Touchstones were stones used to test the purity of precious
metals such as gold and silver. Touchstones were also called Lydian stones,
after the country of Lydia, the birthplace of precious metal coinage and the
first country credited with the use of touchstones. The spread of gold coinage
particularly increased the profits that could be earned from adulterating and
alloying gold coinage, and touchstones offered an inexpensive and useful test
for purity of gold coinage. Both individuals and governments were known to
reduce the purity of precious metals by alloying them with cheaper metals.
Touchstones were cut from black siliceous stone or opaque quartz, brown, red,
or yellow in color, with a smooth surface, and convenient for holding in one
hand. Ancient and medieval assayers tested the purity of gold or silver by
rubbing the metal across a touchstone with sufficient pressure to leave a
streak. Different metals left streaks of different colors. The color of the
streak left on the touchstone by a metal of unknown purity could be compared
with the color of a streak left by a piece of metal of known purity. Nitric acid
was put on the streaks to dissolve impurities, and sharpen the contrast between
the streaks of pure and impure metal. From this comparison an assayer rendered a
judgment about the purity of a metal. Because differences in shades of color can
be slight, the test involved a significant subjective component. Nevertheless,
the test brought to light the more outrageous debasements, and was sufficiently
accurate for most purposes.
Before the development of more advanced techniques the Goldsmiths’ Company of
the City of London kept test metals of known purity, called touch needles, for
use in making touchstone tests. The company made available 24 gold needles for
each of the traditional 24 gold carats. They kept similar pieces for silver.
Touchstone tests are not decisive in detecting silver alloyed with copper,
but can be used to assay gold with some accuracy. By the fifteenth century the
Tower mint in London was using a new method, cupellation, which makes use of the
tendency of various metals to fuse at high temperatures. The new method using
fire grew out of the experiments of the alchemists during the medieval era.
See also:
Trial of the Pyx (England)
References:
Davies, Glyn. 1994. A History of Money.
Marx, Jennifer. 1978. The Magic of Gold.
