Determining the Purity of Precious Metals in Antiquity: Three Methods
A question came up yesterday from a reader/commenter on how the purity of precious metals was tested in antiquity. While we are definitely aware and practitioners of modern precious metals purity testing methods, we were frankly stumped on the method for testing precious metals in antiquity so we did some research to create this summary. There were essentially three methods for testing precious metals purity in ancient times, the Fire Assay method, the Archimedes Method, and the Touchstone Method, and they are all used in similar forms today.
Fire Assay Method — The Fire Assay Method dates back to at least ancient Egypt and Babylon as it is mentioned in writings from this time period. The basics of fire assaying are the addition of lead in excess to the gold which is then melted in a special crucible under an oxidizing flame. The alloying elements (excluding silver) and the lead are oxidized and the resulting composition is absorbed into the ceramic crucible. What is left behind is any gold or silver which it contains. These two metals were then parted by strongly heating in a crucible filled with a mixture of brick dust, copper and iron sulphates and salt, which converted the silver to silver chloride. This silver was absorbed by the brick dust and the gold was recovered unaffected. There were other variations of this method, but they all were very similar. This is not a true assay, of sorts, but was one of the earliest methods of identifying silver and gold composition in a composite metal.
Archimedes Method — The Archimedes Method, developed s based on the fact that gold has nearly twice the specific gravity of silver and more than twice that of copper. As gold or silver is debased, the specific gravity is reduced. The testing for this had variations, but the similarities involved balancing pans that were tested outside and inside of water against known pure silver or gold weights to determine the differential. The core problem was the equations for the calculations of The Archimedes Method are believed to have been inaccurate until modern times.
Touchstone — The Touchstone Method is known to have been used in Ancient Greece to assay precious metal purity. The Touchstone Method’s advantage, besides being the most accurate method in antiquity, is that it is non-destructive in nature. Touchstones required rubbing the alloy under investigation onto the surface of a piece of smooth, fine-grained, slightly abrasive, black stone and comparing the color of the streak produced with those obtained from standard alloys. The accuracy of the method was dependent on the knowledge of whether the alloying element in the gold was silver or copper and on having a suitable range of standard alloys for comparison with the unknown sample. In modern times, acids are used for a more accurate result.
Analysis: The purity testing of precious metals in ancient times was less than exact, but the Touchstone method has survived to this day and been improved on due to its superiority over the other more destructive and less exact methods of assaying. The concern for the purity of precious metals in any metal composition both in antiquity and in modern times demonstrates the intrinsic value of precious metals to humanity as a superior store of value.
