melting

Over the past two years the jewellery trade has become accustomed to the fact that precious metal prices are rising on a daily basis, and many members of the trade have made a significant amount of money from buying and selling scrap. In 2010 an extraordinary 1,650 tonnes of fine gold was fed back into the global gold supply chain. But what happens to the scrap between the customer selling it and it being recycled back into the supply chain?

Recovery of precious metals has always been an integral part of the jewellery industry. Despite involving very high value materials, the physical process is totally unregulated, although the commercial process is controlled by anti money laundering legislation. Melting scrap to recover the precious metal content is not an exact science and it is worth traders understanding the process to avoid financial loss.

Scrap for metal recovery may be clean, even unworn fine jewellery, or precious metal articles mixed with base metal pieces such as watches or gem set pieces. At the other end of the scale are automotive catalytic converters containing a low level of platinum/rhodium and polishing mops and other process ‘sweepings’ discarded by manufacturing jewellers, which inevitably contain tiny amounts of gold and other precious metals. With fine gold at £28 per gram every speck counts.

Whatever the form, the precious metal needs to be raised to its melting point to separate it from the other unwanted materials present. Broken crucibles, sludge, polishing dust, rag and mops etc bearing low percentages of precious metals need to be crushed, broken down or incinerated to reduce the bulk before being melted. The process varies from material to material.

The actual melting is carried out in a furnace, which will heat the scrap material to above its melting temperature – 1,100ºC for gold, 900 to 1,000ºC for silver, 1,500ºC for palladium and 1,550 to 1,800ºC for platinum. Furnaces vary from small ones, which will take a maximum of a kilogram of material, up to cavernous constructions, which will melt tonnes at a time. If a jeweller or pawnbroker wants their scrap to be melted as a separate lot to create an individual saleable bar, it is obviously important to source a melting facility with an appropriate sized furnace.

Depending upon the apparent content of the melt, other substances may be added to improve the process. Copper collects precious metals preferentially from low value material. Flux will be introduced to make a slag to release trapped precious metal and to encourage the scrap to form a homogeneous mass. When the material becomes molten the slag rises to the surface and may be skimmed off, taking with it other non-metallic debris. Depending upon the type of material submitted for melting it may have to undergo two or three melts before it is sufficiently homogeneous to be assayed to obtain acceptable results. The objective of the experienced melter should be to create as clean a separation of the metal and slag as possible, and to create a homogeneous bar. This is a bar that has the same precious metal fineness throughout, rather than patches of unmixed metal where the precious metal content may be higher or lower than in the remainder of the bar.

The method of extracting a sample from the bar for testing or assaying in order to determine its fineness is extremely important. There are various options: small samples can be drilled from various points of the bar, and the results compared and averaged if necessary to offset the impact of non-homogeneity; while other melters may favour cutting samples from the corners of the bar. In ideal circumstances a ‘dip sample’ will be taken from the molten metal when it is being stirred in the furnace, providing the best opportunity to capture a truly homogeneous and representative sample of the melt. The Birmingham Assay Office conducts this process using vacuum-sealed glass tubes to gain the most accurate results.

The resulting bar can be sold on to various different parties and will eventually reach a refinery where it will be refined back to pure fine precious metal and will begin its journey all over again.