touchstone
Sign in to savesmall tablet of dark stone used for assaying precious metal alloys
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Encyclopedic overview
Touchstone set
A touchstone is a small tablet of dark stone such as slate or lydite, used for assaying precious metal alloys. It has a finely grained surface on which soft metals leave a visible trace. Basic requirements for a touchstone: hardness on the mineralogical scale 4.6-6.5, the touchstone must be free of cracks and not react with inorganic acids and their mixtures.
Excerpted from Wikipedia’s “touchstone” article, available under the CC BY-SA 4.0 licence.