What the certificate does not say

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Why a small workshop pays attention to three parts per million

Nearly all the leather in the world is tanned with chromium III, and chromium III is not the problem. The problem is a conversion that can happen afterwards. Under heat, an unfavourable pH, contact with certain fats, or simply over a long period in storage, a small proportion of the chromium III in finished leather can oxidise into chromium VI. Chromium VI is a skin sensitiser: it causes allergic contact dermatitis, and once a person has been sensitised they remain so, reacting thereafter to quantities far below what first provoked it. That is why UK chemicals law restricts it rather than merely labelling it, and why the threshold for leather that touches skin is set at three milligrams per kilogram — three parts per million, close to what the standard test method can reliably detect at all.

The consequence for anyone buying or making leather goods is less obvious than the rule. Because the oxidation happens after tanning, a certificate of analysis records the state of a hide on the day it was tested and says progressively less as time passes. A hide tested at a tannery, shipped across a continent, stored through a warm summer and cut a year later is not described by that document in the way its holder assumes. Treating the certificate as the end of the enquiry is the mistake; treating the age of the result as part of the information is the fix. Alongside chromium sits a second restriction worth the same attention, on azo dyes capable of releasing certain aromatic amines in leather and textile articles that touch skin.

The obligation falls on whoever places the article on the market, and that is worth stating because small makers often assume it stays with a tannery abroad. It does not travel that far. A workshop that brings leather into the United Kingdom may be the importer for these purposes, in which case the duty is its own and a supplier's paperwork is evidence rather than a defence. None of this makes leather a hazardous material or chrome tanning a scandal. It makes the questions specific: what was tested, by which method, when, and who is answerable if the answer turns out to be wrong.