- Metallurgical assay
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A late 19th century Canadian seal used to certify the quality of assayed gold.
A metallurgical assay is a compositional analysis of an ore, metal, or alloy.
Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metals (bullion) are assayed by an assay office. Silver is assayed by titration, gold by cupellation and platinum by inductively coupled plasma optical emission spectrometry (ICP OES).[1][2] Precious metal items of art or jewelry are frequently hallmarked (depending upon the requirements of the laws of either the place of manufacture or the place of import). Where required to be hallmarked, semi-finished precious metal items of art or jewelry pass through the official testing channels where they are analyzed or assayed for precious metal content. While different nations permit a variety of legally acceptable finenesses, the assayer is actually testing to determine that the fineness of the product conforms with the statement or claim of fineness that the maker has claimed (usually by stamping a number such as 750 for 18k gold) on the item. In the past the assay was conducted by using the touchstone method but currently (most often) it is done using X-ray Fluorescence (XRF). XRF is used because this method is more exacting than the touchstone test. The most exact method of assay is known as fire assay or cupellation. This method is better suited for the assay of bullion and gold stocks rather than works or art or jewelry because it is a completely destructive method.
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The touchstone
The age-old touchstone method is particularly suited to the testing of very valuable pieces, for which sampling by destructive means, such as scraping, cutting or drilling is unacceptable. A rubbing of the item is made on a special stone, treated with acids and the resulting color compared to references. Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test. It is not indicated for use with white gold, for example, since the color variation among white gold alloys is almost imperceptible.
X-ray fluorescence
The modern X-ray fluorescence is also a non-destructive technique that is suitable for normal assaying requirements. It typically has an accuracy of 2 to 5 parts per thousand and is well-suited to relatively flat and large surfaces. It is a quick technique taking about three minutes, and the results can be automatically printed out by computer. It also measures the content of the other alloying metals present. It is not indicated, however, for articles with chemical surface treatment or electroplating.
Fire assay / cupellation
The most elaborately accurate, but totally destructive, precious metal assay is fire-assay. (It may also be called by the critical cupellation step that separates precious metal from lead.) If performed on bullion (high purity precious metal alloy) to international standards, the method can be accurate on gold metal to 1 part in 10,000. If performed on ore materials using fusion followed by cupellation separation, detection may be in parts per billion. However, accuracy on ore material is typically limited to 3 to 5% of reported value. Although time consuming, the method is the accepted standard applied for valuing gold ore as well as gold and silver bullion at major refineries and gold mining companies.
1916 photograph of an assayer performing an electrolysis test on a gold sample at the United States Assay Office in New York.In the bullion fire assay process, a sample from the article is wrapped in a lead foil with copper and silver. The wrapped sample, along with prepared control samples, heat at 1650 F (temperature varies with exact method) in a cupel made of compressed bone ash or magnesium oxide powder. Base metals oxidize and absorb into the cupel. The product of this cupellation (doré) is flattened and treated in nitric acid to remove silver. Precision weighing of metal content of samples and process controls (proofs) at each process stage is the basis of the extreme method precision. European assayers follow bullion traditions based in hallmarking regulations. Reputable North American bullion assayers conform closely to ASTM method E1335-04e1. Only bullion methods validated and traceable to accepted international standards obtain genuine accuracies of 1 part in 10,000.
Cupelation alone can only remove a limited quantity of impurities from a sample. Fire assay, as applied to ores, concentrates or less pure metals, adds a fusion or scorification step before cupelation.
Fusion is a melt (typically at 1950 F) in a dry chemical flux designed to precipitate lead and precious metals from the ore sample into lead button. Silicates, carbonates, and other non-precious impurities reject into a glassy slag. The lead button product is typically cupeled to further concentrate the product to pure precious metals, but selected instrument method are able to directly analyze precious metals within the lead button.
Method details for various fire assay procedures vary, but concentration and separation chemistry typically comply with traditions set by Bugby or Shepard & Dietrich in the early 20th century. Method advancements since that time primarily automate material handling and final finish measurements (i.e., instrument finish rather physical gold product weighing). Arguably, even these texts are largely an extension of traditions that were detailed in De re metallica by Agricola in 1556.
Variation from skills taught in modern standard adaptions of fire assay methodology should be viewed with caution. The standard traditions have a long history of reliability; "special" new methods frequently associate with reduced assay accuracy.
Coins
A coin assayer is often assigned to each mint or assay office to determine and assure that all coins produced at the mint have the correct content or purity of each metal specified, usually by law, to be contained in them. This was particularly important when gold and silver coins were produced for circulation and used in daily commerce. Few nations, however, persist in minting silver or gold coins for general circulation. For example the U.S. discontinued the use of gold in coinage in 1933. The U.S. was perhaps the last nation to discontinue the use of silver in circulating coins after its 1970 half dollar coin, although the amount of silver used in smaller denomination coins was ended after 1964. Even with the half dollar, the amount of silver used in the coins was reduced from 90% in 1964 and earlier to 40% between 1965 and 1970. Copper, nickel, cupro-nickel and brass alloys now predominate in coin making. Notwithstanding, several national mints, including the Perth Mint in Australia, the Austrian Mint, the British Royal Mint, the Royal Canadian Mint, the South African Mint and the U.S. Mint continue to produce precious metal bullion coins for collectors and investors. The precious metal purity and content of these coins is guaranteed by the respective mint or government and therefore the assay of the raw materials and finished coins is an important quality control.
In the UK the Trial of the Pyx is a ceremonial procedure for ensuring that newly-minted coins conform to required standards.
Goldaş 50g silver bullion in assay card.Notes
Further reading
Fire assays
- Bugby, Edward E. A Textbook of Fire Assay 3rd ed (1940), Colorado School of mines Press, Golden Colorado.
- Fulton, H.C., A Manual of Fire Assaying, McGraw-Hill Book Company, Inc., New York, NY, 1911.
- Lenahan, W.C. and Murry-Smith, R. de L., Assay and Analytical Practice in the South African Mining Industry, South African Institute of Mining and Metallurgy, Johannesburg, South Africa, 1986.
- Shepard & Dietrich, A Textbook of Fire Assaying, McGraw-Hill Book Company, 1940.
- Taylor, P.R. (ed.), Prisbrey, K.A., Williams, J.F., Sampling, Preparation, Fire Assaying, and Chemical Analysis of Gold and Silver Ores and Concentrates, Department of Mining, Engineering and Metallurgy, University of Idaho, 1981.
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