Assay
2 of the 6 encyclopedias on this shelf carry an entry for Assay. Both are reproduced below, so you can see where they agree and where they differ.
The Encyclopedia of Founding (1892)
The determination of the quantity of any particular metal in an ore, alloy, or other metallic compound, more especially of the quantity of gold or silver in coin or bullion. It differs from analysis thus: The component parts of the mineral or alloy are, by analysis, separated, and an estimate made of their respective quantities; while by assay, it is only the valuable metals that are sought for; as, in the case of silver and gold alloys the inferior metals are dispersed, the quantity being determined by the loss of weight. A gold alloy is assayed by obtaining a certain number of grains, which, after being carefully weighed, are wrapped in sheet-lead and exposed to intense heat in a cupel placed under a muffle. Cupels for this purpose must be very porous, and are simply a small block with a cavity on the upper side to receive the metal. When fusion takes place, the lead is converted into a vitreous oxide, which, acting as a flux, acts powerfully to oxidize and vitrify the inferior metals contained in the alloy, which being changed, are absorbed by the porous cupel, leaving a globule of unoxidable metal behind. The globule will be silver or gold, or a compound of both, which may be separated by the method shown at "Separating Metals from their Alloys." Another method of assaying is described at "Touch-needle." See MUFFLE.
American Watchmaker and Jeweler (1892)
To subject an ore, alloy or metallic compound to chemical examination in order to determine the amount of a particular metal contained in it.
What Has Changed in the Last Century
The cupellation the 1892 Encyclopedia of Founding describes -- lead-wrapped sample, porous cupel, muffle furnace -- is still done, and fire assay remains the legally accepted referee method for gold in ore and bullion. What has changed is everything around it: the weighed gold bead is now usually dissolved and measured by atomic absorption or plasma spectrometry, and routine mine and scrap work is done by X-ray fluorescence, which reads an alloy's composition in seconds without destroying it. The touch-needle, offered as an alternative method in 1892, survives only as a rough field test. The word itself has also travelled: to most readers today an "assay" is a laboratory test in biology or medicine -- an immunoassay or a PCR assay -- rather than anything to do with metal.
Written for this edition, 2026.