Clay is of value not only to increase the weight but also to
render the paper more opaque, so as to prevent type or illustrations
from showing through, while at the same time it makes possible a
smoother surface by filling the pores in the paper. But while it adds to
the weight, clay must, of necessity, weaken the paper. In engine-sizing,
which is done in the beater, the size is thoroughly incorporated with
the fibers as these revolve or flow around the engine. This sizing
renders the paper more nearly impervious to moisture. The difference
between a paper that is sized and that has a repellent surface which
prevents the ink from settling into it when it is written upon, and an
ordinary blotting-paper with its absorbent surface, is due entirely to
the fact that the former is most carefully treated with sizing both in
the beating engine and in the size tub or vat referred to later, whereas
in the latter paper it is omitted. If the paper is to be tinted or
body-colored, colors made from aniline are generally used. Only in the
highest grade of writing-paper and in some few papers that demand colors
fast to the light is any other order of coloring matter employed.
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