Assay, Assaying. The testing of ores or bullion metal to determine desirable metal content. This testing was a major function of mints to determine gold and silver content of metal supplied to them up to the 20th century (when this function was taken over by metal refinery firms). Assay offices, operated by the government, were established to perform this testing, often near the mines. Assaying differs from refining (although assaying actually refines a very small amount of ore to determine gold or silver content). Mints did perform the refining function of gold and silver ore brought to them (in any amount) until this function was assumed by refinery firms.

History of assaying. Man's interest in gold and silver – and how to extract

these metals from ore – had existed since antiquity. By empirical methods (experimentation) man had developed a large body of knowledge that made assaying the most advanced technology by the 16th century. The assayer succeeded in what he was trying to do (isolate the gold) in contrast to the alchemist (who sought to change base metal into gold). Yet both metalworkers had devoted vast resources of time and effort to the study of these precious metals.

Assaying required elaborate containers of glass and metal. A flask made of platinum was used for parting gold with sulphuric or nitric acid. This was introduced in America in 1867 from England.

SCALES were particularly important for weighing gross amounts of ore, and delicate scales for weighing tiny ore samples or purified metals. Often assayers had two or three very sensitive scales. They also employed a number of weights, often making their own. They also created their own weight systems at various times with up to 16 fractional divisions of some arbitrary amount (called unity in America and assay ton in England).

How assays are made. A very small sample of the total quantity received is assayed. Once a sample is selected, it is carefully weighed. Then the test could be conducted by one or a combination of these processes:

? Dry assay is a testing of a sample of ore or mixed metals to ascertain the content of the gold, or of silver. This process is also called the crucible method or cupellation (because of the use of the cupel). The sample is fused with lead (as a flux), then this is melted in a cupel or cupee – a small container of phosphate of lime (made of ground dried bones). When placed in a furnace the cupel absorbs all metals (even platinum) other than gold and silver. It leaves these as a "button" of the mixed elements in the bottom of the cupel. The silver is separated from the gold by dissolving in nitric acid and distilling out the silver from the resulting silver nitrate.

? Humid assay (or wet assay) is a roasting of the ore, with lime if necessary, and an extraction of the gold and silver with sulphuric or nitric acid (aqua regia or aqua fortis), then a precipitation from the solution with mercury or copper.

? Scorificiation method is a modification of the dry assay. Instead of a cupel, a shallow dish of burned clay, called a scorifier, is used. Ground ore is placed in the clay dish, ten to twenty times its weight in granulated lead is added and a little borax glass (as the flux). This is heated in a muffle, an arched enclosure inside a furnace, made of firebrick with openings on the sides and back (to allow entry of the greatest amount of hot air). The lead melts and begins to oxidize; this lead oxide (called a litharge) combines with everything in the ore but gold and silver. The door of the muffle is opened, fresh air hardens the litharge with the desired gold and silver in an "eye" formed as a pool in the slag of undesired material. After removing the dish from the furnace, and allowed to cool, the button of metal easily separated from the litharge by tapping out. The gold is then separated from the silver by use of nitric acid.

? Parting method, also a form of cupellation, was employed when the resulting button was too rich in gold. The physical form of the two combined metals would be beads, fine particles or loose sponge. Silver had to be added to this (process called inquartation) and melted. The silver would then be separated by dissolving the button in nitric acid leaving the gold intact. The silver could then be distilled out of the resulting silver nitrate.

The isolated gold and silver metal could then be weighed (very accurately) and proportions applied to the total weight or gross amount of the ore. If necessary, several samples were taken for large ore deposits. The United States Mint converted from dry assay to humid assay about 1838.

These assaying methods were quite advanced, considering they had changed little since the 16th century. These same methods were in use at the Paris Mint – and reported in a 1696 book: Boizard (J.), Traite des monoyes, de leurs circonstances et dependances. Many other books were written on assaying in 17th and 18th centuries, often detailing minutia of the processes: as calf skulls were the best bones for cupels, lining these with fish-bone ash aided the process, the composition of the fluxes, as lead silicate glass, or adding iron filings to lead sulphide ores.

Assays by mints. Since mints require precious metals to strike coins, this raw material was in constant need. For the most part government mints and assay offices were consistent buyers, whose purpose was not only to assay the metal deposited, but[RWB2] to convert the metal into coin, oftentimes to purchase the entire output of a mine. U.S. mints and Assay Offices did not normally perform assays of raw ore. They received bullion bars from individuals, companies and anyone else. They also accepted old jewelry, dental scrap, and anything else containing gold with a value over $100.

This policy changed in 1896 when silver was produced in such quantity in America that the mints could no longer convert it all. This is the reason for the super abundance of silver dollars during and prior to this time.

Assay offices. Since transportation was a large cost factor in the 19th and 20th centuries, assay offices were established near the source of the metal, often near the mine fields themselves. For this reason mints of the United States were established near mining fields as well.

In 21st century Europe assay offices are still active even in countries that do not produce silver or gold but are necessary for hallmarking precious metal manufactured goods.

American Government Mints and Assay Offices

∙ Philadelphia Mint (1792-date)

∙ Charlotte, North Carolina (1837-1861 as a

mint; until 1913 as an assay office)

∙ Dahlonega, Georgia Mint (1838-61)

∙ San Francisco Mint (1853-date)

∙ Carson City, Nevada Mint (1865-99

continues as an assay office)

∙ Denver Mint (1904-date).

Assay offices also have been in:

∙ Denver Assay Office (1864-1905)

∙ Boise, Idaho (1872-1933)

∙ Helena, Montana (1877-1933)

∙ Saint Louis (1881-1911)

∙ Deadwood, South Dakota (1897-1927)

∙ Seattle, Washington (1898-1955)

∙ Salt Lake City (1908-1933).

References:

C28 {1954} Singer 3:58-71.

CH22 {1966} Taxay, p 84-87.