Last week we took a look at chromium in textiles – and leather. With the increased interest in avoidance of certain chemicals and industrial products that are particularly harmful to our environment, it’s not surprising that manufacturers are becoming ingenious in pointing out attributes that play to this script. So we now see claims for “chrome free” leather as being “eco friendly”.
Although most leather is tanned using chromium (from 80 – 95% of all leather produced uses chrome tanning ) there is a third type of leather tanning, called aldehyde tanning, which like vegetable tanning does not use chromium.
Often leather is advertised as being “pure aniline”, “full or semi aniline”, “top grain” “nubuk”- these are just terms which describe how the dye is applied or in the case of “top grain”, where the hide comes from on the animal. These terms have nothing to do with tanning.
Let’s look at leather tanning for a minute and find out what that means:
Sometimes leather manufacturers will tell you that they don’t use the toxic form of chromium in tanning – the toxic form is called chromium VI or hexavalent chromium. And that is correct: chromium tanned leathers use chromium III salts (also called trivalent chromium) in the form of chromium sulfate. This form of chromium is found naturally in the environment and is a necessary nutrient for the human body. However, the leather manufacturers fail to explain that chromium III oxidizes into chromium VI in the presence of oxygen combined with other factors, such as extremes in pH. This happens during the tanning process. Chromium-tanned leather can contain between 4 and 5% of chromium  – often hexavalent chromium, which produces allergic reactions and easily moves across membranes such as skin. End of life issues, recovery and reuse are a great concern – chromium (whether III or VI) is persistent (it cannot be destroyed) and will always be in the environment. Incineration, composting and gasification will not eliminate chromium.
Vegetable tanning is simply the replacement of the chromium for bark or plant tannins –all other steps remain the same. And since there are about 250 chemicals used in tanning, the replacement of chromium for plant tannins, without addressing the other chemicals used, is a drop in the bucket. Last week I mentioned some of the other 249 chemicals routinely used in tanning: alcohol, coal tar , sodium sulfate, sulfuric acid, chlorinated phenols (e.g. 3,5-dichlorophenol), azo dyes, cadmium, cobalt, copper, antimony, cyanide, barium, lead, selenium, mercury, zinc, polychlorinated biphenyels (PCBs), nickel, formaldehyde and pesticide residues.
Aldehyde tanning is the main type of leather referred to as “chrome-free”, and is often used in automobiles and baby’s shoes. Aldehyde tanning is often referred to as “wet white” due to the pale cream color it imparts to the skins. But aldehydes are a group of chemicals that contain one chemical which many people are familiar with: formaldehyde. And we all know about formaldehyde: it is highly toxic to all animals; ingestion of as little as little as 30 mL (1 oz.) of a solution containing 37% formaldehyde has been reported to cause death in an adult human and the Department of Health and Human Services has said it may reasonably be anticipated to be a carcinogen.
Aldehyde tanning essentially uses formaldehyde, which reacts with proteins in the leather to prevent putrefication. BLC Leather Technology Center, a leading independent leather testing center, states that leathers should contain no more than 200ppm of formaldehyde for articles in general use. If the item is in direct skin contact this should be 75ppm, and 20ppm for items used by babies (<36 months). Typically, with modern tanning techniques, leathers contain 400ppm or less. Yet that far exceeds levels set elsewhere – in New Zealand, for example, acceptable levels of formaldehyde in products is set at 100 ppm – the European Union Ecolabel restricts formaldehyde to 20 ppm for infant articles, 30 ppm for children and adults, while GOTS prohibits any detectable level.
BLC Leather Technology Center commissioned a study by Ecobilan S.A. (Reference BLC Report 002) to do a life cycle analysis to evaluate the various tanning chemicals, to see if there was an environmentally preferable choice between chrome, vegetable and aldehyde based processes. The result? They found no significant differences between the three – all have environmental impacts, just different ones. These LCA’s demonstrate that tanning is just one of the impacts – other steps may have equal impacts. Chrome was cited as having the disadvantage of being environmentally persistent. “Another consideration, in terms of end-of-life leather or management of chrome tanned leather waste, is the possibility of the valency state changing from the benign Cr III to the carcinogenic Cr VI.”
So much for “chrome free” leather. But since all three tanning processes impact the environment to the same degree, the least toxic (vegetable) is the one I’d choose. But there are precious few tanneries doing vegetable tanned leather.
One issue which is a hot topic in leather production is that of deforestation and the sourcing of skins from Brazil – cattle ranching in Brazil accounted for 14% of global deforestation and ranches occupy approximately 80% of all deforested land in the Amazon.  Greenpeace and the National Wildlife Federation (NWF) aims to stop all deforestation in the Amazon by encouraging the meat processors to insist that their suppliers register their farms and map and log their boundaries as a minimum requirement. They also encourage companies to cancel orders with suppliers that are not prepared to stop deforestation and adhere to these minimum requirements. Many of the Leather Working Group (LWG)(for a list of these members, see footnote 9) member brands have made commitments to a moratorium on hides sourced from farms involved in deforestation and LWG itself has a project to identify and engage with the key stakeholders in Brazil, investigate traceability solutions, conduct trials and implement third party auditing solutions.
 Richards, Matt, et al, “Leather for Life”, Future Fashion White Papers, Earth Pledge Foundation
 Gustavson, K.H. “The Chemistry of Tanning Processes” Academic Press Inc., New York, 1956.
 Barton, Cat, “Workers pay high price at Bangladesh tanneries”, AFP, Feb. 2011
 Agency for Toxic Substances & Disease Registry, “Medical management guidelines for formaldehyde”, http://www.atsdr.cdc.gov/mmg/mmg.asp?id=216&tid=39
 BLC Leather Technology Center Ltd, “Technology Restricted substances – Formaldehyde”, Leather International, November 2008, http://www.leathermag.com/news/fullstory.php/aid/13528/Technology_Restricted_substances-Formaldehyde.html
 “Evaluation of alleged unacceptable formaldehyde levels in Clothing”, Wellington, New Zealand: New Zealand Ministry of Consumer Affairs, October 17, 2007.
 “Broken Promises: how the cattle industry in the Amazon is still connected to deforestation…” Greenpeace, October 2011; http://www.leatherworkinggroup.com/images/documents/Broken%20promises%20-%20Oct11FINAL.pdf
(9) Currently the consumer brands involved with the LWG are: Adidas-group, Clarks International, Ikea of Sweden, New Balance Athletic Shoe, Nike Inc, Pentland Group including (Berghaus, Boxfresh, Brasher, Ellesse, Franco Sarto, Gio-Goi, Hunter, KangaROOS, Mitre, Kickers (UK), Lacoste Chaussures, ONETrueSaxon, Radcliffe, Red or Dead, Speedo, Ted Baker Footwear), The North Face, The Timberland Company, Wolverine World Wide Inc including (CAT, Merrel, Hush Puppies, Patagonia, Wolverine, Track n Trail, Sebago, Chaco, Hytest, Bates, Cushe, Soft Style). New brands recently joined are Airwair International Ltd, K-Swiss International, Marks & Spencers and Nine West Group.