Guide · 20 October 2026 · by Zafer

Why leather rusts steel: what a tool roll does to a blade

Somebody brings me a chisel, every few months, with a brown bloom across the back of the blade in exactly the shape of the pocket it was sitting in. They want to know what was wrong with the leather. Nothing was wrong with the leather. The leather was doing what vegetable-tanned leather does to steel — which is the thing nobody tells you when they sell you a roll to keep your tools safe.

An open crazy horse leather tool roll on a dark workbench with chisels and a knife in its pockets, lit by low raking window light

Somebody brings me a chisel every few months with a brown bloom across the back of the blade, in exactly the shape of the pocket it had been sitting in. Sometimes it is a knife, and the mark runs in a clean straight stripe a few millimetres wide. The question is always the same: was the leather faulty?

No. The leather was behaving perfectly normally. The problem is that a tool roll looks like a safe place, and for steel it is one of the least safe places in the building.

This is a short piece of chemistry that I think everyone who owns a leather roll, a knife sheath or a leather-lined case should know, because once you know it the fix takes about four seconds and costs nothing.

Leather is an acid, and it holds water

Two facts, and the whole article is downstream of them.

The first: vegetable-tanned leather is acidic. It typically sits somewhere around pH 3.5 to 5 — roughly between black coffee and tomato juice. That acid arrived with the tanning liquor and it does not leave. It is not a residue you can rinse out; it is part of the chemistry that stopped the hide from rotting in the first place.

The second: leather is hygroscopic. A hide in a normal room carries something like a tenth to a seventh of its own weight in water, and it exchanges that water with the air continuously. This is not a defect either — it is why leather feels warm rather than clammy, why it breathes, and why it can be worked soft at all.

So a tool pocket is a layer of acid that is permanently slightly damp, pressed tight against bare metal, with no air getting in and nothing getting out. If you set out to design a corrosion cell on purpose, that is roughly what you would build.

And then there is the third ingredient, which is the only one you actually control: time.

WHY A SHEATH IS THE WORST PLACE TO STORE A BLADE The leather supplies the acid. The room supplies the water. Only the third input is yours. CROSS-SECTION THROUGH A TOOL POCKET, MAGNIFIED VEGETABLE-TANNED LEATHER pH 3.5–5 · carries a tenth of its weight in water trapped film — no air in, no drying out, no end CARBON STEEL the one part of this you were trying to protect The leather is not a barrier. It is one half of the cell. 1  ACID — IT NEVER LEAVES THE HIDE Free acid from the tanning liquor stays in the fibre. 2  WATER — LEATHER IS HYGROSCOPIC It draws damp from the room and holds it on the steel. 3  TIME — THE ONLY INPUT YOU CHOOSE Hours are nothing. A fortnight is a pit you can feel. WHERE IT STARTS — NOT ON THE OPEN FACE, BUT WHERE THE LEATHER PRESSES HARDEST THE POCKET MOUTH Tightest grip, most handling, and the dampest edge of the roll UNDER THE FLAP Starved of air — corrosion wants less of it, not more ALONG THE STITCH LINE The thread wicks water in and lays it down in a straight line Rust that arrives in a straight line was told where to go. Read the shape and you know the cause. ACIDITY, FOR SCALE — LOWER IS MORE CORROSIVE VEG-TAN LEATHER pH 3.5–5 CHROME-TAN LEATHER pH 3.5–4 · plus salts HUMAN SKIN pH about 5.5 NEUTRAL WATER pH 7 Both tannages are acidic. Neither is a cupboard — and “sealing” the leather only traps the water on the wrong side.
A tool pocket is a layer of acid, permanently slightly damp, pressed against bare metal with no air getting in. The only variable you control is how long you leave it there.

The reaction is a thousand years old

There is a second thing happening alongside ordinary rusting, and it is the reason the stain is often not orange but blue-black.

Vegetable tannins are chelators: molecules that grab metal ions and hold onto them. Tannin plus iron plus a little water produces iron tannate, a dense blue-black compound. This is not an obscure laboratory curiosity. It is iron gall ink — the ink of medieval manuscripts, made for the better part of a millennium by stirring iron salts into an extract of oak galls, which is to say into exactly the tannin we tan hides with.

So when a blade comes back from a sheath with a dark grey-black shadow on it rather than orange pitting, that is the ink reaction, and the leather will usually have a matching dark mark where the iron went the other way. Both substances are telling the same story from opposite sides.

The practical consequence: the black marks tend to be shallow and polish off. The orange ones have eaten into the surface and leave a pit behind. If you find black, you got away with it. If you find orange, you did not.

Close view of the inside of a leather tool pocket showing a dark grey-black shadow in the exact shape of the blade that sits in it, the leather around it sound and unmarked
The same reaction, seen from the other side. Iron and tannin make a blue-black compound, so the hide keeps a shadow of the tool — which is iron gall ink, made by accident, in a pocket.

Chrome-tanned leather is not the escape route

The obvious conclusion — use chrome-tanned leather instead, since it has no tannin — is half right and not much use.

Chrome-tanned leather is also acidic, typically a shade lower than veg-tan rather than higher. And it carries residual neutral salts from the tanning float: sulphates and chlorides. Dissolve salt in the film of water sitting on your blade and you have made an electrolyte, which is the thing that makes a corrosion cell run faster. You have swapped the ink reaction for the mechanism that eats cars in winter.

There is no tannage that makes leather a safe long-term home for bare steel. This is a property of leather, not a fault in a particular hide.

Where the rust actually starts, and why the shape tells you

People expect corrosion on the exposed part of the blade, where the air is. It is almost always the opposite: it starts where the leather is pressed hardest against the metal.

The reason is that rusting needs water and a difference in oxygen, not a generous supply of it. Where the leather clamps the steel, oxygen cannot reach, and that oxygen-starved patch becomes the anode of a small cell while the open, aerated surface nearby becomes the cathode. The attack concentrates in the crevice. Engineers call this crevice corrosion and design expensive things around avoiding it; a tool roll creates it by accident, in every pocket, for free.

Which is why the marks are so legible once you know what you are reading:

A single leather tool pocket held open to show the mouth of the pocket and the stitch line running down beside it, the two places where leather presses hardest against a blade
The mouth and the stitch line. Corrosion does not start where the air is — it starts where the leather grips, because an airless patch beside an aerated one is a cell.

What a tool roll is actually for

Here is the reframe that fixes the problem, and it is not a chemical one.

A roll is not a cupboard. It is a device for keeping edges off each other while they travel — and for keeping the inside of your bag off the edges. It is a carrying material doing a carrying job.

Every good property of a roll is a transport property. It holds each tool in its own pocket so a chisel cannot ride against a knife and take the corner off it. It rolls into a bundle that survives being thrown into a bag. It unrolls flat so that the tools present themselves in order and you can see at a glance that none are missing. A jewellery roll earns its keep on exactly the same principle. It is quiet, it does not slide off a bench, and it will not scratch the work. Not one of those jobs requires the tools to live there.

The moment the roll becomes storage — tied shut and put on a shelf for the winter — every one of those virtues stops mattering, and the one property it has that you did not ask for takes over.

CARRY, OR STORE — ONE ROLL, TWO COMPLETELY DIFFERENT JOBS The variable is not the leather. It is how long the steel is asked to sit in it. HOURS A day out, a job, a commute. Nothing at all happens. This is the job. DAYS A weekend away, a week in the van. Oil the steel first — not the leather. WEEKS Forgotten in a cupboard. A bloom at every line where leather touched. MONTHS “Safe storage.” Pitting, which is permanent. The edge is a worse edge now. Leather is a carrying material. A drawer, a block, or a wrap of oiled paper is a storing material. WHAT THE METAL DECIDES — THE SAME POCKET, FOUR OUTCOMES CARBON STEEL — CHISELS, KNIVES, SCISSORS, NEEDLES Rusts, and it is the tool you care about most. Black marks polish off; orange ones leave a pit. NICKEL- OR CHROME-PLATED STEEL Safe until the plating is scratched — then it corrodes faster, in the shape of the scratch. BRASS AND BRONZE No rust. Tannins darken it to a brown-green patina, which is cosmetic and wipes back. STAINLESS — A PROMISE, NOT A FACT Resists well in the open, then pits in exactly the airless crevice a tight pocket creates. Oil the steel, not the leather. The barrier belongs on the thing you are protecting.
Hours are what a roll is designed for; months are what ruins a blade. And a tight pocket is the one place even stainless steel can pit.

The rules, in the order they matter

None of this is a reason to own fewer leather rolls. It is a reason to use them the way they work.

  1. Carry in leather. Store somewhere else. If the tools are going away for longer than a week, they come out of the roll. This single rule removes about ninety per cent of the problem and is the only one I would insist on.
  2. Oil the steel, not the leather. A wiped film of camellia oil, light machine oil or even a smear of paste wax on the blade is a real barrier between metal and acid. Feeding the leather does the opposite of what people hope: a conditioner is mostly fats and water, so you are adding moisture to the damp side of the problem and softening the fibre so it hugs the steel more closely.
  3. Never put a tool away damp. Not from rain, not from a wet stone, not from your hands on a hot day. Wipe it, let it sit in the air for ten minutes, then roll it.
  4. If it must be stored in leather, interleave. A wrap of oiled kraft paper, or the vapour-inhibiting paper sold for exactly this, between blade and pocket. Not cling film — that traps the water instead of excluding it.
  5. Watch the pocket mouth and the stitch line when you check anything over, because that is where it shows up first.
  6. Check at the seasonal changeover. The same two moments that matter for storing a bag — going into winter and coming out of it — are when workshop humidity swings hardest.

And one thing not to do: do not try to line the pockets with plastic to keep the acid away from the steel. A plastic liner stops evaporation, and a sealed damp space is worse than an acidic breathing one. Every attempt I have seen at this has rusted faster.

The stain on the leather is a different matter

The iron goes both ways, and the mark the steel leaves on the hide is permanent too — a grey-black shadow in the shape of the tool that sits in that pocket.

I would leave it. There is nothing to repair; the fibre is sound and the leather is no weaker. What you have is a record of exactly which tool lived where, written by the tools themselves, and on a working roll it is the most honest patina there is. My own roll has a dark parallelogram where a skiving knife has sat for years, and I can find that pocket in the dark because of it.

That is the asymmetry worth ending on. The same reaction that spoils a blade merely marks a hide. One of the two materials in this story can afford the exchange, and it is not the steel.

What we do about it

Our tool rolls are made in Crazy Horse and in calf. The Crazy Horse is vegetable-tanned, and as you will have gathered from the section above, the tannage barely changes the conclusion anyway — so everything here applies to ours exactly as written, and we would rather say so than let you find out through a chisel. We stitch them with waxed linen and we do not line the pockets with anything synthetic, because a pocket that can dry is better than a pocket that is sealed.

What a roll of ours is for is the drive, the bench, the class, the site, the fifteen years of being unrolled and rolled again. Keep the steel oiled and the roll will outlast several sets of tools. Leave a bare blade in it over a wet winter and no leather on earth will save you.

Further reading