Electropolishing

Mix 3 parts phosphoric acid (85%) to 1 part distilled water — about 600 ml total. Goggles and nitrile gloves.

The viscosity is the mechanism, not an incidental property. A thin, watery electrolyte will etch the surface dull instead of polishing it. Do not dilute this further to make it easier to handle.

Take the copper sheet and sand half of one face with abrasive paper in one direction, leaving clear parallel scratches. Leave the other half as it is.

Degrease and handle by the edges.

Polishing a piece that is already smooth teaches you nothing. The scratched half is the experiment.

Copper sample to the positive (+). A second copper or stainless sheet to the negative (−), facing it, about 30 mm away and parallel.

Positive is the reversal Jacquet made. In electroplating the work is negative and metal arrives; here the work is positive and metal leaves.

Raise the voltage slowly from 0 and record current at each step. Plot current against voltage.

You are looking for the plateau — a range, typically 1.5–2.0 V for copper in phosphoric acid, where current stops rising as voltage increases.

Below the plateau the surface etches dull. Above it, gas evolves and pits the surface. Only on the plateau does it polish. That plateau is the viscous film governing the reaction instead of the voltage, and finding it is Jacquet's actual contribution — before this, results were luck.

Hold on the plateau for 5–10 minutes.

Rinse in running water, then dry.

Expect: the scratched half is now brighter than the half you left alone. The scratches have been levelled, not filled — nothing was added.

Look at it at a glancing angle. Mechanical polishing leaves fine parallel lines whichever way you work; an electropolished surface has no directional structure at all, because dissolution has no direction.

The patent. French patent 707,526, 1930, covering the electrolytic polishing of copper and brass, following his 1929 discovery — made, by his own account, by reversing the polarity of a plating cell. Jacquet published the phosphoric-acid copper theory in 1936 and demonstrated polished high-purity aluminium in 1937.

Why a metallographer cared. To look at a metal's grain structure you need a flat, undamaged surface. Mechanical polishing always leaves a smeared, work-hardened layer — the Beilby layer — that hides the very structure you are trying to see. Electropolishing removes material without touching it, so what is left underneath is the real metal. It became standard specimen preparation and remains so.

Where it went commercially. Pharmaceutical, dairy and semiconductor equipment. Not for looks: a mechanically polished stainless surface has microscopic scratches and folded-over metal that trap product and bacteria and cannot be cleaned out. Electropolished stainless has neither, which is why it is written into hygienic-design standards.

The honest limit. Electropolishing levels, it does not flatten. It will remove scratches and micro-roughness beautifully and will do almost nothing about a dent, a wave or a lack of geometric accuracy — those are long-wavelength features and the viscous film simply follows them. A part that must be flat still has to be ground or lapped first.