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What is Touchstone?

Mary McMahon
Updated Mar 05, 2024
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A touchstone is a piece of hard dark stone such as jasper, slate, or basalt which is used in the assaying process to test the purity of metals. The use of such stones was once quite widespread; although less common today, touchstones are still occasionally seen in assaying offices, and some people keep them around as interesting curios. It is also possible to see examples of historic touchstones, some of which are quite old, on display in museums.

To use a touchstone, someone would take a piece of the metal being tested and rub it against the touchstone. The metals most frequently tested with a touchstone are silver and gold, as these metals are classically soft and they have a high value which serves as an incentive to alloy them with cheaper metals. Once a streak to be tested has been laid down, a “testing pencil” of a known alloy is used to apply a second streak for comparison.

In addition to comparing the physical differences between the streaks, most assayers also use acidic solutions to dissolve impurities in the samples, which can reveal the trace of a cheap metal alloy. Touchstones are typically stored with an assortment of test pencils and acids for the purpose of testing, and several companies continue to produce supplies for using touchstones.

The practice of using a touchstone appears to have arisen in Greece around 500 BCE. The introduction of the touchstone to Greek society radically changed the Greek economic system, creating a real value for coined money by allowing people to actually test it for purity. As a result, the term “touchstone” began to acquire wider implications, which is why you may hear people referring to a means of judgment or measurement as a “touchstone.”

You don't have to be interested in assaying metals to be intrigued by a touchstone. Some scientific stores and mineralogy companies sell touchstone kits, which allow people to test their own samples. If you do acquire such a kit, make sure to supervise younger users, as the acids involved can be dangerous. It is also a good idea to store the acids in a cool dry place well out of the reach of curious children and pets.

Frequently Asked Questions

What exactly is a touchstone?

A touchstone is a fine-grained black or dark stone, such as slate or lydite, used historically to test the purity of gold and silver by observing the streak of metal left on its surface. The color and quality of the streak would help determine the precious metal's composition and purity.

How was a touchstone used in ancient times?

In ancient times, a touchstone was used by merchants and assayers to assess the quality of precious metals. By rubbing the metal against the touchstone, they could examine the streak's color and properties. Comparing these marks to known standards or reference streaks allowed them to estimate the metal's purity without damaging the original piece.

Can touchstones still be used today?

While modern technology has largely replaced touchstones for metal purity testing, they can still be used for educational purposes or by traditional goldsmiths. Touchstones provide a direct and historical method for metal testing, offering a tangible connection to ancient trade and metallurgy practices.

Are there any scientific principles behind how a touchstone works?

Yes, the scientific principle behind a touchstone's functionality is the varying resistance of different metals to abrasion. Softer metals will leave a more apparent streak on the touchstone, while harder metals will leave a fainter one. The color and quality of the streak can then be analyzed to determine the metal's purity.

Is a touchstone related to the metaphorical use of the term in language?

Indeed, the metaphorical use of "touchstone" in language derives from the literal object. It signifies a standard or criterion by which the qualities of a thing are tested. Just as the stone was used to test metal purity, metaphorically, a touchstone refers to anything that serves as a test or benchmark of authenticity or quality.

What are some modern alternatives to using a touchstone for testing metal purity?

Modern alternatives to touchstones include X-ray fluorescence (XRF) analyzers, which can non-destructively determine the composition of metals within seconds. Other methods like inductively coupled plasma (ICP) mass spectrometry and fire assay are also used for precise measurements of metal purity in professional settings.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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