For our day job of making and selling great leather there is one more insidious term that continues to create huge problems, so much so that tanners themselves have started to use it so carelessly that it is almost legitimising this weasel term.

The phrase “heavy metals” is a badly defined scientific term and should be avoided as the lack of an agreed description of what is and is not within the category makes it meaningless. It is often used to create fear with the general public. Definitions can include the nature of the atomic structure, the atomic weight, groupings in the Periodic Table of elements and a number of other things. The most common definition refers to metals with a specific gravity of more than 5 especially those like lead and mercury, which are also poisonous. Chromium has an SG of 7.1 and titanium 4.5 yet when a titanium leather came to the front at the Best of APLF awards a year or two ago it was moved from the Environment to the Innovation prize because one judge categorised titanium as a “heavy metal” which for him outplayed its positives as a hypoallergenic material that can be put on the land without problems.

The definition of Heavy Metal by Anne Marie Helmenstine, PhD on Chemistry is:

A heavy metal is a toxic metal. There is no standard definition assigning metals as heavy metals. Some lighter metals and metalloids are toxic and thus are termed heavy metals, which some heavy metals, such as gold, typically are not toxic. Most heavy metals have a high atomic number, atomic weight and a specific gravity greater than 5.0 heavy metals include some metalloids, transition metals, basic metals, lanthanides and actinides.


Heavy metals

Examples of heavy metals include lead, mercury, cadmium, sometimes chromium. Less commonly, metals including iron, copper, zinc, aluminum, beryllium, cobalt, manganese and arsenic may be considered heavy metals.

Helpful or confusing?

How can scientists just add that throwaway first line? Almost daily we come across comments about chromium being heavy and toxic yet we know that chromium is needed in our daily diet to help hold off illnesses like diabetes. It is found naturally in beef, eggs, chicken, green peppers, bananas and spinach. Chromium helps to maintain normal blood glucose levels by enhancing the effects of insulin. The body needs chromium for healthy carbohydrate, protein and fat metabolism. Tanners are aware of the significant difference between CrIII and CrVI but the extraordinarily loose use of the term heavy metal makes it exceptionally difficult to explain this to the public or to many designers.

John Duffus of The Edinburgh Centre for Toxicology prepared a report on the subject of Heavy Metals a decade ago. His starting position was 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”.

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”.

So as an industry we should support the scientific conclusions that we abandon the classification of metals using terms such as “heavy metals”, which have no sound scientific or terminological basis. These terms of “organic” and “toxic” need to be banned from our vocabulary and our literature. Let’s restrict the use of the word ‘heavy’ to the groceries we take home and the beer that we drink. 

Mike Redwood

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