Touchstone (assaying tool)

A touchstone, also called touchstone is a small grinding stone, which is used to determine the composition and the degree of purity of precious metals.

Selection

A touchstone must be sufficiently hard to grind and take a little of the metal can be checked. For better visibility of the left by the abraded metal lines it should be black, without being dissolved by aqua fortis or aqua regia. These properties have black hard mudstones. A good touchstone is uniformly dyed jet black or dark red, also fine grained, has no stains or other inclusions like veins on. Since the metal lines do not adhere to overly smooth surfaces, the surface must not polished, but only satin finish.

Most are made from polished stones tasting black chert, called Lydit. But even colored stones of black agate are suitable for it. In contrast, poorly suited glass or ceramic plates. Quite unsuitable, however, is calcareous rocks such as marble, as this is not resistant to nitric acid, foams on contact and flares up. - The touchstone previously used for the elderly should have been white.

Treatment of the stone

The touchstone must be carefully kept clean. To clean moistened with water, the tasting bars are removed carefully without to press with a pumice stone or fine emery stone to not carve the touchstone Otherwise, its surface would be useless. Also suitable is cork with which it is cleaned using water and a non- ritz scouring agent. The resulting slurry is rinsed, then dried the stone with a soft cloth. Acid is not suitable for cleaning.

A non- greased touchstone would lose too much material, in addition, the metal lines would be only imperfect and usually assumed heavy. For this reason, the test surface should have a light coat of oil or grease film. Therefore, the stone is final, extremely thin rubbed with light oil, such as almond oil. (Even the human skin fat is suitable. ) Previously it was rubbed with a whole amygdala or sanded a soft coal. However, to replace excessive fat, the cleaned stone for several hours is placed in a dilute ammonia solution. Since a dusty no reliable test would permit, he should be kept free of dust.

Use

To determine the degree of purity, the bar sample is applied by means of touchstone. In this case, the test piece ( putative precious metal ) is rubbed on the touchstone so that a metallic bar is left to originate with him from tasting needles to compare. Touchstones are used mainly jewelers, goldsmiths and jewelery shops.

Others

In a figurative sense since the early modern period refers to the phrase " bring to the test " an empirical study according to which can be confirmed or rejected a (theoretical) assumption; " Danne hero we keep the world before a try stone god, on which the Almighty the people, even as otherwise a rich man, the gold or silver tried " ( Grimmelshausenmuseum: Simplicissimus ).

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