Touchstone
Encyclopedia
A touchstone is a small tablet of dark stone such as fieldstone
, slate
, or lydite, used for assay
ing precious metal
alloy
s. It has a finely grained surface on which soft metal
s leave a visible trace.
in the second episode of his 1978 BBC television series Connections
.
) 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 differently to specific concentrations of nitric acid
or aqua regia
, thereby identifying the quality of the gold. Thus 24 Carat (purity)
gold is not affected but 14 Carat gold will show chemical activity.
Fieldstone
Fieldstone is a building construction material. Strictly speaking, it is stone collected from the surface of fields where it occurs naturally...
, slate
Slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
, or lydite, used for assay
Metallurgical assay
A metallurgical assay is a compositional analysis of an ore, metal, or alloy.Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metals are assayed by an assay office...
ing precious metal
Precious metal
A precious metal is a rare, naturally occurring metallic chemical element of high economic value.Chemically, the precious metals are less reactive than most elements, have high lustre, are softer or more ductile, and have higher melting points than other metals...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
s. It has a finely grained surface on which soft metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s leave a visible trace.
History
The touchstone was used in ancient Greece. Its role in the introduction of monetary economy was explored by science historian James BurkeJames Burke (science historian)
James Burke is a British broadcaster, science historian, author and television producer known amongst other things for his documentary television series Connections and its more philosophical oriented companion production, The Day the Universe Changed , focusing on the history of science and...
in the second episode of his 1978 BBC television series Connections
Connections (TV series)
Connections is a ten-episode documentary television series created, written and presented by science historian James Burke. The series was produced and directed by Mick Jackson of the BBC Science & Features Department and first aired in 1978 and 1979...
.
Usage
Drawing a line with gold on a touchstone will leave a visible trace. Because different alloys of gold have different colors (see goldGold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
) 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 differently to specific concentrations of nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
or aqua regia
Aqua regia
Aqua regia or aqua regis is a highly corrosive mixture of acids, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and hydrochloric acid, usually in a volume ratio of 1:3, respectively...
, thereby identifying the quality of the gold. Thus 24 Carat (purity)
Carat (purity)
The karat or carat is a unit of purity for gold alloys.- Measure :Karat purity is measured as 24 times the purity by mass:where...
gold is not affected but 14 Carat gold will show chemical activity.