Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations:
- wetting back- semi-processed leather is rehydrated.
- sammying – 45-55%(m/m) water is squeezed out the leather.
- splitting – the leather is split into one or more horizontal layers.
- shaving – the leather is thinned using a machine which cuts leather fibres off.
- neutralisation – the pH of the leather is adjusted to a value between 4.5 and 6.5.
- retanning – additional tanning agents are added to impart properties.
- dyeing – the leather is coloured.
- fatliquoring – fats/oils and waxes are fixed to the leather fibres.
- filling – heavy/dense chemicals that make the leather harder and heavier are added.
- stuffing – fats/oils and waxes are added between the fibres.
- stripping – superficially fixed tannins are removed.
- whitening – the colour of the leather is lightened.
- fixation – all unbound chemicals are chemically bonded/trapped or removed from the leather
- setting – area, grain flatness are imparted and excess water removed.
- drying – the leather is dried to various moisture levels (commonly 14-25%).
- conditioning – water is added to the leather to a level of 18-28%.
- softening – physical softening of the leather by separating the leather fibres.
- buffing – abrasion of the surfaces of the leather to reduce nap or grain defects.