Testing Needles
Small strips of steel with gold points, usually running from 4k. to 20k. inclusive, and used in conjunction with a touchstone for determining the quality of gold. The gold to be tested is first rubbed upon the touchstone, and the needle which most closely approximates to it in quality, in the judgment of the operator, is also rubbed upon the stone. The two marks are then treated with nitric acid and the difference in color indicates the difference in quality of the two marks. See Touchstone.