touchstone (assaying tool)
{{short description|Small tablet of dark stone used for assaying precious metal alloys}}
{{about|the assaying tool|the metaphor|Touchstone (metaphor)|other uses|Touchstone (disambiguation)}}
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.{{cite dictionary |title=Touchstone |dictionary=Oxford English Dictionary |url=http://www.oed.com/view/Entry/203901 |accessdate=2013-03-10}}{{Cite journal |last=Moore |first=D. T. |last2=Oddy |first2=W. A. |date=1985-01-01 |title=Touchstones: some aspects of their nomenclature, petrography and provenance |url=https://www.sciencedirect.com/science/article/pii/0305440385900159 |journal=Journal of Archaeological Science |volume=12 |issue=1 |pages=59–80 |doi=10.1016/0305-4403(85)90015-9 |issn=0305-4403}} Retrieved 2025-07-14
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.{{Cite book|title=Gold: A Cultural Encyclopedia|last=Venable|first=Shannon L.|publisher=ABC-CLIO, LLC|year=2011|isbn=978-0313-384318|location=Santa Barbara, CA|pages=264}} Retrieved 2025-07-14 It was also used in Ancient Greece.{{Cite book |last=Bisht |first=R. S. |year=1982 |chapter=Excavations at Banawali: 1974–77 |editor-last=Possehl |editor-first=Gregory L. |title=Harappan Civilization: A Contemporary Perspective |location=New Delhi |publisher=Oxford and IBH Publishing Co. |pages=113–124}} Retrieved 2025-07-14
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.{{Cite web |title=The History of Touchstone Testing For Gold {{!}} PDF {{!}} Metallurgical Assay {{!}} Gold |url=https://www.scribd.com/document/32031595/The-History-Of-Touchstone-Testing-For-Gold |access-date=2025-07-14 |website=Scribd |language=en}}
Operation
Drawing a line with gold on a touchstone will leave a visible trace. Because different alloys of gold have different colors (see gold), the unknown sample can be compared to samples of known purity. This method has been used since ancient times. In modern times, additional tests can be done. The trace will react in different ways to specific concentrations of nitric acid or aqua regia, thereby identifying the quality of the gold: 24 karat gold is not affected but 14 karat gold will show chemical activity.{{Cite web |title=Touchstone {{!}} Metallurgical Testing, Alloy Analysis & Quality Control {{!}} Britannica |url=https://www.britannica.com/technology/touchstone-metallurgy |access-date=2025-07-14 |website=www.britannica.com |language=en}}
The touchstone test is not suitable for gold-filled or gold-plated items because it does not work as well with alloys and white gold. Silver can be tested using a touchstone, but it requires some skill, as the metal used to alloy the silver can affect the results. There is no reliable touchstone for other platinum group metals.{{Cite web |title=Precious Metals Testing |url=https://www.thenaturalsapphirecompany.com/education/precious-metal-standards-regulations-marks/precious-metals-testing/ |access-date=2025-07-14 |website=The Natural Sapphire Company |language=en}}