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Lapis Lydius, Touchstone, Assaying

Lapis Lydius, Touchstone, Assaying Page of 453 Lapis Lydius, Touchstone, Assaying Text size:minus plus Restore normal size   Mail page  Print this page
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LAPIS LYDIUS.
Pliny (xxxiii. 43) refers to this description by Theophrastus, adding that the Touchstone, besides its Latin name " coticula," was called the Lydian stone, and the Herculean. He further states, what seems incredible, that experienced assayers, by rubbing off a piece of any ore on this stone, " cum e vena ut lima rapuerunt experimentum," were enabled to declare without mistake how much gold, silver, or copper it contained within one scruple, " scriptulari differentia."
The present Touchstone is a black Jasper of a somewhat coarse grain, and the best pieces come from India. The Italian gold­smiths employ it in the following ingenious manner. They have a set, strung on a ring, of 24 " needles," little bars of gold, each of a known and marked standard from one carat up to twenty-four (or fine). Taking the gold to be assayed, they rub it on the stone ; by the side of the streak it leaves, they rub the needle which seems to the eye the nearest in quality. Next they pour " aqua regia " on the two streaks, and if the solvent produces the same effect on each, it proves that the gold in the piece and in the needle is of the same standard. If there be a difference perceptible, they try another needle, until an exact coincidence is obtained. It is very singular that Tavernier should have known nothing of this method before he saw it generally employed by the Banyan goldsmiths : for as the Italian craftsmen are as immutable in their processes as the Hindoos themselves, they must have had this simple method of assaying in Tavernier's days. To exhibit the economy of the Banyans he adds that they carefully wipe the stone, after using it, with a lump of wax, which thus in course of time becomes charged with an appreciable quantity of fine gold.
The assay of gold by fire has been described under Obryza (Aurum). The Roman assay of silver was equally simple. Filings of the article to be tested were thrown upon an iron chafing-dish made white-hot : if the silver kept its colour it was proved fine ; if it turned red (containing copper), of the second quality; but if black it was pronounced base. Pliny exposes an ingenious fraud of the silversmiths intended to baffle this test. They kept the chafing-dish beforehand steeped in urine, the salts of which, when the iron was heated, fixed upon
Lapis Lydius, Touchstone, Assaying Page of 453 Lapis Lydius, Touchstone, Assaying
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