Why the assay sample comes from the bar, not the scrap

A question we hear from clients sending their first lot: why can’t the refinery just test the scrap as it arrives, and skip the melt? The answer is the whole reason melting comes first, and it is worth understanding, because it is also what protects the client’s settlement.

Scrap has no single fineness

A bag of scrap jewellery is a collection of different alloys. One ring is 18 karat, the next is 22, a chain is hollow with a solder seam, a clasp has a steel spring. Even within one piece, solder joints are a lower fineness than the metal around them. Whatever piece you pick to test, the result describes that piece and nothing else.

An X-ray fluorescence reading makes the problem worse rather than better, because it only sees the surface. A plated or gold-filled item reads as gold on the outside and is mostly something else underneath.

A melt averages everything

When the whole lot is melted in an induction furnace, the metals mix into a single liquid alloy, and the non-metal content burns off. Poured into a mould, it becomes one homogeneous bar whose fineness is the same at every point. Now a sample taken from anywhere in that bar describes the whole batch.

This is the step that turns a bag of unknowns into something that can be assayed by fire, and it is why the intake weight and the melted weight are both recorded. The difference between them is the loss to slag, crucible and burn-off, and it is visible to the client rather than hidden in an estimate.

The sample is drawn from the pour

At Meridas the sample for assay is taken from the poured bar, not from the original material. It is logged against the client and the lot before it enters the laboratory, screened by XRF, and then run by cupellation, the referee fire-assay method, with atomic absorption, potentiometry or density testing brought in where the material calls for it. The result is issued within 24 hours and is the figure settlement is based on.

Why this protects the client

It might seem that testing the scrap directly would favour the client, since a good piece could be picked. In practice it does the opposite. A refinery that priced scrap on a spot reading would have to build in a wide margin against the unknowns, and the margin always falls on the seller. A refinery that melts first and assays the bar can settle on a confirmed number, with the loss shown as a weight difference rather than assumed as a discount.

That is the arrangement we would want if we were the ones sending the metal, so it is the one we run. The melting and assaying pages set out each step.