Practical Lithography Part 7

[Ill.u.s.tration: FIG. 28.--Convenient method of stacking decorated metal plates to distribute their weights and prevent injury.]

Take, for instance, a course of eight printings, each one absolutely necessary to the design, and each one to be dried by stoving. It is only reasonable to suppose that the first and second printings will be seriously affected by the subsequent stovings. They will undoubtedly harden to such an extent as to render the super-position of other colours a difficult matter, and their purity of tone will most probably be affected. According to the same line of reasoning, some sheets would be more affected by the heat than others, owing to their position in the racks, the heat of stoves being greatest near the top.

Air-drying can only be accomplished by adding to the printing ink a proportion of some suitable drier. (_See_ Transfer Printing, p. 71.) Naturally, it requires a much longer time, but it is equally effective, much less troublesome, and generally more satisfactory.

CHAPTER XIV

TIN-PLATE DECORATION

Suitable Designs--A Variety of Effects--Gold Lacquer-- Super-position of Colours--Embossed Effects--Embossing Plates--Lacquers.

It is usually and rightly supposed that the most effective results in Tin-plate Decoration are produced from designs which are lithographed for that specific purpose.

Designs which are specially arranged for Paper Printing can be used so long as the effect produced by the transposition from right to left does not affect its application, or render the same impossible. This, of course, applies more particularly to designs in which lettering appears, but at the same time it may affect designs of an essentially pictorial character in an equally important manner; for it must be remembered that a drawing for Tin-plate Printing must appear on the lithographic stone exactly as it is impressed on the metal, and not, as in Paper Printing, reversed from right to left. This naturally simplifies the drawing on stone, and to some extent enables the draughtsman to dispense with the reversing mirror when copying.

The primary object of this short chapter will be to point out some of the characteristic features of Tin-plate Decoration, so that such methods as are usually adopted by the artist and the printer may be modified or amplified to meet any peculiar requirements of work in hand.

A greater variety of effects can be attained on the polished metal plate than it is at all possible to produce on paper.

A gold effect, the result of lacquer printing, is especially striking.

In a similar manner an excellent translucent l.u.s.tre can be imparted to almost any colour by taking away the white opaque ground, and thereby producing a peculiar semi-transparency which is both pleasing and effective. Lacquer printed over _white_ produces a _buff_ colour, which can be used as a second yellow or to form the base of a flesh. The colour of the lacquer is softer and less obtrusive when printed under instead of above the white. The super-position of colour generally, as described in Chap. XII. page 70, is peculiar to tin-plate printing, and suggests the advisability, if not the necessity, of a special design.

The advantages of such super-position are obvious and substantial. Under ordinary commercial conditions it is almost impossible in tin printing to obtain the same intensity of tones in the printed colours as in paper printing. Some such strengthener as the super-position of suitable colours is therefore necessary. The work of the lithographic draughtsman is in this respect of a somewhat unusual character; but a little intelligent consideration will render its execution on these lines comparatively easy and satisfactory.

As already stated, yellows can be accentuated by a super-position of lacquer, and in the same manner blues and greys add intensity to black.

Red can be strengthened by a foundation of lacquer, also flesh and yellow, either singly or in combination. The drawing of lacquer and white formes should receive the most careful attention. They should fit each other accurately, even to the most minute details; for the slightest overlapping will be revealed by the presence of a very a.s.sertive buff colour, while any deficiency in combination will leave a not less striking margin of bright tin exposed.

In some instances it might be an advantage to transfer one forme from _black_ to _white_ to produce the opposite colour, and thus ensure perfect register.

In decorative designs particularly, tin-plate printing suggests almost unlimited possibilities for brilliant effects, and in this respect it offers fair lat.i.tude for individual originality and manipulative skill.

In the production of show tablets especially, considerable attention has been given to embossing, in order to suggest and represent relief.

Though not actually produced by the artist, this is in effect part of the scheme or plan of his design, and will consequently influence his work to some extent. For simple ornamentation only, metal embossing presents little that is new or novel; but for the production of relief effects in the pictorial elements of a design its application becomes a more important and influential matter. By its aid a flat and otherwise uninteresting subject can be made attractive and vigorous, and for general purposes of effective display its value will be considerably enhanced and its a.s.sertive character emphasised.

Without entering too minutely into matters which affect the lithographic printer in an indirect fashion only, it will yet be useful to him to know how an embossing die is produced which registers accurately with the design to be operated on. A black impression of the outline forme of the design is made on transfer paper similar to that described in Chap.

XII. page 68, and re-transferred, by pressure only, to another sheet of the same paper. The re-transfer is to be the impression required, and this in its turn is re-transferred again to a prepared bra.s.s plate. The only preparation necessary is the levelling and planing of the plate, and, if desired, it can be coated with a thin layer of white paint or enamel. This white ground makes the work easier, by rendering the impression more distinct. With such a guide as this the cutting or engraving is a comparatively simple matter.

The engraved plate is placed in a casting box, and a stereo-metal casting is made from it. A little tr.i.m.m.i.n.g may be necessary for the completion of these two tools, and they will require to be suitably mounted, so that they may be accurately adjusted in the embossing press.

Metal embossing has certain limitations which must be recognised as an essential condition of its effective application.

The plates vary considerably in texture and temper, and the depth and character of relief will, to some extent at least, be controlled by the quality of the metal. Sharp lines and abrupt terminations impose a strain under which many plates split, therefore such features ought to be avoided. Easy, rounded lines, rather than those of the straight furrow description, produce the best results and give least trouble during operations. Where abrupt terminations are quite unavoidable the design should, if possible, be so arranged as to evade the super-position of colour over these parts.

Lacquer, if properly mixed and applied, is probably the toughest pigment used by the tin printer, and is generally suitable for embossed work. It prints an exceedingly thin layer or film on the face of the metal, which under average conditions rarely interferes with the working of the embossing tool.

CHAPTER XV

PHOTO-LITHOGRAPHY

Early Experiments--An a.n.a.lysis--The Direct Process-- Transfer Process--Line and Half-tone--Some Difficulties-- A Natural Grain--Ink Photo-screen Effects--Essential Features.

One of the most promising features of lithography is its co-partnership with photography as a rapid and accurate method of reproduction. The resources offered by this combination are very extensive. For facsimile copying and proportionate enlargement or reduction photography stands unrivalled, and, although in certain phases it may be somewhat mechanical in its effects, its relation to lithography as a reproductive art is nevertheless of an intensely practical nature, and far from inartistic in character.

The first idea of inking up a photographic print so that it might be transferred to the lithographic stone was suggested in the simplest possible manner. A brief account of its inception will be instructive as well as interesting, inasmuch as it will lead to a clearer conception of the elementary principles involved.

During the early experiments in carbon printing it was discovered that a gelatinous film sensitised with certain bichromates could be charged with a coloured pigment, and a picture developed thereon. At first it was not realised that images produced by the action of the light on such a surface could be inked up with a greasy composition and afterwards transferred to the lithographic stone, but it was not long before this important point became apparent. It was found that after exposure under the negative the transfer ink would only adhere to such portions of the gelatinous surface as had been acted upon by the light.

Photo-lithography will best be considered under two sections, namely:--

1. The _direct_ process, in which the actual printing surface is prepared and exposed under the negative.

2. The _transfer_ process, in which a gelatine-coated paper is sensitised in a solution of bichromate of pota.s.sium and the photographic print made upon it.

The _direct_ process in its application to the lithographic stone is uncertain in its results. It is impossible to secure sufficiently close contact between the negative and the stone, particularly when large surfaces are under operation, and consequently the print is rarely if ever an unqualified success. The erasure of defective work is also a serious matter, and can only be effected by polishing and preparing the stone again.

In the _transfer_ process absolutely close contact can be a.s.sured by the use of the transfer paper; and should the print from any cause whatever prove defective, another can be made immediately without any serious loss of time or material.

The successful application of the _direct_ process to zinc and aluminium plates is, however, an accomplished fact. The metal plate is sufficiently elastic to adapt itself to any inequalities on the surface of the negative. Under such conditions as these this process offers at least one very important advantage. There is not the slightest possibility of distortion such as might occur in the development of a transfer. The metal plate also lends itself to easy manipulation.

_Photo-lithography_ in _line_ is simply the reproduction of line drawings or prints in which the design is represented in black and white with only such gradations as may be suggested by lines or dots.

_Half-tone_ photo-lithography is the reproduction of a design or copy which has in its composition gradations of tone in the form of flat tints.

It is sometimes described as the translation of the graduated light and shade of the original copy into a surface which can be printed from by mechanical means, for which purpose the ink-bearing surface is broken up into the most minute sections, and thus forming an almost imperceptible grain. The first attempts to reproduce the half-tones of a copy, in the form of a grain consisting of minute dots of varying size and contiguity according to the gradation of tone required, were made with a screen of open textile fabric. This screen was placed between the lens and the sensitive plate, but the results were crude and unsatisfactory.

The invention of cross-lined screens, in which the lines were cut on gla.s.s and filled with black or other suitable colouring matter, was a decided advancement in the half-tone photo processes.

The "screeny" effect produced by the "unvarying uniformity of grain" in half-tone work is undoubtedly the chief drawback to its more extensive adoption for photo-lithography. Fine etching cannot be resorted to as in photo-engraving, neither is it possible, to emphasise effects by skilful overlay and underlay; consequently half-tone impressions from a lithographic stone are frequently disappointing. There are no insurmountable obstacles to hinder the production of excellent transfers, nor is it a difficult matter to transfer them to stone. The trouble is, as already pointed out, the unvarying uniformity of the grain.

This effect, or rather this lack of effect, has been to some extent overcome by the use of a "four-line" screen in lieu of the usual "crossed" screen, but even this is merely a _remedy_ and not a _cure_.

It has been confidently a.s.serted that the highest degree of excellence in photo-process work will be attained by the adoption of what may be termed a natural grain. Several processes have been introduced which are undoubtedly based upon collotype methods in which a reticulated grain is produced more or less suitable for lithographic printing. Unlike the mechanical screen grain the texture of these processes reproduces the original copy with but little, if any, loss of expressive power. This is indeed a feature of considerable importance, and suggests many possibilities in the way of artistic reproduction.

To reproduce an old chalk drawing so that it might be successfully transferred to stone and printed in the usual way, would be practically impossible by any other process. In copying through a ruled screen many of the delicate contrasts of light and shade would be so reduced as to become almost valueless, consequently the print loses both in artistic and expressive power. In contradistinction to this a natural grain exhibits no harshness or indistinctness in the gradations of tone, and retains its clearness and sharpness throughout the printing operation.

Reverting again to the half-tone ruled screens, it may be well to state that small prints, being usually subjected to a closer inspection than large ones, must be reproduced with great attention to the finer details to ensure a certain amount of fidelity, and for this reason a screen with fine rulings must be employed. Naturally, stronger and more vigorous reproduction can be secured with the coa.r.s.er rulings, but the _screen_ effect will be too p.r.o.nounced for close scrutiny.

There is still much to achieve in photo-lithography, and it is probably owing to a full recognition of this fact that the progressive character of the process is maintained. Its commercial value is undoubted, and its successful application is chiefly a question of _how_ and _where_ it can be most effectively introduced.

The essential features of photo-lithography are:--

1. A copy or original in which the modelling is well defined, and the light and shade well emphasised, even to a point of slight exaggeration.