Melt & Assay Explained
Published: 19th January 2022
Whether you have old jewellery, dental scrap, sweeps from the jewellery manufacturing process or other precious metal containing alloys such as catalytic converters, we can melt these items down to produce a solid bar of metal along with a test certificate detailing its precious metal content.
Sheffield Assay Office has a proven track record for its melt and assay services, combining years of experience and UKAS accredited test methods to give reports you can trust, with our Analytical Services department melting and assaying between 25-50kg of scrap metal weekly. The chemistry of the melts varies significantly from pure platinum, gold and/or silver, to dental and jewellery scraps in addition to ferrous and steel sub-products.
Using our skills and experience, we create the optimal conditions to produce a bar which can ultimately be sold, with approximately 50% of the precious scrap we process being dental scrap and mixed metal materials.
The melt process of these scraps is challenging due to differences in melting points and densities of the different metals contained in these materials. Melting precious metals requires achieving the melting temperatures of all elements present to produce a homogenous liquid phase, which will solidify after being left to cool in a mould.
Precious metals have different melting temperatures of 1,064ºC, 962ºC, 1,555ºC and 1,768ºC for gold, silver, palladium and platinum respectively. Consequently, if there is a mixture of precious metals containing platinum, the furnace must be able to reach at least 1,768ºC. Furthermore, the use of fluxes and/or additives is a well-known practice during melting metals in order to facilitate separation of unwanted, non-metallic components, and therefore improve the homogeneity of the final melt. Fluxes encourage the formation of a slag, which contains original flux (e.g. borax Na2B4O7) and non-metallic material, and releases precious metal to the molten metal phase. Slag then rises to the surface and may be skimmed off while metallic phases remain on the bottom of the crucible. In addition, copper metallic or its oxides are used as a collector of precious metals from material containing ceramics and/or porcelains.
The use of an appropriate furnace, fluxes and additives, and the knowledge of an experienced melter, plays a very important role in producing a homogenous final bar. The bar will then be stamped with an identifying mark to associate it with the final report.
Here at our Sheffield analytical services laboratory our test methods include fire assay, potentiometric titration and ICP-OES analysis. The most appropriate methods will be used to issue a test report which can then be used to trade the precious metal bar.
Although the most optimal melting procedures are used, there are some instances when is not possible to produce a fully homogenous bar due to separation of metals during the solidification (cooling process). In these instances, the bar assay must show the bar heterogeneity clearly, which will be taken into account during the trading agreement.
Learn more about our Precious Metals Analysis services here.
Do you have any precious metals that you would like to sell? For further information, please call our Analytical Services department on +44 (0)114 231 2121 or email us at analytical@assayoffice.co.uk.
The Sheffield Assay Office was established in 1773, under an Act of Parliament and today the company assays and hallmarks the precious metals - silver, gold, platinum and palladium. Sheffield Assay Office is one of only four UK assay offices who all work to uphold the Hallmarking Act of 1973 and continue to ensure consumer protection for customers purchasing precious metals.
To find out more about the whole range of services offered by Sheffield Assay Office, such as our hallmarking and analytical services, please email us at info@assayoffice.co.uk or complete the contact form on our website at http://www.assayoffice.co.uk/contact-us,
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