Burnishing and oxidation

Burnishing and oxidation are chemical conversion treatments: the surface of the metal reacts and changes colour in depth instead of receiving a pigment. The difference shows immediately on an edge — a burnished piece, when worn, still shows metal, not paint flaking off.

The result depends on three things that have to be held together: the starting alloy, the preparation of the surface and the contact time. The alloy decides the range of tone available: the same treatment on brasses with different copper content gives different colours, which is why a sample must always be re-checked on the actual material of the order. Preparation decides evenness: a trace of grease or a fingerprint leaves a mark that only appears once the reaction has happened. Time decides depth: the same products take you from light brown to black, passing through the warm intermediate tones.

The families of the range come from this. Clouded finishes are obtained by applying the product with a pad in a deliberately uneven way, so the tone gathers into patches. Deep oxidised finishes work on long contact times. Graded finishes need a continuous spread in a single pass, because every restart leaves a step. Aged finishes add a mechanical pass after the reaction, which brings the raised areas back into the light as wear would.

Variation is part of the result, but it has to be governed. For pieces that will sit side by side it is best to run the whole order in the same cycle and the same bath, and to keep a control sample for the batch: two identical pieces produced months apart can differ, and saying so beforehand avoids a dispute afterwards.

Once the reaction is complete the surface has to be stopped and protected, normally with a matt clear coat or, where requested, with wax. Without protection the oxidation continues, and a piece delivered today is no longer the same piece six months later.