for use with light half-tone blocks may pick off when employed with blocks giving heavy dark shades. The production of a dense black print is always a severe test on an art paper.

Influence of Mineral Matter on Printing Results. In a hand-made paper containing little or no mineral matter, the picture is blurred and indistinct, the depression of the water-mark being brought into prominence by the absence of ink along the lines of the mark itself. (See Fig. 77.)

In a cheap printing paper with a small proportion of 5 to 10 per cent. of loading, there is a slight improvement in the result.

With a machine-finished paper containing about 16 to 18 per cent. of loading or mineral matter a marked improvement is obtained.

With an imitation art paper containing about 30 per cent. of mineral matter, much of which lies on the surface of the sheet, the picture shows more detail.

Finally, in the art paper having surfaces composed entirely of mineral matter, the clearness of the picture leaves nothing to be desired. (Fig. 78.) But it must not be overlooked that the surface is artificial and the paper itself plays quite a secondary part, being sandwiched in, as it were, between two layers of mineral coating. In consequence of this the quality of the "body" paper is often considered a matter of no importance, and many art papers are merely composed of ingredients in which the conditions essential for durability are chiefly remarkable for their absence.

The rationale of the process is therefore perfectly clear. In a normal highclass writing paper, the absence of any mineral matter absolutely precludes an

even surface, and the glazing or polishing of these surfaces as produced by processes perfectly familiar to the papermaker, viz., the use of the supercalender, the plate-glazing rolls, and so on, is not sufficient to give the desired effect. Consequently, the print is blurred, indistinct, and lacking in detail, and the curious outlining of the water-mark is a clear indication of unevenness of surface. The examination of the surface of a high-class linen paper reveals the existence

of numerous fibres crossing and recrossing one another in every conceivable direction. It is obvious that since even minute fibres occupy a certain volume, the surface of the paper at all such points of intersection must be slightly above the spaces between them, and that no amount of pressure will bring them into one common plane. (Fig. 79.)

Now the addition of a small amount of mineral matter tends to fill up the interstices of the paper. The particles of mineral matter, such as china-clay, which is the usual loading for cheap papers, distribute themselves around the fibres and fill up, more or less, the air spaces, and thus reduce the irregularity of

surface. The pressure due to the action of the rolls in the supercalender reduces the minute undulations on the surface of the sheet just as in the case of a high-class rag paper, but the interstices are not so pronounced. As the percentage of mineral matter is increased the improvement in the paper judged from the printing standpoint is noticeable, and apparently the particular characteristic necessary in an "art" paper is only to be obtained by a complete

degradation of the actual paper by a mineral substance which has nothing in common with the fibrous constituents.

The "imitation art " paper, which is a compromise between an ordinary cheap printing paper and the so-called high-class "art paper," contains a very high percentage of mineral matter which is brought up to the surface by a peculiar process of treatment, so that the sheet of paper is rendered more suitable for half-tone printing. To use the word "paper" in connection with a sheet which consists of 35 per cent. china-clay held together by a few low-class vegetable fibres is an abuse of terms, but it is one which has some commercial significance.

Suggestions as to Surface. With such a purely artificial surface upon a sheet of paper, which can easily be removed by friction with the moistened finger, it is evident that durability of the pictures is scarcely to be expected.

The resistance of the art paper to the ravages of time is a matter which should not be difficult to determine, since all the elements for rapid depreciation are present in the shape of a large percentage of a cheap glue mixed with a common clay lying on the surface of a more or less inferior body paper, so that the moisture and damp of an ordinary atmosphere find books of this class an easy prey in course of time. The preservation of books containing the fine art illustrations obtained by process printing can be prolonged by careful storage in a properly dried room, and having regard to the large number of expensively prepared books which are being continually published, this question is of the utmost importance.

In this case, as in many similar instances, the rule that "prevention is better than cure" holds good, but so far, the introduction of a fine art paper in which the smoothness of surface shall be an inherent quality of the actual paper, and not a mere artificial characteristic, has not been successfully accomplished.

There is a big field open for a systematic research into this question, which would afford considerable scope for chemical and mechanical ingenuity. No doubt the discovery of some method for obtaining a smooth surface without the employment of such adventitious substances as those which obtain at present would be extremely profitable to the inventor. Attempts have been made to obtain a coating by means of some form of dissolved cellulose, but so far without success. Investigation should probably proceed along the lines of the "Willesden process." In the manufacture of waterproof Willesden paper and similar goods, the body paper is passed through a bath containing cellulose dissolved in Schweitzer's reagent, an ammoniacal solution of cupric oxide. When this solution evaporates, a greenish mass of cellulose and copper oxide coats the whole surface, and the coating becomes intimately associated with the body paper by reason of the partial dissolution of the surface of the paper which takes place as the ammonia evaporates.

The colour of the final product is greenish, but the substitution of the copper by zinc brought about by treating the Schweitzer's reagent with metallic zinc, produces a colourless solution of the corresponding zine salt, which has somewhat similar properties.

Other soluble forms of cellulose offer considerable attractions for research in this new field. The work of Messrs. Cross and Bevan, with the thiocarbonates and acetates of cellulose, is well known in this connection, but the application of these soluble derivatives as a coating pure and simple has not yet met with any wide measure of success.

The attempt to produce a glossy surface on paper by the reprecipitation of a soluble cellulose seems to be a correct and scientific procedure, eliminating to a large extent the conditions which favour disintegration and the disunion of three irreconcilable elements, the body paper, the mineral coating and the printers' ink.

Many of the difficulties experienced by printers using art papers may be overcome by careful attention to the conditions of working. The most important condition for successful work is that of an equable temperature. It is a matter of common experience that printing with art papers on Monday gives far more trouble than the work later on in the week, and that in the winter this difficulty is more pronounced. The general principle to be observed is the desirability of having the art paper, the ink and printing presses all at the same temperature, not only during the process of printing, but also for some time before use. The regulation of the heat of the room to some fixed standard would do away with

many troubles which are at present ascribed to the " paper" in some vague manner. Thus, for example, a given sample of ink may give satisfactory results. one week because the supply has been kept in the printing-room, while the next week, being brought direct from a cold store room, it may give rise to endless trouble.

With regard to the ink which is used for printing on art paper, no systematic work has been done, and it is evident that a proper investigation into this question would result in some definite information. As the enamel on the art paper in its natural condition has a dull surface the ink must be of good quality. In general terms, the ink should have sufficient consistency to be tacky, the

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rollers should be hard and well seasoned, with a proper distribution of the ink by the inkers. In addition to the property of consistency, the ink should be manufactured in such a way as to dry quickly when placed on the surface of the paper. The consistency is determined by grinding down the colouring-matter with as much strong varnish as possible, the ink being thinned down if necessary afterwards, though the addition of too much medium varnish for this purpose tends to make the ink when dry flake away from the surface of the paper. The addition of driers is not always essential as the constituents of the ink pigment possess in themselves the necessary qualities for this purpose. Considerable technical knowledge is necessary in the task of making up ink varnish to the required consistency and with the required tackiness and drying properties, because so many factors enter into consideration, many of which are of a mechanical nature, as already intimated.

Probably, just as important a factor is the nature of the paper itself. Successful printing can only be accomplished by practical experiment, and this