Touch Needles
by Universal History Archive/uig
Title
Touch Needles
Artist
Universal History Archive/uig
Medium
Photograph - Photograph
Description
A set of 24 touch needles. These were made of the purest obtainable gold and each was alloyed with different known amounts of silver and copper, or of both. The sample of gold to be tested was rubbed on a hard dark stone, and the colour and thickness of the trace left was matched by one of the touch needs and the value of the sample obtained. From De re metallica, by Agricola, pseudonym of Georg Bauer (Basle, 1556). Woodcut.
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August 5th, 2016
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