Why bark tannage takes weeks

Chrome tanning takes a day. Ours takes five to eight weeks. People ask, reasonably, what the skin is doing in all that time.

The answer: it's drinking. Slowly, because it has no choice.

Collagen is a crowd

A skin is a dense weave of collagen fibres — in a salmon skin, a fine crosshatch, layered like plywood. Tanning means getting tannin molecules in between those fibres and bonding them there, so bacteria can no longer take the structure apart. That's all preservation is: making the material indigestible to everything that would eat it.

Tannins are big molecules. They move through a wet skin the way honey moves through a sponge — by degrees, from the outside in. Put a raw skin in a strong tannin liquor and the outer layers bind up immediately, forming a tanned shell around a raw core. The trade has an old phrase for it: dead-tanned outside, raw inside. Such leather cracks, delaminates, and rots from the middle.

The vats are a staircase

So the old method does the opposite. The skin starts in a liquor so weak it barely tastes of bark, and moves — over weeks — through vats of rising strength. The weak liquors penetrate to the core first; the strong ones finish the binding afterward. Every vat is a step, and the staircase can't be skipped.

Time isn't a cost of the method. Time is an ingredient of it, as much as the bark.

Temperature matters too, and it works against impatience. Warm liquors would move faster, but warm liquor also ferments and can scald a delicate skin. Cold liquors — and on the Côte-Nord, cold is not hard to arrange — work slowly and cleanly. The fish skins we tan are thinner and finer-grained than cowhide, which shortens the calendar a little. It does not change the principle.

What the weeks buy

A skin tanned through its full thickness behaves like a single material instead of a laminate. It flexes without cracking, takes dye evenly, and ages by darkening rather than by decaying. The Metta Catharina hides survived two centuries of seawater because there was no raw core for the sea to find.

That is what the weeks buy. When a Smolten skin is numbered and entered in the archive, its batch record shows the dates in and out of every vat — the staircase, written down. It is the least glamorous page of the provenance record, and the most important.