Metallurgical assay
Encyclopedia
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
, metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
, or alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
.
Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metal
Precious metal
A precious metal is a rare, naturally occurring metallic chemical element of high economic value.Chemically, the precious metals are less reactive than most elements, have high lustre, are softer or more ductile, and have higher melting points than other metals...
s (bullion) are assayed by an assay office
Assay office
Assay offices are institutions set up to assay precious metals, in order to protect consumers. Upon successful completion of an assay, Assay offices are institutions set up to assay (test the purity of) precious metals, in order to protect consumers. Upon successful completion of an assay, Assay...
. Silver is assayed by titration
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
, gold by cupellation
Cupellation
Cupellation is a metallurgical process in which ores or alloyed metals are treated under high temperatures and carefully controlled operations in order to separate noble metals, like gold and silver, from base metals like lead, copper, zinc, arsenic, antimony or bismuth, that might be present in...
and platinum by inductively coupled plasma optical emission spectrometry (ICP OES).
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 hallmark
Hallmark
A hallmark is an official mark or series of marks struck on items made of precious metals — platinum, gold, silver and in some nations, palladium...
ed, 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.
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 goldWhite Gold
White Gold is a 2003 Russian action film directed by Viktor Ivanov from a screenplay by John Jopson and Viktor Ivanov. The story begins with the actual events of 1919 when a White Army train carrying the bulk of Czar Nicholas' gold reserves arrives empty at Siberia's Irkutsk station...
, for example, since the color variation among white gold alloys is almost imperceptible.
X-ray fluorescence
The modern X-ray fluorescenceX-ray fluorescence
X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...
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 cupellationCupellation
Cupellation is a metallurgical process in which ores or alloyed metals are treated under high temperatures and carefully controlled operations in order to separate noble metals, like gold and silver, from base metals like lead, copper, zinc, arsenic, antimony or bismuth, that might be present in...
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.
Hallmark
A hallmark is an official mark or series of marks struck on items made of precious metals — platinum, gold, silver and in some nations, palladium...
ing 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
De re metallica
De re metallica is a book cataloguing the state of the art of mining, refining, and smelting metals, published in 1556. The author was Georg Bauer, whose pen name was the Latinized Georgius Agricola...
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 mintMint (coin)
A mint is an industrial facility which manufactures coins for currency.The history of mints correlates closely with the history of coins. One difference is that the history of the mint is usually closely tied to the political situation of an era...
or assay office
Assay office
Assay offices are institutions set up to assay precious metals, in order to protect consumers. Upon successful completion of an assay, Assay offices are institutions set up to assay (test the purity of) precious metals, in order to protect consumers. Upon successful completion of an assay, Assay...
to determine and assure that all coin
Coin
A coin is a piece of hard material that is standardized in weight, is produced in large quantities in order to facilitate trade, and primarily can be used as a legal tender token for commerce in the designated country, region, or territory....
s produced at the mint have the correct content or purity of each metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
specified, usually by law, to be contained in them. This was particularly important when gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
and silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
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
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
alloys now predominate in coin making. Notwithstanding, several national mints, including the Perth Mint
Perth Mint
The Perth Mint is Australia's oldest currently operating 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
Trial of the Pyx
The Trial of the Pyx is the procedure in the United Kingdom for ensuring that newly minted coins conform to required standards. Trials have been held from the twelfth century to the present day, normally once per calendar year; the form of the ceremony has been essentially the same since 1282 AD...
is a ceremonial procedure for ensuring that newly-minted coins conform to required standards.
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.