Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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splitting

1. The process of dividing a hide or skin horizontally into two or more layers. Before the introduction of splitting machines, a hide was reduced in thickness by SHAVING (2) after tanning. The modern splitting machine has a flexible knife in the form of an endless band moving at high speed over a pair of large pulleys. When the leather to be split is placed on the bed of the machine and pushed forward, it is gripped by pairs of rollers and propelled forward in such a manner that the band knife cuts it into two layers. The propelling rollers are made up of a large number of small ring rolls with rubber centers which allow for initial variations in the thickness of the hide. The knife can be adjusted to slice through the thickness of the hide at any desired depth below the grain surface by adjusting the level of the rollers. While the machine is in operation, the band knife is automatically sharpened by grinders on its underside.

Splitting may be done while the hide is in the limed or partially limed condition, or following tannage. Hides are sometimes split in the limed condition so that a grain layer of desired thickness may be taken off and tanned separately from the splits. A gain in yield of the resultant grain leathers may be expected as a result of the slight "letting-out" of the fiber structure consequent to splitting before tannage. In certain tannages, such as vegetable tannage of leather for purposes not requiring the full thickness of the hide, savings in materials and shortening of processing time may also be effected.

When the hides are to be split in the unhaired or limed condition, the stock is plumped to a condition of sufficient firmness or solidity so as to be fed through the splitting machine smoothly in order to split off the grain layer of desired thickness uniformly across the full width and length of the hide. The fibers have a more vertical position when the hide has been plumped and will stand up against the beveled cutting edge of the band knife with less tendency to leave gouges or uneven ridges and valleys.

Although it may be highly desirable (or necessary) to reduce the substance of a hide or skin by splitting, the operation may also have undesirable effects. The strength of leather is not uniform throughout its thickness, and the grain layer, which contains the thermostat mechanism of the living skin, is relatively weak. As nearly all of the strength of leather lies in the corium, the strength of a grain split will depend upon the thickness of the corium layer which it contains. Splitting always causes a loss of strength per unit width and the sum of the strengths of the splits is always less than the strength of the unsplit leather. In fact, the breaking strength of the grain split of a vegetable-tanned calfskin, for example, will be between 3.5 and 12% of the unsplit skin, while the breaking strength of the flesh split will be between 54 and 70% of the unsplit skin; thus the leather loses between 18 and 42.5% of its strength because of splitting.

2. Separating the plies of a paper or board. (291 , 306 , 363 )




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