Ageing:
After chrome tanning tanned leather are taken out from tanning bath & piled-up on a wooden horse & kept for 2-3 days.
It allows greater fixation of chrome complex & uniform distribution of Cr throughout the leather.
The chrome complex goes on a changing by liberating more & more acid. Some of the acid fixed to basic side of the protein. Actually the acid is increased due to hydrolysis of the chrome-complex.
The basicities of Cr-complex/compound present inside the leather are increased. So, Cr-fixation & tendency to form new cross linkage are increased.
Conditioning:
After Cr-tanning & ageing the leather are conditioned to the required moisture content & to make them suitable for certain mechanical operations to achieve the desired thickness. The conditioning of the leather is known by the term "sammying" - which aims at re-tanning certain moisture content about 40-45% of its weight. This is done by partial drying to reduce moisture content either by open drying or by mechanical means using a machine called sammying machine. It should be noted that uniform moisture content is just enough to make them suitable for further mechanical operations to obtain the required thickness.
The sammying machine are used in tannery to squeeze the hides/skins. The combine action of cylinders covered by absorbent material & pressure the liquid of the bath etrappend into the fibres are removed. The sammying machine model are alternative type (two action) or continuous (single action).
After sammying, the leather are further processed in machines to obtain the correct thickness uniformity throughout the leather. This achieved by using mechanical means-using splitting & shaving machines.
Splitting:
Usually splitting is done in the splitting machine, keeping a slightly higher thickness uniformity through out the hide/skin.
Shaving:
Shaving is done later to obtain the uniform desired thickness in the shaving machine & checked with the thickness gauge for uniformity.