The artist is frequently compelled in such cases to put a layer of heavy white colour on the sheet of paper before proceeding to paint the picture.
The storage of books under favourable conditions has a great deal to do with the permanence of the paper, and the degradation of a paper in relation to the tub-sizing qualities is much hastened by the presence of moisture in the air.
Starch.—The same is true of starch, which is largely employed as a binding or sizing material in paper. The degradation of gelatine, starch, and similar nitrogenous substances is due to the action of organisms, and the following experiments, suggested by Cross, are interesting in this connection.
If strips of paper are put into stoppered bottles with a small quantity of warm water and kept at a temperature of about 80° F., fungus growths will be noticed on some of them after the lapse of fourteen days. Rag papers sized with gelatine will show micro-organisms of all kinds. A pure cellulose paper, like filter paper, will not produce any such effects. The result in the first case is due to the nitrogenous substance, viz., the gelatine used in sizing, since the two papers are identical as far as the cellulose fibres are concerned. High-class wood pulp papers, unless sized with gelatine, would not show similar results. The action of the organisms upon the nitrogenous material by a process of hydrolysis is in the direction of the production of soluble compounds allied to the starch sugars capable of being assimilated by organisms.
The cellulose of esparto and straw are readily attacked, and it is on this account that the tissues of the various straws are digested more or less when eaten by animals. It is for this reason that the celluloses from straw and esparto are inferior to the cotton cellulose in producing a paper likely to be permanent.
Chemical Residues.—The necessity for manufacturing a pure cellulose half-stuff is fully recognised by paper-makers. This was not the case in the early days of the manufacture of wood pulp, for it is a matter of common experience that many of the books printed on wood pulp paper between 1870 and 1880 are in a hopeless condition, and it is quite easy to find books and periodicals of that date the pages of which crumble to dust when handled. This serious defect has been proved to be due to the presence of traces of chemicals used in manufacture which have not been thoroughly removed from the pulp.
The precautions necessary in bleaching pulp by means of chloride of lime, in order to prevent (1) any action between the fibre and the calcium hypochlorite, (2) the presence of residual chlorine or soluble compounds derived from it, and (3) the presence of by-products arising from the use of an antichlor, are also well known to paper-makers. The subject has been closely studied by chemists, who have shown that the deterioration of many modern papers may be ascribed to carelessness in bleaching.
The questions relating to the chemical residues of paper can only be adequately dealt with by a discussion of actual cases which arise from time to time. There are certain conditions in manufacture, common to all papers, which may give rise to the presence of chemical residues, of which two have already been mentioned.
The acidity of papers is frequently quoted as an instance. It is true that the presence of free acid in a paper is most undesirable, as it seriously attacks the cellulose, converting it into an oxidised form. This in course of time renders the paper so brittle as to destroy its fibrous character.
The change is brought about by the acid, which itself suffers no material alteration, so that the process of deterioration is continued almost indefinitely until the cellulose is completely oxidised. Most papers, however, show an acid reaction when tested with litmus, the usual reagent employed by those not familiar with the proper methods of testing paper. All papers which have been treated with an excess of alum for sizing purposes would show an acid reaction with litmus without necessarily containing any free acid.