To Distingmish Genuine Silver
American Watchmaker and Jeweler (1892)
Important: Safety Warning
This page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.
This information has been left in for historical purposes but should not be acted upon.
File or scrape the surface of the articles to be tested, rub the exposed portion on a touchstone and apply a test water consisting of 32 parts of distilled water and 16 parts of chromic acid. Rinse the stone in water and if the article is genuine silver a red spot will be left upon the stone, but if it is an imitation the mark will be unaffected. The finer the quality of the silver the more intense will be the red spot Silver Assay with Testing Tube. Place in the tube enough of the pulverized mineral to fill one inch of space, and on this pour nitric acid to occupy two inches more, and hold the mixture over the flame until the acid boils. The acid will dissolve whatever silver may be present, and must be passed through filtering paper to remove extraneous matter, and returned to the tube. Next add a few drops of water saturated with salt; any silver or lead that may be present will be precipitated in a cloudy form to the bottom. Drain off the acid, place the precipitate in the sunlight, and in a few minutes, if it contain silver, it will turn to a purple color, and may be again liquified by the addition of spirits of ammonia. The testing tube is formed of thin glass, about five inches long, and less than one inch diameter; bottom and sides of equal thickness. Where the tube is lacking, a cup may be used instead. Silver Assay by Smelting. If no lead is present, mix 600 grs. of the pulverized ore with 300 grs. carbonate of soda, 600 grs. of litharge, and 12 grains charcoal in a crucible; add a slight coal of borax over all, put on the furnace, melt, take off, give it a few taps to settle the metal, let it cool, and remove the button.
What Has Changed in the Last Century
The chemistry in this 1892 piece is sound: silver does give a brick-red spot of silver chromate with chromic acid, and the silver chloride precipitate really does darken in sunlight, which is the same light-sensitivity that made photography possible. What has changed is that jewellers no longer need to file the piece at all — handheld X-ray fluorescence analysers, standard in trade shops and scrapyards since the 2000s, read the alloy through the surface in seconds and report the fineness as a number. Chromic acid itself is now the problem: hexavalent chromium is classed as a human carcinogen and its use is tightly restricted, so touchstone acids sold today are more often nitric or dichromate-free formulations. The one method here that survives essentially intact is the last one — fire assay by cupellation with litharge and soda remains the legally recognised referee test for precious metal content.
Written for this edition, 2026.