In recent years, in response to the demand from fashion brands for metal-free leathers to overcome issues related to metal tanning, alternative chromium tannings that use various molecules or organic salts have been added to the traditional vegetable tanning.

The official definition (see standard “UNI EN 15987: 2015 Key definitions for the leather trade”) states: “Metal-free leather: Animal skin/hide converted into leather, whose total content of all tanning metals in leather (Cr, Al, Ti, Zr, Fe) is less than or equal to 0.1% (mass of the sum of all metals/total dry weight of the leather)”.
The definition concerns, therefore, what is not present in leather (metals), rather than characterising what is there, namely the non-metallic-based tanning agent.
This opens the field to the use of numerous substances (which have tanning activity) and to the need to evaluate their environmental sustainability in a much broader horizon than just the tanning phase, which must include the entire life cycle (LCA), taking account of related environmental criteria. The analysis must therefore start from the production of the tanning agent, with a careful assessment of the sources, whether renewable or not, in order to understand the various stages in the tannery, and end with the degradation of the molecule in purification plants.
A currently known problem of non-metallic tanning is that they stress the efficiency of purification plants. This may be due to the nature of the wastewater, because, having a very different composition, can cause imbalances in plants designed to work with the chrome tanning cycle. This can, however, be considered a transitory problem that can be solved in the long term with system adjustments. It is though also necessary to consider the possibility that the new tanning agents do not bring the expected benefits in an environmental key: that is because it does not happen automatically that an organic substance is naturally biodegradable: indeed, many of the synthetic organic substances are poorly or not at all biodegradable, with the consequence to increase the number of persistent organic pollutants, or those chemical substances that persist in the environment, bio-accumulate through the food chain and carry the risk of causing negative effects on human health and the environment.
The use of a metal-free tanning substance that wants to be truly eco-sustainable and not the result of a greenwashing operation must therefore take into account, in addition to the fundamental feature of making non-perishable leather, also the principles of Green Chemistry in its synthesis, and finally the ability to be efficiently degraded in purification plants or at the end of the leather’s life cycle.

*Ph.D. Tiziana Gambicorti, Head of the industrial Tuscany district SSIP- Italian Leather Research Institute