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Scouring (30-45 minutes) - Surfactants are used at 38oC to remove dirt and grease (lanolin) from wool.
Pickling (16 hours) - Prior to tanning, skins are pickled in a solution containing acid and salt. The salt is added to prevent swelling of skins by acid. The internal pH of skins is lowered to approximately 2.8-3.0, thereby preparing skins for penetration by tanning agent.
Tanning (16 hours) - Tanning is generally achieved by using chromium salts which form cross-links with collagen, stabilising skin structure and preventing putrefaction. The tanning step is conducted at about 25oC and pH 2.5-3.0 to allow for penetration of chrome. Once penetrated, chrome is fixed to collagen by raising pH to about 3.6 using sodium bicarbonate, and heating to about 35-40oC. This step raises shrinkage temperature of skin from about 60oC to around 100oC.
Wool Dyeing / Fatliquoring (3-4 hours) - After tanning, wool may be dyed a variety of colours. Wool dyeing is performed at about pH 4.5-6 and at 60-65oC, "Pelt reserve agents" being added to prevent wool dye staining pelt. After dye is exhausted, pH is lowered to about 4.0 to fix dyes to wool, and fatliquor is added to bath. Fatliquors are emulsified oils which are used in leather manufacture to lubricate collagen fibres, allowing them to move freely when skin is dried, thereby imparting softness to skin.
Drying (4-24 hours) - Once tanned and wool dyed, skins are then dried in heated forced-air dryers by "toggling" skins to a frame. Drying is conducted at temperatures of 50-80oC.
Drycleaning (4-24 hours) - The dried skins are "drycleaned" in either white spirit (a high-boiling petroleum fraction) or perchloroethylene. The drycleaning removes natural fat and grease present within skins.
Pelt Dyeing / Syntanning - Once pelt grease is removed, skins are returned to paddle for pelt dyeing. This step is performed at low temperature (less than 30oC) in order to minimise staining of wool by pelt dye. After pelt dyeing, skins are "syntanned" using synthetic tanning agents in order to give pelts added fullness and firmness. The skins are then redried.
Step 2. Finishing
Once all tanning and processing steps have been completed, skins are ready for final finishing prior to being used in ugg boot manufacture.
Staking - After "conditioning" skins to approximately 20% moisture content, skins are staked in a staking machine. This softens and stretches skin pelt, and puts a "nap" on pelt surface.
Combing / Ironing / Clipping - The wool is combed in a combing machine to remove tangles and any burrs or grass seeds present in wool. An iron (a hot, 150oC revolving cylinder) straightens wool, removing natural wool crimp. Finally, a cylinder clipping machine is used to clip wool pile to desired length, usually about 12-15 mm.
Step 3. Manufacturing
Panels are cut from skins using a "clicking press". Once cut, panels are sewn together using industrial sewing machines, then sole is glued to boot. Once this is done, boots are ready for sale!
Hopefully this article has helped you understand a little on the process of sheepskin hide preperation and ugg boot manfacture. Please feel free to browse our range at:
Australian Ugg Boots