Leather lab dip approval: how to sign off a colour

Every lab dip guide on the internet was written for fabric. Leather is a natural substrate that varies across a single hide, and the approval routine that works on a woven cotton will let bulk through that you would never have signed.

A guide by TL San Martín, a tannery of origin in Elda (Alicante, Spain) since 1995, LWG Gold certified under the Leather Manufacturer standard.

Why do the fabric rules break on leather?

Because they assume the substrate is uniform, and a hide is not.

A woven cotton is manufactured to be identical across the roll. That is the premise the whole lab dip routine rests on: dye a small swatch, match it, and the bulk will behave the same way because the bulk is the same cloth. Leather has no equivalent. Within one hide, the butt — the back and rump — has the densest, most even fibre structure. The belly and flank are looser and more open. Looser fibre takes up more dye, so the same drum, the same recipe and the same day produce a belly that reads darker and a butt that reads cleaner.

That is not a defect and no tannery can dye it away. It is the reason a leather colour is approved as a range, not as a point.

Two more differences that catch people out on the first order:

• The colour you see is often two layers, not one. On an aniline article the dye is what you are looking at. On a pigmented one there is a coating on top, and the crust underneath can shift a long way without the finished surface moving at all. Which of the two you bought changes how tightly the colour can be held — see the families of leather finish.
• Substance changes colour. The same recipe on 1.0 mm and on 1.6 mm will not read the same, because penetration relative to thickness changes. Approve the dip at the substance you are actually buying, not at whatever offcut was to hand.

What should a leather lab dip physically be?

Not a card. A piece of the article.

Ask for that last row explicitly. A single clean swatch cut from the best part of a hide is a flattering sample, and it is the most common reason a founder feels ambushed at bulk. If you have already hit that problem, the diagnosis is in when bulk does not match the sample.

Vocabulary, translated once: crust is leather that has been tanned and dried but not yet finished — the semi-finished stage where colour is set. A drum charge is one load of the dyeing vessel. Substance is thickness in millimetres. Article is the tannery's own product name; "black calf" is a description, not an article.

Under what light do you approve it?

Under a named one, in a light booth, and never at your desk.

Leather finishes are pigmented, dyed or both, and pigment systems are prone to metamerism — two surfaces that match under one light and diverge under another. The industry convention is to assess under a defined set: D65 (average daylight), a store fluorescent such as TL84, and an incandescent source. If the dip matches under D65 and separates under store light, you have a metameric pair and the recipe has to be reworked. Approving in your office, under whatever is in the ceiling, means you approved one of the three at random.

Write the light into the approval. "Approved under D65" is a specification. "Approved" is not.

Three things worth specifying in the same breath, because they are measured and they are what a colour is actually judged on in service:

• Rub fastness. EN ISO 11640 runs a wool felt back and forth across the surface — around 40 cycles a minute under a load in the order of a kilogram — and the result is read on the grey scales: ISO 105-A02 for change in the leather's own colour, ISO 105-A03 for what came off onto the felt. A dark leather that transfers onto a white lining is a returns problem, not an aesthetic one.
• Light fastness, assessed under xenon arc to ISO 105-B02. Relevant for anything pale, and for anything that will sit in a shop window.
• Wet and dry, both. Rub results diverge sharply between the two, and footwear lives in both conditions.

Your supplier should hold these numbers for the article already. If the answer is vague, that tells you something about the supplier — the wider list is in 12 questions to ask a leather supplier.

Why does colour decide your minimum order?

Because the minimum is set by the drum, not by you.

This is the single piece of arithmetic that reframes a first order. A tannery dyes a batch, and the vessel has a working charge. Below it the recipe is not reliably reproducible. So the practical floor is not "how much leather do I want" — it is one drum charge per colour.

The consequence is blunt and worth stating before you design the range:

Three colours is not one order with three options. It is three minimums, three lab dips, three approvals and three separate risks of a batch-to-batch shift.

A founder who launches in two colours instead of five reaches the same shelf with roughly 60 % less cash tied up in material, and with two colour relationships to manage instead of five. The full cash picture is in your first leather order, and the material decisions that sit upstream of all of this are in the hub, leather for shoes.

There is one way around it that most founders never think to ask for: share a drum charge. If the colour is close to something already running, a tannery can often pull your quantity from a scheduled batch. You lose the ability to specify the colour exactly. You gain access to a batch you could not have commissioned. That trade is worth asking about out loud, and it is one of the things brands post to find each other for on the open board.

What has to be written down at approval?

Four lines. They cost nothing now and settle every argument later.

1. Which reference governs. You will end up holding an approved lab dip, a Pantone code and, eventually, a production sample. Name the one that wins when they disagree — in writing, at approval, before anything is dyed. If you do not, the argument happens during production, when the leather already exists.
2. The tolerance, as a range. Two swatches — the lightest and the darkest you accept — not a single target. Physical, signed, one copy each. This is the leather-specific version of the fabric routine and the step most often skipped.
3. The light source. As above.
4. Batch continuity. Whether a repeat order will be matched to the original approval or only to itself. These are different commitments, and the second one drifts over a year.

Retain your half of every signed swatch somewhere that is not a desk drawer. It is the only physical evidence in the relationship, and what counts as a normal variation versus a genuine fault is covered in defect or normal factory flaw. What the words on the invoice actually commit your supplier to is in leather grades and what they mean.

🔗 Post it on the open board

Looking for a tannery that will run a lab dip for a first-order quantity, or for another brand to share a drum charge with so a colour becomes reachable? That is exactly what the Materials category of the open board is for — free to post, open to read. If what you need is a finished-goods partner rather than the material itself, post it under Partners & factories.

TL San Martín tans and finishes leather in Elda, Alicante, since 1995: LWG Gold under the Leather Manufacturer standard — the only standard under which a Gold medal exists at all; a Trader tops out at “Approved”. Request samples before you specify anything.