Leather tanning is the process of treating raw animal hides to prevent decay and make them suitable for use. It involves soaking, hair removal, pickling, and tanning the hides through either the more common chrome tanning process or vegetable tanning. Chrome tanning uses chemicals containing trivalent chrome to tan the hides over 8 hours, while vegetable tanning uses tannic acid extracted from tree bark over 2-4 days. After tanning, the hides undergo further processing like dyeing, rolling, and finishing to produce leather. Tanning can cause environmental impacts through air and water pollution from chemicals used. Methods to reduce impacts include reusing water, recovering chrome, using more efficient processes, and choosing vegetable tanning
2. What is tanning? Leather is made from animal skins or hides which have been chemically treated to preserve quality and natural beauty. The chemical procedure used to ready raw animal hides for use is called "tanning." A piece of hide or skin which has been tanned produces a strong, flexible leather which is able to resist decay and spoilage. (http://www.essortment.com/all/leathertanning_rdcu.htm)
3. Step 1: Soaking Hides are re-hydrated or re-soaked and washed in large rotating drums (C.C. Leathers Inc., “History and Process of Leather”) http://www.cotance.com/process.htm
4. Step 2: Hair Removal Hair is removed by chemical digestion Lime and sodium sulphide solution Hairless hides are then neutralized with acids and treated with enzymes Removes deposits Increases softness (C.C. Leathers Inc., “History and Process of Leather”)
5. Step 3: Deliming Hairless hides are then neutralized with acids and treated with enzymes Removes deposits Increases softness (C.C. Leathers Inc., “History and Process of Leather”)
6. Step 4: Pickling Hides are soaked in a solution of water, salt, and hydrochloric (or sulphuric) acid (C.C. Leathers Inc., “History and Process of Leather”) http://www.cotance.com/process.htm
7. Step 5: Tanning Tanning Two main methods: Chrome and Vegetable Chrome is more common Vegetable tanning produces stiffer leathers; chrome tanning produces softer leather (C.C. Leathers Inc., “History and Process of Leather”)
8. Step 5: Chrome Tanning Most upholstery, shoes, garments, bags Hides placed in rotating drums and washed in a chemical containing trivalent chrome After 8 hours, the chrome is “fixed” with an alkaline chemical (sodium carbonate) (C.C. Leathers Inc., “History and Process of Leather”)
9. Step 5: Vegetable Tanning Used on shoe soles, luggage, belts, and some upholstery Slower, 2-4 days Uses tannic acid, which is extracted from tree bark (C.C. Leathers Inc., “History and Process of Leather”)
10. Step 6: Dyeing Placed in rotating drums with hot water, dyes, and synthetic tanning materials to obtain desired color (C.C. Leathers Inc., “History and Process of Leather”) http://www.cotance.com/process.htm
11. Step 6: Rolling Rolled through a machine to make stronger Dried by hanging or dry tumbling (C.C. Leathers Inc., “History and Process of Leather”) http://www.cotance.com/process.htm
12. Step 7: Finishing Finished with coatings of acrylic, urethane, vinyl, wax, nitrocellulse, dye, or other materials (C.C. Leathers Inc., “History and Process of Leather”) http://www.cotance.com/process.htm
13. Environmental Impact Air pollution ammonia gas, hydrosulphuric gas and volatile organic compounds Water contamination residual baths for hide treatment and washings containing chemical products Contamination of the soil flesh, hairs, hide chippings and scrapings Large amount of water consumption Chrome has high level of contamination (Siddharth Singh, “Project Report on Environmental Impact Assessment (in Leather Industry)”)
14. Ways to Reduce Reuse water Recover and reuse chrome* New process- improved chrome syntan with more than 90% uptake of chrome Use ammonia-free deliming Use less salt (although will decrease “shelf-life”) (Siddharth Singh, “Project Report on Environmental Impact Assessment (in Leather Industry)”)
15. Vegetable vs. Chrome Study of toxicity was evaluated by multiple bioassays including developmental defects and loss of fertilization rate in sea urchin embryos and sperm, and algal growth inhibition Giovanni Pagano and Giuseppe Castello, “Complex Mixture-Associated Hormesis and Toxicity: The Case of Leather Tanning Industry”
