Mike Redwood


International Leather Maker

There is too much careless talk these days. A decade ago, there were increasingly angry comments about opponents talking about “vegan leather” and using all manner of inaccurate terms.

There was also rising concern that the leather industry had itself become part of the problem. A wander round any trade show saw signage and promotional wall hangings demonstrating how even leather was deep into greenwashing and bad terminology.

I presented a list of some “weasel words” in regular use by the leather industry at the 2014 American Leather Chemists Association (ALCA) annual meeting to demonstrate the need to be more accurate and precise. These were:

  • Heavy metal
  • Toxic
  • Eco
  • Green
  • Sustainability
  • Biodegradable
  • Organic, chrome-free
  • Carbon footprint.

I had previously been particularly struck when a very creative titanium tanned leather was given an innovative award but refused an environmental one because one judge noted that titanium was a heavy metal. Later I checked the definition of heavy metals and quickly racked up a list of 36 separate meanings.

Many of these I found in a study published by the Human Health division of the International Union of Pure and Applied Chemistry set up to look at the term. The lead author was John Duffus of The Edinburgh Centre for Toxicology, who noted that “there is a tendency to assume that all so-called ‘heavy metals’ have highly toxic or ecotoxic properties. This immediately prejudices any discussion of the use of such metals, often without any real foundation”.

Abandon terms such as “heavy metals”

His conclusion is quite clear. The term “heavy metal” has never been defined by any authoritative body such as IUPAC. Over the 60 years or so in which it has been used in chemistry, it has been given such a wide range of meanings by different authors that it is effectively meaningless. No relationship can be found between density (specific gravity) and any of the various physicochemical concepts that have been used to define “heavy metals” and the toxicity or ecotoxicity attributed to “heavy metals”.

He concludes: “It is clear that we should abandon classification of metals using terms such as ‘heavy metals’, which have no sound terminological or scientific basis.”

Most of the terms I identified as frequently subject to misuse fell into similar positions. Given the wide acceptance of the Brundtland definition of “sustainability”, and the paper explaining it being quickly found on the Internet and an easy read, I have always been concerned when people turn to the nearest dictionary to find whatever definition suits them on the day. In an increasing polarised and tribal world, these definitions matter.

So, I was not surprised to see the latest edition of International Leather Maker (January/February 2024) featuring an editorial feature by Professor Will Wise on metal-free leather. His concern was that a slack approach to interrogating the definition of metals was leading some brands to exclude benign metal salts that actually make the leather better; noting that “better” is a leather term that has itself evolved!

He said: “It is remarkable to me that some brands would exclude a new process/chemical that offers an improvement in sustainability credentials or performance simply because it contains a benign metal salt.”

Duffus does address the issue of defining metals in his paper, and he uses the careless use of the term carbon as another example. Not everyone involved in making and using leather is an expert in the sciences. We certainly struggle with so few “sustainability” staff in the materials field having chemistry qualifications or even much knowledge of leather in the way the leather buyers used to have in all sectors.

A world of third-party accreditation to meet oversimplified goals based on scientific terms that are not capable of rigorous definition and are anyway often misunderstood is an unsatisfactory way to proceed. Professor Wise makes sense in asking the industry to look to definitions that better reflect real word use and actual outcomes.


2002, Duffus, John H., “Heavy Metals”—A Meaningless Term?, Pure Appl. Chem., Vol. 74, No. 5, pp. 793–807

International Union of Pure and Applied Chemistry, Chemistry and Human Health Division, Clinical Chemistry Section, Commission on Toxicology*


Follow Dr Mike Redwood on Twitter: @michaelredwood

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