Touch Needle

a small bar made of specific precious metal alloys that is used for determining the fineness of an article on a touchstone. Touch needles are prepared from the same precious metal as the article that they are designed to assay for fineness.

The touch needles manufactured in the USSR have the following precious metal content (in percent): gold, 37.5, 50.0, 58.3, 75.0, and 95.8; silver, 75.0, 80.0, 87.5, 91.6, 92.5, and 96.0; platinum, 95.0; and palladium, 50.0 and 85.0. Usually Ag and Cu (more rarely Ni, Zn, Pd) are used as hardeners for gold needles, Cu for silver and platinum needles, and Ag and Ni for palladium needles. Fifteen types of 14-carat gold needles are manufactured, varying in hardener content and in color. Type no. 1 contains 41.7 percent Ag and no Cu, and type no. 15 contains 41.7 percent Cu and no Ag. Needles no. 2 through 14 contain decreasing amounts of Ag and increasing amounts of Cu.

Touch needles for determining the fineness of dental plates contain 90.0 percent Au, 4.0 percent Ag, and 6.0 percent Cu.