There are two groups of operations, which affect the leather thickness.
- Machine operations
- Chemical operations
Variations in any or all of these operations can cause variations in the final thickness of leather.
The obvious operations are splitting and shaving. Splitting whether done after liming or in wet blue stage reduces the leather thickness by splitting it into two layers, grain and flesh split.
Shaving is mostly done in wet blue and in some cases in crust or after finishing. Shaving carves out material from the leather and thus reduces the thickness.
There are a number of splitting, shaving parameters, which can affect the thickness of final leather such as the water content of the leather before the operations, machine settings etc.
Buffing is another mechanical operation that removes material (with sand paper/emery paper).
The reduction in thickness is not much. However, in case of products involving heavy buffing (corrected grain) or repeated buffing (Nubuck, suede) this may have an impact.
Less obvious are operations that press the leather and reduce the apparent thickness.
- Setting is done after the drum operations. By applying pressure this machine removes the wrinkles and flatten out the leather.
- Drying option can influence the thickness. For example any drying which involves stretching out and/or pressure (such as, vacuum drier, toggle drier, paste drier etc) will reduce the apparent thickness.
- Embossing, Ironing/Plating, or Printing use pressure to flatten out or apply specific patterns on the leather surface.
Any variations and lack of uniformity in these operations may cause deviations in final thickness from the specification.
Some operations like dry drumming (milling) can relax the leather structure showing minute increases in apparent thickness of leather.
Finally, the moisture content of the leather also has an impact on the thickness. You can think of a number of situations in which leather is exposed to different humidity conditions during leather making or during its use. All these situations have to be taken into account for controlling the leather thickness.
As we know leather is a fiber net work. If we cut a leather fiber and look under magnification we can see that they are made of fibrils which are in turn made of micro fibrils.
- Liming can influence the final thickness of leather
In raw hide, the space among these fibrils and fibers are filled with water and other material. During liming the fiber structure is relaxed by water entering the network. Long and thorough liming will ensure that the network is opened into fibrils/micro fibril levels. Otherwise, they might be sticking together. Also during liming all other materials in the voids of fiber structure are removed.
If the opening up is complete, tanning and re-tanning material can enter the voids in the fiber network and plumb the leather increasing its thickness. Otherwise, the leather will be relatively thinner.
Any problem in thickness in the finished leather may be related back to improper or inconsistent and non-uniform opening-up during liming.
- Tanning and re-tanning material can affect the leather thickness.
Some tanning material can fill the fiber network and increase the leather thickness others can not. Chrome tanning is a non-filling type tanning. Therefore, the fiber network in wet blue after chrome tanning is filled mostly with water. When dried the fibers will stick together and the leather will be thin and hard.
Tanning materials such as vegetable tannins, syn-tans, resins etc can fill up the fiber net work and keep the fibers apart increasing the apparent thickness of the final leather.
Thus leather processes, which use these types of re-tanning material, can plumb up the leather and give higher thickness compared to those processes, which do not use these materials.