melting

Whilst the Laboratory boasts some of the most up-to-date induction furnaces, which can heat up rapidly using the lowest possible amount of energy, the procedures that the Melters are following are centuries old. The Melters pride themselves on recovering the most possible precious metal from a melt and producing a bar that is smooth and free of air bubbles. Experience and instinct are everything and talking to them about melting is reminiscent of talking to an expert chef about how to create a perfect gourmet dish. Control of the temperature, careful addition of the right ingredients at the right time, and attending to the mixture throughout the process are all crucial. As with all procedures at Assay Office Birmingham, the first vital step to a melt is visual inspection by an expert. No two melts are the same and one melter will always take responsibility from start to finish, to ensure that he or she spots any peculiarities. The initial assessment will identify the level of non-precious metals present, including items that may contain pitch or iron, or sometimes even pennies to add to the weight of the product. This will determine the temperature at which the melt is initiated, the addition of the appropriate melting flux, and the percentage of precious metal contained in the melt, providing an important ‘sanity check’ when the bar is assayed. The melt is also checked for items that are likely to explode such as watch batteries and lighters, which are carefully removed.

The high price of silver over the past year has driven a significant increase in the number of silver melts and care must be taken to identify modern silver, which could contain cadmium, a dangerous element when vaporised. Any suspect item would either be tested by XRF prior to melting, or a tiny sample would be tried first, with the melter carefully watching for the tell tale thick yellow fumes which immediately appear. If cadmium is found to be present, the batch will be rejected, due to the potential harm to human health. Some batches of scrap are very ‘dirty’. This is not necessarily in the literal sense, although they may contain floor sweepings and other residues from manufacturing; more commonly batches of scrap contain high quantities of non-precious materials such as stones and glass, or functional components such as springs and internal watch workings, which are more likely to be steel. In this case the melt may have to be run twice – sometimes three times – in order to fully separate the precious from the non-precious metal. Once the batch has been assessed it is approved for melting. This takes place in a graphite pot. Pots are kept separate for different metals and finenesses to avoid any contamination, however small.

The content of the scrap determines the initial melting temperature. That which is ‘dirty’ or contains significant levels of iron will be melted initially at a lower temperature – around 950 degrees – to allow the melt to take place more slowly, and then the iron can be collected by skimming it off the surface. The non-precious metal elements, including stones and base metal, rise to the surface and the colour and consistency of the surface of the melt provides important information to the experienced melter. Such melts will have melting fluxes added to achieve a cleaner result. Fluxes assist the binding of the metals, absorb some of the unwanted elements and assist the flow, leaving both the bar and the melting ‘pot’ cleaner. Once the melt is considered homogeneous, a ‘dip’ sample is taken from the centre of the molten metal by drawing it up a glass tube.

This is the most accurate method of sampling, as it takes a representative sample from the centre of the pot as opposed to drilling a piece from the finished bar. When asked about adding fluxes and pouring the metal, the Melters descend further into expert ‘chef ’ mode. They have already explained the dangers of producing “Swiss cheese”, referred to as very porous, if the temperature is raised too rapidly. Now they gesture with their hands to describe the quantity of flux required, which depends on how the melt is behaving, and reiterate how important the control of temperature is. Having achieved a homogeneous melt it is then necessary to increase the heat to achieve a ‘pouring’ temperature so that the molten metal can all be poured into the warm mould before it starts to solidify. This will vary according to the size of the melt and the elements it contains, so it is once again a matter of judgement. The induction furnace includes the technology to do this automatically, but the Melters also use their own skill and judgement to achieve a smooth, homogeneous bar with the smallest amount of air bubbles. Watching them pouring the melt, shimmering at a temperature well above 1,000 degrees C, it is easy to understand why they want to be in complete control.