Short answers, pulled from the story.
Vegetable tanning dates back more than 7,000 years and relies on tannins extracted from tree bark to stabilize proteins within the hide. This ancient process transforms raw animal skins into durable materials that resist rot and remain flexible unlike raw hides which putrefy when rewetted.
Chrome tanning was developed in the latter half of the 19th century and uses chromium sulfate to reduce production time from months to approximately one day. This method produces a pale blue material known as wet blue and is now the most common form of leather used globally despite environmental costs.
The government shut down 49 high-polluting tanneries out of 404 in Kanpur India in July 2009 due to severe pollution levels. The waste contains chromium levels of 100 to 400 milligrams per liter and sulfide levels of 200 to 800 milligrams per liter creating toxic sludge that threatens local ecosystems.
Cattle hides constitute about 65% of all leather produced while horse hides make durable shell cordovan and kangaroo leather is used for bullwhips. Lamb and deerskin create soft leather for apparel and reptilian skins like alligator and crocodile offer distinct patterns for luxury goods.
Full-grain leather contains the entire grain layer without any removal of the surface and develops a patina during its useful lifetime. Bonded leather uses leather scraps that are shredded and bonded together with polyurethane or latex onto a fiber mesh with the amount of leather fibers varying from 10% to 90%.
Religious restrictions have shaped the industry for centuries with Judaism forbidding leather shoes on Yom Kippur and Jainism prohibiting leather use since it is obtained by killing animals. In response to these restrictions and environmental concerns many forms of artificial leather have been developed using polyurethane or vinyl coatings applied to a cloth backing.