Editing 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)}}
[[file:Pierre de touche.jpg|thumb|Touchstone set]]

A '''touchstone''' is a small tablet of dark stone such as [[slate]] or [[lydite]], used for [[Metallurgical assay|assay]]ing [[precious metal]] [[alloy]]s. It has a finely grained surface on which soft [[metal]]s leave a visible trace.<ref>{{cite dictionary |url=http://www.oed.com/view/Entry/203901 |title=Touchstone |dictionary=Oxford English Dictionary |accessdate=10 March 2013}}</ref>

==History==
The touchstone was used during the [[Indus Valley civilization#Chronology|Harappa period of the Indus Valley civilization]] ca. 2600–1900 BC for testing the purity of soft metals.<ref>{{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}}</ref> It was also used in [[Ancient Greece]].<ref>{{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}}</ref>

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 [[gold#Color|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 (purity)|karat]] gold is not affected but 14 karat gold will show chemical activity.

== See also ==
* [[Litmus#Uses|Litmus test]]
* [[Spot analysis]]
* [[streak (mineralogy)|Streak test]]

== References ==
{{reflist}}

* {{cite journal |last1=Moore |first1=D.T. |last2=Oddy |first2=W.A. |title=Touchstones: some aspects of their nomenclature, petrography and provenance |journal=Journal of Archaeological Science |date=January 1985 |volume=12 |issue=1 |pages=59–80 |doi=10.1016/0305-4403(85)90015-9|bibcode=1985JArSc..12...59M }}

[[Category:Materials science]]
[[Category:Jewellery]]
[[Category:Gold]]
[[Category:Lithics]]
[[Category:Inventions of the Indus Valley Civilisation]]
[[Category:Indian inventions]]

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