Touchstone (assaying tool)

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Touchstone set

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. [1]

Contents

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. [2] It was also used in Ancient Greece. [3]

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 colours (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.

See also

Related Research Articles

Alloy Mixture or metallic solid solution composed of two or more elements

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Gold Chemical element, symbol Au and atomic number 79

Gold is a chemical element with the symbol Au and atomic number 79, making it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile metal in a pure form. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements and is solid under standard conditions. Gold often occurs in free elemental (native) form, as nuggets or grains, in rocks, veins, and alluvial deposits. It occurs in a solid solution series with the native element silver, naturally alloyed with other metals like copper and palladium, and mineral inclusions such as within pyrite. Less commonly, it occurs in minerals as gold compounds, often with tellurium.

Metallurgy Domain of materials science that studies the physical and chemical behavior of metals

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Electrum Alloy of gold and silver

Electrum is a naturally occurring alloy of gold and silver, with trace amounts of copper and other metals. The ancient Greeks called it "gold" or "white gold", as opposed to "refined gold". Its color ranges from pale to bright yellow, depending on the proportions of gold and silver. It has been produced artificially, and is also known as "green gold".

Metalworking Process of making items from metal

Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale: from huge ships, buildings, and bridges down to precise engine parts and delicate jewelry.

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A goldsmith is a metalworker who specializes in working with gold and other precious metals. Nowadays they mainly specialize in jewelry-making but historically, goldsmiths have also made silverware, platters, goblets, decorative and serviceable utensils, and ceremonial or religious items.

A hallmark is an official mark or series of marks struck on items made of metal, mostly to certify the content of noble metals—such as platinum, gold, silver and in some nations, palladium. In a more general sense, the term hallmark can also be used to refer to any distinguishing mark.

The fineness of a precious metal object represents the weight of fine metal therein, in proportion to the total weight which includes alloying base metals and any impurities. Alloy metals are added to increase hardness and durability of coins and jewelry, alter colors, decrease the cost per weight, or avoid the cost of high-purity refinement. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry. Coin silver, which was used for making silver coins in the past, contains 90% silver and 10% copper, by mass. Sterling silver contains 92.5% silver and 7.5% of other metals, usually copper, by mass.

Gold plating

Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.

Glossary of numismatics

This glossary of numismatics is a list of definitions of terms and concepts relevant to numismatics and coin collecting, as well as sub-fields and related disciplines, with concise explanations for the beginner or professional.

Hardened steel Carbon steel quenched and tempered after a heat treatment

The term hardened steel is often used for a medium or high carbon steel that has been given heat treatment and then quenching followed by tempering. The quenching results in the formation of metastable martensite, the fraction of which is reduced to the desired amount during tempering. This is the most common state for finished articles such as tools and machine parts. In contrast, the same steel composition in annealed state is softer, as required for forming and machining.

Karatmeter

The Karatmeter is a scientific instrument which uses X-rays to give an exact reading of the purity of gold. Due to its very high precision and fast result, X-ray analysis has been adopted by international agencies in India as part of the certification process used to hallmark gold. It is an accurate, non-destructive means of testing the purity of gold and other related elements. X-Ray method of Gold analysis gives the purity of gold up to 10-12 microns and hence it gives the analysis of coating only.

Depletion gilding is a method for producing a layer of nearly pure gold on an object made of gold alloy by removing the other metals from its surface. It is sometimes referred to as a "surface enrichment" process.

Black Hills gold jewelry is a type of jewelry manufactured in the Black Hills of South Dakota. It was first created in the 1870s during the Black Hills Gold Rush by a French goldsmith named Henri LeBeau, who is said to have dreamed about the design after passing out from thirst and starvation. Black Hills gold jewelry depicts leaves, grape clusters and vines, and is made with alloys of gold with standard yellow gold as well as green and pink gold. In 1980, the 8th Circuit affirmed an injunction ruling that if a manufacturer was to call its jewelry Black Hills Gold, then it must be made in the Black Hills. The state of South Dakota designated Black Hills gold as the official state jewelry in 1988.

Colored gold Various colors of gold obtained by alloying gold with other elements

Pure gold is slightly reddish yellow in color, but colored gold in various other colors can be produced by alloying gold with other elements.

As a metaphor, a touchstone refers to any physical or intellectual measure by which the validity or merit of a concept can be tested. It is similar in use to an acid test, litmus test in politics, or, from a negative perspective, a shibboleth where the criterion is considered by some to be out-of-date. The word was introduced into literary criticism by Matthew Arnold in "Preface to the volume of 1853 poems" (1853) to denote short but distinctive passages, selected from the writings of the greatest poets, which he used to determine the relative value of passages or poems which are compared to them. Arnold proposed this method of evaluation as a corrective for what he called the "fallacious" estimates of poems according to their "historic" importance in the development of literature, or else according to their "personal" appeal to an individual critic.

Metallurgical assay Compositional analysis of an ore, metal, or alloy

A metallurgical assay is a compositional analysis of an ore, metal, or alloy, usually performed in order to test for purity or quality.

Gold parting

Gold parting is the separating of gold from silver. Gold and silver are often extracted from the same ores and are chemically similar and therefore hard to separate. Over the centuries special means of separation have been invented.

Gold coin Coin made from gold

A gold coin is a coin that is made mostly or entirely of gold. Most gold coins minted since 1800 are 90–92% gold, while most of today's gold bullion coins are pure gold, such as the Britannia, Canadian Maple Leaf, and American Buffalo. Alloyed gold coins, like the American Gold Eagle and South African Krugerrand, are typically 91.7% gold by weight, with the remainder being silver and copper.

Mask of Tutankhamun Gold mask of Egyptian Pharaoh Tutankhamun

The mask of Tutankhamun is a gold mask of the 18th-dynasty ancient Egyptian Pharaoh Tutankhamun. It was discovered by Howard Carter in 1925 in tomb KV62 in the Valley of the Kings, and is now housed in the Egyptian Museum in Cairo. The death mask is one of the best-known works of art in the world and a prominent symbol of ancient Egypt.

References

  1. "Touchstone". Oxford English Dictionary. Retrieved 10 March 2013.
  2. Venable, Shannon L. (2011). Gold: A Cultural Encyclopedia. Santa Barbara, CA: ABC-CLIO, LLC. p. 264. ISBN   978-0313-384318.
  3. Bisht, R. S. (1982). "Excavations at Banawali: 1974–77". In Possehl, Gregory L. (ed.). Harappan Civilization: A Contemporary Perspective. New Delhi: Oxford and IBH Publishing Co. pp. 113–124.