All papers are improved in strength by glazing up to a certain point, but over-glazing crushes the paper, renders it brittle and liable to crack. Unfortunately, the maximum strength of a paper is generally reached before the maximum of finish, with the result that the former is frequently sacrificed to the latter. The usual result of glazing is found in an increase of 8 to 10 per cent. in the tensile strength, but a diminution of elasticity to the extent of 8 to 10 per cent. With supercalendered magazine papers, the high surface is imparted for the sake of the illustrations which are produced by methods requiring it. The addition of considerable quantities of clay or mineral substances improves the finish, so that the question of the relation of glazing to strength, surface, and loading is one which affects the subject of deterioration of paper very materially. With writing paper the false standard of an "attractive" appearance is almost universally accepted by the public as the basis of purchase without any reference to actual quality. _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.