Natural Black Touch Stone used for the testing of precious metals in conjunction with testing acids.
Size 100mm x 80mm x 10mm.
Touchstones are a traditional method of determining the type and approximate purity of metals, particularly gold, with minimal physical damage to the article itself (compared with acid testing directly onto the item). This natural touchstone is made from a low reactive material that is evenly black in colour, and is designed for use with metal testing acids. The item with unknown metal or purity is scraped along the touchstone, leaving a visible trace line. Other articles of known purity can also be scraped along the touchstone as a control measure for comparison. Test acids are then used in turn, beginning with the lowest purity test acid, and the reaction of the metal trace lines are observed. The trace line will React or dissolve when the acid appropriate to its approximate purity is used, enabling the tester to determine the carat purity of gold items.