From Paper-mill to Pressroom

by William Bond Wheelwright

Again, a great difference is noticeable in the flexibility of books, dependent largely on whether the grain runs parallel or at right angles to the binding. If flexibility is desired, the grain should run parallel to the back of the binding. Occasionally a wide-paged pamphlet, especially of light-weight paper, is improved by the rigidity to be gained from having the fibers run at right angles to the binding. It is also true that this increases the strength of the binding, as the sewing or wire stitching passes around more fibers than if the grain ran up and down the page.

Not infrequently does the middle signature of a pamphlet pull loose from the binding. Usually in such cases the paper is not strong anyway, but it could have had more resistance had the grain run at right angles to the binding.

The tensile strength of a strip of paper is greater with the grain, but its elasticity is greater across the grain.

A convenient way to ascertain the direction of the grain in papers that do not show it clearly by folding is to cut two narrow strips a few inches long, hold them by one end so that they coincide. When held horizontally, if the loose ends do not part, it indicates that the lower paper has its grain in the long dimension. If the lower paper has its grain crosswise, the loose end will sag away from the top strip, because, as above remarked, a paper is more flexible across the grain. This test may be applied either to sized or unsized papers.

Another test is to cut a small square and moisten one side; the paper will curl into a little cylinder and the grain runs parallel to the length of the cylinder. This test cannot be applied to an unsized paper.

This leads us to a consideration of the effects of moisture and humidity on paper.

It will be recalled from the chapter on Paper-Making (No. [VI]) how plastic paper is in its moist stage, and how tenacious of water are the cellulose fibers. It will also be recalled that there is considerable shrinkage across the web of the paper from the time it leaves the wire to the moment it is reeled. In fact, the very thing which makes paper-making a possibility is the shrinking of each individual fiber, occasioned by the expulsion and evaporation of the water, which has served as a carrier from the machine chest to the wet end of the machine.

This propensity of each individual fiber does not cease when the paper is made, but persists forever. A cellulose fiber will absorb moisture from the air in proportion to the relative humidity, just as the hair in a barometer is continually shrinking or expanding as the weather changes.

A definite percentage of moisture is normal to a cellulose fiber in proportion to the moisture in the air. The fiber swells as it absorbs, and shrinks as it gives off water.

Herzberg gives as the results of investigation with a good writing-paper made of rags, sized with rosin, the following report of the percentage of moisture retained under various degrees of relative humidity: