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

1. Leather TanningSpencer Black

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: SoakingHides 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 RemovalHair is removed by chemical digestion Lime and sodium sulphide solution Hairless hides are then neutralized with acids and treated with enzymesRemoves depositsIncreases softness(C.C. Leathers Inc., “History and Process of Leather”)

5. Step 3: DelimingHairless hides are then neutralized with acids and treated with enzymesRemoves depositsIncreases softness(C.C. Leathers Inc., “History and Process of Leather”)

6. Step 4: PicklingHides 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: TanningTanningTwo main methods: Chrome and VegetableChrome is more commonVegetable tanning produces stiffer leathers; chrome tanning produces softer leather(C.C. Leathers Inc., “History and Process of Leather”)

8. Step 5: Chrome TanningMost upholstery, shoes, garments, bagsHides placed in rotating drums and washed in a chemical containing trivalent chromeAfter 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 TanningUsed on shoe soles, luggage, belts, and some upholsterySlower, 2-4 daysUses tannic acid, which is extracted from tree bark (C.C. Leathers Inc., “History and Process of Leather”)

10. Step 6: DyeingPlaced 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: RollingRolled through a machine to make strongerDried by hanging or dry tumbling (C.C. Leathers Inc., “History and Process of Leather”) http://www.cotance.com/process.htm

12. Step 7: FinishingFinished 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 ImpactAir pollutionammonia gas, hydrosulphuric gas and volatile organic compoundsWater contaminationresidual baths for hide treatment and washings containing chemical productsContamination of the soilflesh, hairs, hide chippings and scrapingsLarge amount of water consumptionChrome has high level of contamination (Siddharth Singh, “Project Report on Environmental Impact Assessment (in Leather Industry)”)

14. Ways to ReduceReuse waterRecover and reuse chrome*New process- improved chrome syntan with more than 90% uptake of chromeUse ammonia-free delimingUse less salt (although will decrease “shelf-life”)(Siddharth Singh, “Project Report on Environmental Impact Assessment (in Leather Industry)”)

15. Vegetable vs. ChromeStudy of toxicity was evaluated by multiple bioassays including developmental defects and loss of fertilization rate in sea urchin embryos and sperm, and algal growth inhibitionGiovanni Pagano and Giuseppe Castello, “Complex Mixture-Associated Hormesis and Toxicity: The Case of Leather Tanning Industry”