Touch Needles and Touchstone: Graded Gold, Silver and Copper Alloy Sets for Assay by Streak (Agricola, 1556)

"Although the assay made by fire is more certain, still, since we often have no furnace, nor muffle, nor crucibles ... we can always rub gold or silver on the touchstone, which we can have in readiness."

Select a touchstone that is thoroughly black and free of sulphur; the blacker and the freer of sulphur, the better.

For each needle weigh the metals in the proportions of his tables (notebook), counting the bes or mark as 24 duellae: the gold-silver set runs from 1 gold + 23 silver to pure gold; the gold-copper set from 12 + 12 to pure gold; the silver-copper set from 1 silver + 23 copper to pure silver. Melting each charge in a small crucible and hammering it into a small marked bar is our method (he gives only the compositions). Number the needles and hang them on a ring (Illustration 255).

Rub the gold or silver (bar, mine gold or coin) on the touchstone. Rub beside it the needle you judge by colour to be similar; if its streak is too pale, try one of stronger colour; if too deep, a paler one. The matching needle gives the proportion of silver or copper in the gold, or of copper in the silver. The first eleven gold-silver needles test the gold in a silver bar; the other thirteen the silver in a gold bar.

From Georgius Agricola, **De Re Metallica** (1556), Book VII, translated by Herbert C. and Lou H. Hoover, 1912 (Gutenberg 38015; checked on the archive.org scan georgiusagricola00agri, pp. 252-259). His sets for gold with unequal silver and copper (13, 25, 37 and 28 needles) and his second silver-copper scale of 31 needles by half-semunciae are not reproduced here. How to melt and form the needles, and the carat and per-mille figures, are ours.