ON A RECENT VACATION in Morocco, my husband and I watched one of the oldest chemical processes in action: the conversion of animal hides into leather by a process that was already well-developed by the 11th century.
Although Morocco is known for fine leather, the chemical engineering we observed in the tannery at Marrakech wasn't elegant. This was strictly a batch process, with men in tall rubber boots sloshing hides around in vats containing various solutions. Featured high on the list of chemicals used were lime and ammonia—and even pigeon dung.
Around the world, the tanning industry is modernizing and implementing higher standards for protection of workers and the environment. But the basic chemical process of converting a raw hide into leather remains largely the same as it was in the 11th century. Essentially, leather makers impregnate raw hides with a variety of chemical substances that prevent them from decaying, make them resistant to wetting, and keep them supple and durable.
At the materials heart of the process are the fibers of collagen proteins, which make up the bulk of the hide and which the tanning process seeks to protect and preserve. Removing oils, fats, contaminant proteins, and surface dirt is also important in the conversion of hide into leather.
The conventional tanning process takes the hide through as many as 15 steps—from soaking, liming, and pickling to tanning, dyeing, and fatliquoring, the step in which oils are introduced into the skin before the leather is dried to replace the natural oils lost in processing. The result is leather in its final form.
The process starts with dry-cured or wet-salted skins, which are cleaned and rehydrated. The hides are soaked in a solution of detergents, bacteria-killing agents, and proteolytic enzymes that help clean the pelt.
Cleaning is followed by a lime bath to remove hair from the hide. The lime bath, traditionally sodium sulfide or hydrosulfide, breaks down fibrous structural proteins known as keratins in the hair and skin and removes other proteins that would otherwise stiffen into a hard, inflexible glue and clog the leather. Liming also causes osmotic swelling, which loosens bundles of collagen fiber in the skin so that tanning agents can thoroughly penetrate.
De-liming, with neutralizers such as ammonium chloride, in turn prepares the hide for bating. Bating imparts further softness, stretch, and flexibility, by removing any unwanted hide components that remain. Today, bacterial enzymes have replaced the animal and poultry dung once used as a source of enzymes.
The hide is then "pickled," or soaked in a sodium chloride brine, with acidity adjusted by sulfuric acid to a pH of about 3. Pickling promotes absorption of the tanning agents into the hide.
For the tanning step, leather makers treat the skins with mineral agents, such as trivalent chromium compounds, or vegetable tannins extracted from tree bark. Vegetable-tanned leather tends to be used for shoe soles and other heavy applications such as luggage and some furniture. Leather produced in a chrome-tanning process—used in the bulk of the industry nowadays—tends to be lighter weight and much more pliable, making it well-suited for high-fashion applications such as handbags and garments.
The pH of the solution is then raised to neutralize the tannins and prepare the hide for posttanning treatments.
All told, it's a process that generates an enormous amount of waste: 1 metric ton of raw hide yields 250-300 kg of leather but also leaves 600 kg of solid waste, including sludge.
For that reason, researchers are trying to develop processes that cut the production of wastes such as chromium and cut the use of water. One of the latest efforts has been dubbed "reverse tanning" by its developers, a team at the chemical laboratory of the Central Leather Research Institute in Chennai, India (Environ. Sci. Technol. 2006, 40, 1069).
Their process avoids cycling through several acidification and basification/neutralization steps used in conventional leather processing in favor of combining several steps at once.
In particular, the new process reverses the conventional sequence of steps. The Chennai team points out that both the de-limed pelt and the chemicals used for posttanning are negatively charged. Hence, the de-limed hides can be directly treated with posttanning chemicals prior to the tanning agents.
The process slashes consumption of processing materials, the team says. For example, they claim that the reversed process reduces water consumption by 65%, effluent discharge by 64%, and total chemical consumption by 54%—all with a significant reduction in costs and a product that is on a par with conventionally processed leathers. The Chennai team concedes that a commercial-level study may be required to validate the economic and technical benefits of their methods, but they argue that the sustainability of leather production depends on the development of an alternative system for leather-making.
"The global leather industry is looking for a viable, cleaner leather-processing methodology to overcome environmental and economic constraints," the team notes.
That may mean less demand for pigeon dung in the future, although there is little likelihood that the historic tanneries of Marrakech and Fez will close down soon.