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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. In the case of fire assaying of gold and platinum ores, the lengthy time required to carry out an assay is generally offset by carrying out large numbers of assays simultaneously, and a typical laboratory will be equipped with several fusion and cupellation furnaces, each capable of taking multiple samples, so that several hundred analyses per day can be carried out. The principal advantage of fire assay is that large samples can be used, and these increase the accuracy in analyzing low-yield ores in the <1g/T range of concentration.
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200:, 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
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carbide heating elements. A flow of air through the muffle assists oxidation of the lead, and carries the fumes for safe collection outside the furnace unit. The lead melts and oxidises to lead oxide, which in turn melts and is drawn into the pores of the cupel by capillary attraction. The precious metals remain in the base of the cupel as a 'prill' which is sent for final analysis of precious metal content.
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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
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inside an electric furnace fitted with silicon carbide heating elements, and heated to between 1,000 and 1,200 °C. The temperature required, and the type of flux used, are dependent on the composition of the rock in which the precious metals are concentrated, and in many laboratories an empirical approach based on long experience is used.
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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
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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, heated at 1,650 °F (or 898.9 °C; temperature varies with exact method) in a cupel made of compressed bone ash or magnesium
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A complex reaction takes place, whereby the carbon source reduces the lead oxide to lead, which alloys with the precious metals: at the same time, the fluxes combine with the crushed rock, reducing its melting point and forming a glassy slag. When fusion is complete, the sample is tipped into a mold
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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
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Fusion: the process requires a self-generating reducing atmosphere, and so the crushed ore sample is mixed with fluxes and a carbon source (e.g. coal dust, ground charcoal, flour, etc.) mixed with powdered lead oxide (litharge) in a refractory crucible. In general, multiple crucibles will be placed
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The modern X-ray fluorescence (XRF) 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
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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
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circulating coins after its 1970 A.D. 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,
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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 one of the last nations to discontinue the use of silver in
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Cupellation: the lead bullets are placed in porous crucibles (cupels) of bone ash or magnesium oxide and heated in air to about 1,000 °C. This is usually carried out in a 'muffle' furnace, containing a refractory muffle (usually nitride-bonded silicon carbide) heated externally by silicon
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One process for X-ray fluorescence assay involves melting the material in a furnace and stirring to make a homogeneous mix. Following this, a sample is taken from the centre of the molten sample. Samples are typically taken using a vacuum pin tube. The sample is then tested by
359:(usually iron) where the slag floats to the top, and the lead, now alloyed with the precious metals, sinks to the bottom, forming a 'button'. After solidification, the samples are knocked out, and the lead bullets recovered for cupellation, or for analysis by other means.
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Variation from skills taught in modern standard adaptations 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 and
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method is most common by far and does not damage the item in question. A rubbing of the item is made on a special stone, treated with acids and the result is compared to the result of the same process done on a sample of gold with a known purity. Red
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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 of art or jewelry because it is a completely destructive method.
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were used for this. Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test, using acids and gold samples both of a specific, known concentration.
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Cupellation 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 cupellation.
196:(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
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363:(i.e., instrument finish rather than physical gold product weighing). Arguably, even these texts are largely an extension of traditions that were detailed in
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The most elaborately accurate, but totally destructive, precious metal assay is fire assay. (It may also be called by the critical
401:. Only bullion methods validated and traceable to accepted international standards obtain genuine accuracies of 1 part in 10,000.
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Smith, Ernest A. "The
Sampling and Assay of the Precious Metals", Met-Chem Research, Inc., Boulder Colorado, 2nd ed. 1947.
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government, and, therefore, the assay of the raw materials and finished coins is an important quality control.
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306:. Metallurgical assay is typically completed in this way to ensure that an accurate assay is performed.
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Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw
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Sampling, Preparation, Fire
Assaying, and Chemical Analysis of Gold and Silver Ores and Concentrates
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is a ceremonial procedure for ensuring that newly minted coins conform to required standards.
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alloys now predominate in coin making. Notwithstanding, several national mints, including the
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specified, usually by law, to be contained in them. This was particularly important when
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602:, Department of Mining, Engineering and Metallurgy, University of Idaho, 1981.
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regulations. Reputable North
American bullion assayers conform closely to
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269: in this section. Unsourced material may be challenged and removed.
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Assay and
Analytical Practice in the South African Mining Industry
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produced at the mint have the correct content or purity of each
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3rd ed (1940), Colorado School of mines Press, Golden
Colorado.
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step that separates precious metal from lead.) If performed on
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and platinum by inductively coupled plasma optical emission
169:, usually performed in order to test for purity or quality.
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Taylor, P. R. (ed.), Prisbrey, K. A., Williams, J. F.,
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results can be automatically printed out by computer.
581:, McGraw-Hill Book Company, Inc., New York, NY, 1911.
49:. Unsourced material may be challenged and removed.
16:Compositional analysis of an ore, metal, or alloy
204:(XRF). XRF is used because this method is more
145:) seal used to certify the quality of assayed
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494:WaarborgHolland, Europe's No. 1 Assay Office
338:1916 photograph of an assayer performing an
384:Cupels for assays and refining noble metals
584:Lenahan, W. C. and Murry-Smith, R. de L.,
550:"How refiners assay precious metal scrap"
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413:A coin assayer is often assigned to each
285:Learn how and when to remove this message
141:A model of a late 19th-century Canadian (
109:Learn how and when to remove this message
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127:Tombstone Courthouse State Historic Park
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318:Colorado assay office – circa 1870 A.D.
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267:adding citations to reliable sources
47:adding citations to reliable sources
125:A 19th-century assay laboratory in
157:is a compositional analysis of an
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595:, McGraw-Hill Book Company, 1940.
421:to determine and assure that all
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304:X-ray fluorescence spectroscopy
254:needs additional citations for
34:needs additional citations for
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342:test on a gold sample at the
593:A Textbook of Fire Assaying
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518:"Touching precious metals"
344:United States Assay Office
579:A Manual of Fire Assaying
484:. The Goldsmiths' Company
591:Shepard & Dietrich,
572:A Textbook of Fire Assay
475:The Hallmarking Process
184:. Silver is assayed by
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399:ASTM method E1335-04e1
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58:"Metallurgical assay"
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226:or black siliceous
155:metallurgical assay
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634:Metallurgy
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505:(in Dutch)
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448:Perth Mint
219:touchstone
213:Touchstone
198:hallmarked
188:, gold by
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99:May 2009
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