Tanning is the process by which animal hide ('raw hide') is transformed from a material which will rot and decay, into leather. It is one of human’s earliest technological activities, dating from at least 7,000 BC.
Traditionally, tanning used tannin, an acidic substance derived primarily from oak trees. Vegetable tanning is a complex, lengthy process which can take up to two months to complete, consequently vegetable tanning now accounts for only 15% of current leather production. Chrome tanning was invented in 1858 by a German technologist, Friedrich Knapp and a Swedish scientist, Hylten Cavalin but was first patented by an American scientist, Augustus Schultz.
The Chrome Tanning process can be completed in a day and swiftly became the dominant form of tanning. Today, it accounts for 95% of shoe leather production, 70% of leather upholstery production and 100% of leather clothing production. Sodium dichromate is the principal raw material used in the Chrome Tanning process, it is a carcinogen and causes damage to blood, kidneys, eyes, heart and lungs. Sun drying and boiling can oxidise and convert the chromium 111, used in the tanning process, into the highly toxic hexavalent chromium, chromium V1.
The main threat to the environment results from the dumping of liquid and solid waste that contains leftover chromium. In some commercial operations 50% of the chromium used in the tanning process is finding its way into the local environment. The run off and scraps are then consumed by animals and subsequently by humans: 25% of the chickens in Bangladesh were found to contain harmful levels of hexavalent chromium. According to the ‘Journal of Cleaner Production’ 2014: ‘Highly polluted sediments resulting from discharge of chemicals adversely affect the ecological functioning of rivers. High concentration of heavy metals has been found in the river Ganga and its tributaries. Increased salinization of rivers and groundwater has led to the loss of agricultural production and reduced the quality of drinking water in Tamil, Nadu, India’.
When chromium enters the water system in large quantities it can cause respiratory problems, infertility and birth defects. The use of chromium also puts workers at risk during the tanning process. If the dust, which is produced when chrome tanned leather is buffed or ground up, is inhaled, it can cause respiratory problems and increase the risk of lung cancer. When chromium comes in contact with the skin it causes dryness and cracking and sores, known as ‘chrome holes’. Even chrome tanned leather in its finished form poses an environmental threat: if the leather seats from scrapped cars are burnt then toxic chromium V1 may be released into the atmosphere. This can even occur as a result of car upholstery being exposed to prolonged strong sunlight. New, ‘green technologies’ do hold out the prospect of converting toxic waste into useful products and purifying and recycling contaminated waste water, however it is unlikely that in poorer countries, where the bulk of chrome tanning takes place, there will be an investment in costly alternative ‘green technology’.
The United Nations Industrial Development Organization report, ‘Chrome Management in the Tanyard’ argues that: ‘even though the chrome pollution can be reduced by 94% on introducing advance technologies, the minimum residual load 0.15kg/t can still cause difficulties when using landfills and composting sludge from wastewater treatment on account of the regulations currently in force in some countries’. Clearly, by using the best available technology the Chrome leather industry can become more environmentally friendly but leather production using traditional vegetable tanning will always be the more environmentally clean process.
The tanning process, carried out in our own tannery, is entirely chrome-free and conforms to strict pollution regulations. You can read more about our workshop and process here.