DURABILITY AND STORAGE	339
a thin tissue of good quality over it, but it is doubtful how long a newspaper
will be readable even with this plan.
On the other hand, news-sheets printed 100 years ago on hand-made rag
printing paper are as good as ever. One of the greatest factors in the deteriora-
tion of paper is the use of resin size. Resin, when exposed to light and air,
becomes friable and is reduced to dust, and all papers sized with resin lose their
ink resistance, colour, strength and finish. When resin size is supplemented
by a coating of gelatine the life of the paper is lengthened, by how long we
cannot tell yet; but it is certain it will not in 100 years be in the same perfect
preservation as when it was made, as the hand-made papers of a century ago
are at the present day.
By substituting silicate of soda for resin we get a much more durable and
light-resisting paper, such as may be used for printing with printer's ink.
As regards dyes, many cannot stand light at all without fading out and
taking all the good of the paper with them. The aniline or coal-tar products
are mostly of this class. Smalts blue is, of course, a permanent colour, so also
in a great measure is good ultramarine blue and most of the mineral colours,
except those which owe their shades to oxides of iron, and which tend to go
darker in colour and destroy the cellulose.
Chemical residues of alkalis, bleach, acids and possible combinations of
chemical traces in mill water many cause or hasten deterioration of paper. A
very curious effect is produced by age in the particles of copper or brass that
are found in a great many good papers that have been beaten with bronze roll
bars, or that have been made from rags in which pieces of copper have been
retained. In a few years from each of these particles irregular lines spread
outwards through the fibres, sometimes covering a space J inch in diameter.
These are called 'dendritic growths'. The products of the oxidation of the
metal travel along the fibres and form fern-like designs.
While the paper-maker knows that many of his products are more or less
perishable, he is also aware that some or all can be greatly improved by being
kept in stock for varying lengths of time, under proper conditions, before being
sent out from the mill. All machine-made papers are produced under a certain
amount of strain, and the fibres ought to get time to relax.
Most printing papers are improved by being kept in a cool, dimly-lighted
stock room for a few weeks. Papers intended for litho work especially will
benefit by absorbing atmospheric moisture and getting rid of their electric
charge.
On gaining a normal air-dry condition finish goes off more or less. A
plate-glazed finish is most permanent; next in order is a super-calender finish.
Any glaze or finish obtained by hot rolls is most liable to- go back. But a