Before an item can be hallmarked the Assay Office will test or assay it to ensure that the precious metal composition meets the legal requirements . No negative tolerance is allowed, so if, for example, an item of 9ct gold which should be 375 parts per thousand fine gold, proves to only be 374 it will not be hallmarked.
Historically in order to assay or test the item small amounts of metal were very carefully scraped or cut from articles to produce laboratory sized samples for assay. This process was known as “sampling”.
Until the late 20th century most articles were received in the unfinished state and in the case of articles which were scraped to remove the metal, the scraping marks were easily removed by the manufacturer in subsequent finishing operations.
Some articles in the form of stampings and castings allow the removal of surplus material by small cuttings to produce samples.
However, since 2000 the number of imported items has risen dramatically. These are highly finished, packed in separate mini grip bags or tissue and the importer rarely has any facility to refinish the item.
The Assay Office therefore had to reconsider its assaying process as scraping a sample from finished product is obviously an unacceptable process for customers as it visibly damages the article. From 1998 major investment was made in exploring X Ray Fluorescence as an alternative assaying method. This became established as an accepted process and further investment in the hi-tech equipment necessary moved all assaying to XRF by 2005. The traditional cupellation and titration methods for gold and silver respectively still remain the referee method in case of uncertainty.
ASSAY BY XRF
Assay by XRF has the benefit of not only removing the need to scrape but it is also cleaner, green and quicker as it removes the need to subject samples to high temperature furnaces and the use of lead to extract impurities.
The XRF machine emits a concentrated beam of X rays onto a focussed spot. The size of this target is adjusted by the operator who has different sizes to select from, always aiming to choose the largest possible “spot size” where the item has a flat surface to receive the beams evenly.
The X Rays penetrate the surface of the material and “excite” the atoms in the alloy . The intensity of energy which these atoms emit is then analysed and collated automatically to provide a reading as to the percentage of each element in the item.
When used and calibrated correctly by an experienced operator, XRF produces extremely accurate results. Accuracy is maintained by continual calibration against extensive reference standards and subsequent adjustment of the reading. The machines used by samplers at Assay Office Birmingham are programmed to deliver confirmation that the article tested complies with a given test standard eg 9ct yellow gold. The same technology with a stronger electric current and a longer exposure to the beams is used by The Laboratory to identify every element in complicated and unusual alloys. In this case a highly trained technician will use his expertise to interpret the results from a graph and challenge every irregularity to ensure nothing has escaped scrutiny.
Assay is carried out on a random selection of samples. The number of samples taken depends on the quantity of articles in the parcel and also the nature of the articles, the number of parts to an article and the different materials from which they have been made; for example, castings, sheet, wire, rod or tube.
Separate samples have to be taken as far as is practical from each category of articles (rings, brooches, earrings, bracelets etc) and each type of material. Composite samples can be obtained from groups of articles of the same category, pattern and type of material.
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