Mineral Substances.—China clay, sulphate of lime, agalite and other inert mineral substances are important factors in lowering the quality of paper, not so much in promoting the actual deterioration of paper by any chemical reaction with the fibres, as in making the paper less capable of resistance to the influence of atmospheric conditions and ordinary usage. Clay in small, well-defined quantities serves a useful purpose, if added to some papers, because it favours the production of a smooth surface, but when the combination of mineral substances is carried to an extreme, then the result from the point of view of permanence is disastrous. This is well recognised by all paper-makers, and in Germany the limits of the amount of clay or loading in high-grade paper have been rigidly fixed. In the case of imitation art paper, which contains 25 to 30 per cent. of its weight of clay, the strength and resistance of the sheet is reduced to a minimum. The paper falls to pieces if slightly damped, the felting power of the fibres being rendered of no effect owing to the weakening influence of excessive mineral matter. This paper is used chiefly for catalogues, programmes, circulars, and printed matter of a temporary and evanescent character, and so long as it is confined to such objects it serves a useful purpose, being cheap, and suitable for the production of illustrations by means of the half-tone process; but its lasting qualities are of the lowest order. The addition of 10 per cent. of any mineral substance must be regarded as the maximum allowance for papers intended for permanent and frequent use.
Coating Material.—The ingenious method for producing an absolutely even surface on paper by the use of a mixture of clay or other mineral substance and an adhesive like glue or casein brushed on to the surface of the paper, is responsible for many of the complaints about the papers of the present day.
The sole merit of this substance is the facility with which half-tone process blocks can be utilised for the purpose of picture production. Beyond this, nothing can be said. The paper is brittle, susceptible to the least suspicion of dampness, with a high polish which in artificial light produces fatigue of the reader's eye very quickly, heavy to handle, and liable to fall to pieces when bound up in book form.
As the fibrous material is completely covered by mineral substances, it is frequently considered of secondary importance, with the result that the “value” of the paper is judged entirely by the surface coating, with little regard to the nature of the body paper. In such cases, with an inferior body paper, the pages of a book very quickly discolour, and the letterpress becomes blurred.
Analysis of a Typical Art Paper.
| — | Per Cent. by Weight. | — | Volume Composition per Cent. |
| Fibre | 77·5 | Fibre | 68·3 |
| Ash, etc. | 22·5 | Ash | 12·0 |
| Air space | 19·7 | ||
| —— | —— | ||
| 100·0 | 100·0 | ||
Rosin.—The presence of an excess of rosin is a well-known factor in the disintegration of the paper, even when the fibrous composition is of the highest order. The decomposition is largely due to the action of light, many experiments having been made by Herzberg and others to determine the nature of the reactions taking place. One of the chief alterations is the change brought about in the ink-resisting qualities of the paper.
The actual character of the chemical reactions as far as the effect on the fibre is concerned is not accurately known. The degradation of a hard-sized rosin paper by exposure to strong sunlight, for example, is probably due to the alteration in the rosin size, and not to any material change in the cellulose. It is hardly conceivable that in a pure rag paper sized with rosin and yielding readily to ink penetration, after about one year's exposure to light, the cellulose itself had undergone any chemical changes capable of detection.
Gelatine.—Papers properly sized with gelatine are preferable to those sized with rosin for the majority of books and documents preserved under normal circumstances. But the nature of a tub-sized paper may be, and often is, greatly altered by unusual climatic conditions. In hot, damp countries papers are quickly ruined, and high-class drawing papers sized with gelatine often rendered useless. The change is scarcely visible on the clean paper, and is only observed when the paper is used for water-colour work, the colour appearing blotchy in various parts of the sheet where the gelatine has been decomposed by the united action of heat and damp.