model, pattern.

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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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      black siliceous stone used to ascertain the purity of gold and silver. assaying by “touch” was one of the earliest methods employed to assess the quality of precious metals. The metal to be assayed is rubbed on the touchstone, adjacent to the rubbing on the touchstone of a sample of a metal of known purity. The streaks of metal left behind on the touchstone are then treated with nitric acid, which dissolves impurities, and thus, when the streaks are compared, the contrast between pure and impure metal is heightened. Because other metals, such as copper, can be alloyed to silver without significantly changing its colour, the touchstone method is not usually employed now to assay silver. It is still used, however, to assay gold and provides a reasonably accurate guide to quality.

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Universalium. 2010.

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