From the Bible to State of the Art - Cupellation Test Methods Explained
Published: 13th November 2019
The process of fire assay by cupellation is the industry standard method for determining the gold and silver content of precious metal alloys. But what are the origins of this technique, and how does the Sheffield Assay Office use it to deliver highly accurate results for its hallmarking and analytical services customers?
Originally used to extract precious metals from their ores, there is evidence of fire assay being used by ancient civilisations throughout Asia Minor around 3,000 BC. Some of the cupel beads still survive today in a museum in Ankara, a product of the process of extracting silver from iron ore.
Assaying is even mentioned in the Bible in Jeremiah 6:27-30!
I have made you an assayer and tester among my people
That you may know and assay their ways
They are bronze and iron, and they act corruptly
The bellows blow fiercely, and the lead is consumed by the fire
In vain the refining goes on; refuse silver they are called
For the Lord has rejected them.
Analysis of lead pipes manufactured by the Roman Empire demonstrate that they could remove any traces of silver down to levels as low as 0.01%, they even stamped them with the words “Ex Arg” to signify that they had been refined.
The art of assaying was re-kindled in the 16th century and has since been perfected into the reference method used by governments, mints, refiners, Assay Offices and laboratories all over the world.
The Modern Method
The Sheffield Assay Office has been testing precious metals for over 240 years. Our old logo even contained the motto “Ex Flammae Veritas” or “Truth from the Flames”.
Items of gold jewellery submitted for hallmarking are now routinely tested by XRF, but fire assay remains the reference method used to verify the results. Samples as small as 50mg can be carefully taken from the item if required. At the other end of the scale, we may receive several kilograms of scrap gold which require melting into a homogeneous bar and with subsequent analysis of drilling or dip samples.
Advances in the technology used for weighing have greatly improved the accuracy achievable by modern fire assay methods, with more sensitive balances enabling us to weigh samples to within 0.001mg. The sample is then wrapped in a piece of lead foil along with a measured quantity of silver, which should be between 2 and 3 times the expected amount of gold present. If the sample has a low copper content, a small piece of copper is also added to prevent the molten sample from spitting. This part of the process is called inquartation, and dilutes the gold in the refinable material to around 25% (literally ‘quartering’ the gold) to enable the metallic elements to be parted completely.
The next stage is scorification, where samples are shaped into a ball and placed into a cupel, which is made of a refractory material such as magnesite or bone ash. They are placed into a cupellation furnace at 1100°C at which the metals become molten. Any non-precious metals are either oxidised or absorbed into the cupel leaving behind just the gold and silver, which forms into a bead when cooled.
At this point, any traces of platinum, palladium, iridium, rhodium, ruthenium or osmium may still be present in the bead, and further treatment may be necessary to remove them.
The beads are rolled out to a thickness of around 0.25mm to increase their surface area and annealed at a temperature of 700°C. They are then wrapped into a cornet shape and placed into small individual baskets made of platinum. These baskets are then immersed in two different strengths of nitric acid and heated. This process is called parting, and removes any silver present in the cornet by dissolution.
Once cooled, the remaining cornet is now pure gold and is weighed and compared to the initial weight of alloy sample. This gives us the percentage of gold present. For example, 9 carat gold is nominally 37.5%.
The key to an accurate assay is the skill and experience of the staff who are able to balance all the factors which may contribute to any variations.
These factors include the inquartation ratio, the mass of copper and lead added, the cupel type, cupel pre-burn, fire assay duration, the position of the cupel inside the furnace, the local temperature inside the furnace, casting of the assay, brushing of the button, flattening of the button, button rolling and lamination thickness, thermal treatment of the assay to improve the assay rolling, efficiency of acid dissolution, composition and position of proof samples.
Respected worldwide, our cupellation test method is now based on the international standard ISO 11426:2016 and is accredited by UKAS to the rigorous requirements of ISO 17025:2017.
To ensure the highest quality results, all our equipment is regularly calibrated and the method has been validated many times over. With every batch of samples we run at least two control samples consisting of 99.99% pure gold. These are subjected to the same process as the samples and by monitoring the results, we can demonstrate that the process is working efficiently to an accuracy of around 1 part per thousand. For refining companies, this accuracy is critical to reduce the financial risk in an industrial environment.
We also regularly score highly in proficiency testing schemes, which compare our results to those of other laboratories.
So whether you are sending in articles for hallmarking, scrap or bullion for testing, the Sheffield Assay Office stands out above the rest for its combination of years of experience with state of the art equipment to give results you can trust.
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 email@example.com or complete the contact form on our website at http://www.assayoffice.co.uk/contact-us,