Tanning turns perishable skin into leather by rewiring its proteins
Raw hide rots. Tanning changes the structure of its collagen for good, locking the protein chains together so the material resists decay. For most of history that meant tree bark, urine and dung. Today, chromium salts do the job for 85 to 90 per cent of all leather.
People were working hides very early: tools for it found at Hoxne in England date back some 400,000 years. A vegetable-tanned shoe around 5,500 years old survives from the Areni-1 cave in Armenia, and Ötzi, the iceman from the Austria–Italy border, was found wearing tanned footwear too. Mehrgarh in Pakistan was tanning between 7000 and 3300 BCE, and around 2500 BCE the Sumerians studded leather onto chariot wheels with copper. The word itself traces to medieval Latin tannum, oak bark.
The old trade stank so badly that tanneries were banished to the poor fringes of towns, and where traditional methods survive they still are. Hides were soaked, pounded clean, then dehaired by steeping in urine, painting with lime or leaving them to putrefy for months. Next came bating, a softening step that relied on bacterial enzymes from pounded-in dog or pigeon dung, or on a bath of animal brains. Bark tannins, cedar oil or alum finished the work.
Modern tanneries follow a similar logic with cleaner chemistry. Hides are first cured in salt, which draws out water until bacteria cannot grow, then go through beamhouse steps. Lime and sulfide agents break the disulfide bonds in keratin so hair lets go, purified enzymes replace dung for bating, and an acid pickle prepares the hide. Chrome tanning grew out of medicine: after 1840 medicine took up soaking gut sutures in chromium solution, and tanners borrowed the trick.
Chromium works by bridging collagen chains, widening the gap between them from 10 to 17 ångströms. It goes in at very low pH so the complexes are small enough to penetrate, then sodium bicarbonate lifts the pH to around 4 to lock the bonds. The result, bluish-grey and called wet blue, takes under a day, against the slower vegetable route favoured for wallets and handbags.
Source: Tanning (leather)
