An assay is a procedure where a property or concentration of an analyte is measured.
There are numerous types of assays, such as an antigen capture assay, bioassay, competitive protein binding assay, crude oil assay, four-point assay, immunoassay, microbiological assay, stem cell assay, and many others, including concentration assays.
There are methods of assay suitable for use on raw materials and other methods which are more properly suited 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).,
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.
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 1553.
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.
In the UK the Trial of the Pyx is a ceremonial procedure for ensuring that newly-minted coins conform to required standards.
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