Properties of paper
Paper is made of cellulose fibres. These fibres are hygroscopic which means they
‘suck’ moisture from the surrounding atmosphere. Water has the effect of
‘plasticising’ the cellulose fibres, which is another word for weakening them.
Ordinary paper, when wet, has very little strength. The inter-fibre bonds relax,
so weakening the sheet. Toilet tissue can claim ‘wet-strength’ if it has just
15% of its strength remaining when it is saturated.
A book has to adapt to its environment and in the process can change shape. If
you live in a northern climate, have a look at the map you keep in the car. The
high humidity normally encountered in cars makes the pages swell and buckle.
One symptom that is frequently observed is ‘cockling’. Cockling is the wrinkled
or puckered condition found in a sheet of paper or board and is caused by
non-uniform drying and shrinkage. When a book comes from the printers, the pages
can often develop ‘waves’ in the pages. Printers go to great lengths to ensure
that paper is exactly the right size so that the images are all in precisely the
right place on the sheets they print. Some print processes even allow for
spraying a fine mist onto the surface of the paper to ensure the correct
dimensions.
A book normally adjusts within a week to domestic conditions and the cockling
vanishes. However it is worth storing the book flat while it is making this
adjustment.
Cockling is magnified if the grain of the paper is not parallel to the binding
edge. Paper expands more across the grain, and with machine-made papers the
width to length expansion ratio can be 10:1. A typical piece of paper will
expand by 1-2% when wet.
Paper benefits from a small percentage of moisture. The moisture content in
paper varies from 2-12% depending on the type of pulp and processing used, the
degree of refining and the chemicals used. A ‘normal’ water content is between 5
and 7 percent by weight. A sheet of wet paper might contain 20% moisture.
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