246 PAINTING MATERIALS
principally for poplar, birch, and similar hard-woods. In its general lines it is not
very different from that used for esparto and straw. (A full description is given by
Sutermeister, pp. 106 ff.) Wood that is to be made into pulp by any of the three
chemical processes is first cut into logs eight to twelve inches in diameter and four
feet long; these are reduced to chips each under an inch in size. The sulphate
process is essentially alkaline, and the chips go into a soda bath. The lost alkali,
however, is made up in this method by sodium sulphate instead of soda ash. The
sulphite process is acid. In this treatment of the chipped wood, intercellular
matter is decomposed by boiling at high temperatures in the presence of sul-
phurous acid. Magnesia or lime is added and the steam pressure is 90 to 150 pounds
per square inch. Sulphite pulp has strong and hard fibres whereas the soda pulps
are comparatively soft and mellow.
Paper, conservation and treatment. It has been noticed (see also Paper) that
pure cellulose fibre is the most resistant and permanent part of paper. This is
affected only by very high temperatures, by acids, alkalis, strong bleaching
solutions (mainly oxidizing agents) and a few other means of destruction most of
them very rare under normal housing conditions. Added materials in paper are
apt to cause general discoloration, brittleness, and an aggravation of mold or
bacterial growth. To prevent autoxidation of a resin, for instance, is impossible
by any methods known, but many other ill effects on paper can be prevented or
kept safely down by general care in the mounting, exhibition, and storage of such
objects. High humidity is to be avoided and dry, well-aired storage is indispen-
sable. Similar conditions must be maintained for exhibition. Bad mounting has
been responsible for much loss and many blemishes in. paper objects* Drawings,
prints, or water colors, if they have good care, are given mats, usually of the ' win-
dow ' type, and made of rag pulp. They are attached to the mats only by small
hinges of adhesive at the upper corners, are kept, with slip papers for protection,,
in light, dry boxes, and, when exhibited, are put under glass.
Certain types of treatment for paper have a value that is largely preventive.
They are to stop any tendency toward deterioration that may have been ob-
served. Fumigation is one of these and is done largely to destroy any mold or
bacterial growth or to kill spores before they develop. Thymol has been much
used for this purpose and so has formaldehyde. Another preventive treatment is
removal of paper from mounting boards that tend to discolor or embrittle it.
Restorative treatment is also frequent. Removal of grease spots or of dirt can
usually be done with organic solvents. Removal of water stains or foxed marks—
discoloration of the fibre itself—is more difficult and less certain, but there are
now established methods for it. In all methods oxygen is the active agent. It
has been employed as a vapor, the evaporation from a plaster block saturated
with hydrogen peroxide and placed with the paper in a sealed chamber being
enough to effect bleaching. As a rule, however, the paper is immersed in a bath
of bleaching solution followed by a neutralizing bath and careful washing. One