Finishing by hand

A little red powder

Look closely at a jeweller's polishing mop and you may find it stained a rusty red. That is rouge, and it is nothing more exotic than a very fine powder of iron oxide. The name is simply French for red. It brings up the final shine on gold and silver, and it is often the last thing a piece touches before it leaves the bench.

Rouge is the end of a long journey, though. Before it comes filing, sanding, often tumbling, and a coarser compound with a more workmanlike job to do. And older than all of them is burnishing: rubbing metal with something harder and smoother, a piece of polished steel or a stone. One jewellery writer calls it probably one of the oldest polishing techniques there is.

Why finishing matters

However a piece is made, cast, cut by laser or built from wire and sheet, it does not come out ready to wear. Cast pieces have a rough skin. Laser-cut parts come off the machine with slightly burred edges and often some discolouration. Soldered joints leave marks. All of it has to be taken back.

Part of the reason is comfort. A burr is a sharp little lip of metal, and an edge that has not been softened will catch on knitwear and feel scratchy against skin. Part of it is looks. And part of it is plating, because plating does not hide what is underneath. Finishing specialists put it plainly: plating is better at emphasising scratches than filling them. A plated layer is a fraction of the depth of an ordinary scratch, so every file mark left under it shows straight through, gilded. The surface also has to be thoroughly clean for the plating to grip at all.

Filing

Filing is the first and coarsest step: shaping an edge, taking off a casting sprue or a burr, levelling a solder joint. Jewellers use small needle files in many profiles, flat, half round, round, square and triangular, so they can reach into the curves of a letter or the inside of a loop.

Files are graded by their cut, from coarse to fine, and every file cuts on the push stroke, away from you. Dragging it back under pressure does nothing useful and dulls the teeth, so the return stroke is a light one.

Sanding through the grits

A file leaves its own marks, and the next job is to take them out. That is done with abrasive paper, often wrapped around a sanding stick (a fingernail emery board does the same job on a flat edge), working from a coarser grit to a finer one.

The logic is simple. Every abrasive leaves its own set of grooves, and each finer grit is there to remove the grooves of the one before. Skip a step and the deeper scratches survive, hidden until the polish shows them up. Jewellers' own advice varies, but one starts at 320 grit and works down to at least 600 for gold and silver, and finer still, 1,200 to 2,000, for platinum before polishing. Another starts at 220, moves to 400 and then goes on to the compounds.

Tumbling

Some pieces are finished not one at a time but in batches, in a tumbler. The pieces go into a barrel or bowl with water, a little compound and a mass of media, and the machine keeps them moving against one another for hours. The media decides what happens.

The machines differ too. Jewellers often describe rotary tumblers as best for burnishing and vibratory tumblers as best for abrasive work. Disc finishers spin a disc at the bottom of a fixed bowl, throwing the load round in a rolling flow, and are used for fast, controlled smoothing of small parts.

Tumbling is not magic. Pins that strike point-first can leave a pitted, orange-peel surface, and on soft precious metals, cutting media can clog with metal until it only burnishes. It saves hours on a chain, but it does not replace careful work on a visible face.

Compounds and mops

Polishing on a motor is done with soft mops, also called buffs, of cotton, felt or similar, charged with a polishing compound. There are two families.

• Cutting compounds, such as tripoli, which remove the last fine scratches.
• Colouring compounds, such as rouge, which bring up the final bright shine.

The rule at the bench is one mop per compound, and per metal. A trace of tripoli carried onto a rouge mop drags fine scratches back into a finished surface. As one jeweller puts it, once a rouge wheel is contaminated with tripoli, "it is no longer a rouge wheel."

Burnishing

A hand burnisher is a hardened steel tool with a smooth, highly polished tip. Rubbed firmly over metal, it flattens and compresses the surface instead of cutting it, so it brightens an edge or closes up fine marks without removing material. Engineers describe burnishing as plastic deformation of a surface by sliding contact, which is exactly what the steel shot in a tumbler is doing in miniature.

Because it only pushes metal around, a burnisher has limits. Fine marks disappear. Deep scratches do not.

Electropolishing

Electropolishing is plating in reverse. The piece is made the anode in a bath of electrolyte, and the current removes a microscopically thin layer of its surface instead of adding one. The high points dissolve faster than the low points, so the surface ends up smoother and brighter. It is widely used on stainless steel, and there are electropolishing machines made specifically for silver jewellery.

It cannot work miracles either. Electropolishing evens out microscopic roughness, but specialists warn that if you can feel a scratch or defect, electropolishing won't get rid of it. Those have to be filed or sanded out first.

Back to the red powder

By the time a piece meets the rouge mop, almost all of the work is done. The filing shaped it, the papers took out the file marks, the tumbler or the burnisher brightened what hands could not reach, and the tripoli took out the last fine lines. The red powder only reveals what was done before it. That is the quiet truth of finishing: the shine is the last thing you see and the least of the work.