Tin-plate Printing Machine.Fig. 25.—Rapid tin-plate printing press.
This rubber covering is a most important feature, and requires both careful adjustment and intelligent use. Careful adjustment is requisite to ensure the tension being perfectly uniform over the whole surface, when the material is drawn taut by means of a tooth-and-ratchet arrangement. Were it not so, the printing surface presented would naturally vary in thickness and resilience, in consequence of which the pressure would be variable and the impression distorted.
A most remarkable peculiarity of this rubber covering is that it has a very decided influence upon the size of the impression. If, for example, its pressure upon the printing forme is increased, the impression will be slightly less than the work on the stone, whilst a lighter pressure will of course produce the opposite effect. It is advisable, therefore, when making a first impression, to measure it from back to front and compare it with the printing forme. This method will ensure a print of the exact size, and avoid any risk of complications in the subsequent printings. Such distortion—for a distortion it really is—may frequently be turned to good account, and under certain conditions it may prove to be a help rather than a hindrance. Some slight inaccuracy in the fitting of any part of the design may occasionally be corrected by inserting patches of thin paper under the cylinder covering, so as to almost imperceptibly increase the pressure over the required area. This idea in its application to “making ready” at a tin-printing machine will suggest many possibilities to the resourceful printer, and if intelligently treated will rarely prove troublesome. In a lesser degree the same system of packing may be applied to the upper cylinder, and the size of the impression to some extent controlled during its transference to the metal plate. The speed of the machine also affects the impression, and in a somewhat peculiar manner. Several theories have been set forth to prove why an increase in speed should produce a slightenlargement of the impression, andvice versâ; but it is doubtful if any of them are altogether satisfactory. Most probably it is due to a momentary change in the resilience of the rubber. The character of the pressure, which is certainly influenced by increasing or diminishing the speed, would of course effect such a change. The effect, as just indicated, is, however, only perceptible when a very pronounced variation in the speed takes place. A rubber blanket is also affected in a somewhat similar manner by atmospheric changes, though not appreciably by moisture.
One other point in connection with the cylinder covering is worth noting. Whenever it is necessary to remove the impression from the blanket,—and the necessity may arise many times during the course of a day’s run,—let it be done with turpentine or benzole, and as rapidly as possible. After the superfluous turpentine or other cleanser has been wiped off, dust over the blanket with French chalk. This will completely absorb any of the cleanser which may have permeated the rubber, and thus minimise any injurious effect.
Unlike the Transfer Process in Direct Printing, the colour sequence is practically the same as for paper printing. There are, of course, essential points of difference, and these may fitly mark the next stage of progress in the discussion of this subject. In the first place, a bright metal plate does not present an altogether suitable printing surface, and for several reasons. The sheen of such a plate will show through many of the printed colours with a dull metallic lustre. The surface, again, is hard and excessively smooth, or, as it is sometimes expressively described, withouttooth. As can easily be seen, this is far from an ideal printing surface.
White also plays a prominent and effective part in many designs, and is frequently indispensable. Its presence, when necessary in Direct Tin Printing, must therefore be arranged for in some form or other. One white printing isseldom sufficient to produce a perfectly clean and solid ground. Two printings, or even three, may be necessary. Purity of tone is a most important point, and therefore a pigment should be chosen which will not only remain unchanged by its contact with the metal, but which will be unaffected by the heat applied during stoving. The appearance of this white may be improved by the addition of blue, as in Transfer Printing (p. 70), and here also the smallest possible quantity will be sufficient.
Gold Lacquer Printing is peculiar to Tin-plate Decoration, and its advent indicated a vast progress in artistic display and effect.
Printing lacquer is a transparent pigment of about the same consistency and character as printing ink. It is usually prepared in three shades of colour—red, citron or orange, and pale gold. A combination of these in suitable proportions will produce almost any strength or shade of gold which may be required. Except as regards their unusually brilliant transparency, these lacquers much resemble yellow lakes, and in paper printing might even be used as economical substitutes for the more expensive pigments. This, however, is only a suggestion, and has little if any practical bearing upon their application to Tin-plate Printing, where they completely transform the bright sheen of the highly polished metal plate into a brilliant and most effective gold. Gold lacquer is an exceptional pigment in many respects. It works exceedingly well, and gives a sharp, flat impression where most other pigments would fail. It also has the power of conveying many of its own good qualities to any printing ink with which it may be incorporated.
In Transfer Printing the question of rapid drying is very important, and in Direct Tin Printing it is even more so. Arrangements of a somewhat extensive character must be made for the reception of metal plates immediately afterprinting, so that the air may freely circulate about them, and thus assist in the drying. The room in which the printing sheets are thus stored must be free from dust, for it will readily adhere to the tacky printing ink, and afterwards prove a source of endless trouble.
Tin-plate Racks.Fig. 26.
Drying racks of various descriptions are used for the storage of printed metal plates. One of the best is constructed on the lines shown inFig. 26. The shelves A A are adjustable so as to carry plates of different sizes. The printed sheets are set up on end in the grooves B B, and by arranging them back to back in pairs a large number can be accommodated without hindering the drying. Other types of drying racks are shown in the sectional drawings (Figs.27Aand27B).
Tin-plate Racks.Fig. 27a.
The question of air-dryingversusstoving has ever been a contentious one, and admits of considerable diversity of opinion. Air-dryingispreferable where convenient. It is much more economical than stoving, of course; but, on theother hand, it might be a better plan to stove a printing than to wait, perhaps for some hours, until it dried naturally. With some printing inks and under certain climatic conditions both may be necessary, so it is almost impossible to lay down any hard-and-fast rules as to the adoption of either plan.
Tin-plate Racks.Fig. 27b.
Some knowledge of their respective disadvantage should be acquired, however, in order to avoid, or at least minimise, them.
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. (SeeTransfer Printing,p. 71.) Naturally, it requires a much longer time, but it is equally effective, much less troublesome, and generally more satisfactory.
Method of Stacking Plates.Fig. 28.—Convenient method of stacking decorated metal plates to distribute their weights and prevent injury.
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 thepolished 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 lustre 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 overwhiteproduces abuffcolour, 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 assertive 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 fromblacktowhiteto 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 latitude 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 assertive 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 brass 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 trimming 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.
Early Experiments—An Analysis—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 longbefore 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. Thedirectprocess, in which the actual printing surface is prepared and exposed under the negative.
2. Thetransferprocess, in which a gelatine-coated paper is sensitised in a solution of bichromate of potassium and the photographic print made upon it.
Thedirectprocess 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 thetransferprocess absolutely close contact can be assured 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 thedirectprocess 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-lithographyinlineis 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-tonephoto-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 glass 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 aremedyand not acure.
It has been confidently asserted that the highest degreeof 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 coarser rulings, but thescreeneffect will be too pronounced 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 ofhowandwhereit 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.
2. Anegativein which the whites of the original appear opaque, with clear glass to represent the lines and solids.
3. Aprintwhich can be developed or inked up with a pigment sufficiently greasy in nature to transfer to the lithographic stone.
The Copy—Gradations of Tone—Scraper Boards—Description and Effect—Shading Mediums—Crayon Drawings—Half-tone Copy.
It has already been pointed out that well-defined modelling is most desirable in the original copy. To secure this a considerable degree of artistic perception and discretion, as well as manipulative skill, is requisite. In photo-process work it is almost impossible to produce artistic effects from an indifferent or unsuitable copy.
Pen-and-ink sketches and wash drawings are entirely under the control of the artist, and characteristic effects are chiefly due to bold and vigorous conception and skilful drawing. Few photographs are suitable for photo-mechanical reproduction without some previous preparation. Accentuation or modulation of the high lights and shadows will in all probability be necessary to secure a sufficient contrast of light and shade. The middle tints may require but little attention, unless it be to work down any tendency to abruptness in the gradations of tone. A bold and well-defined silver print usually copies well when clamped between two pieces of glass to take out the grain, and photographed by artificial light.
Scraper boards offer most remarkable possibilities for black and white and half-tone sketches. A careful examination ofFig. 29will serve to demonstrate their peculiar fitness for process drawing. A light wood pulp board forms a convenientbase upon which a thick coating of white composition is laid. Black ruled lines are printed on this surface, and lines of a similar texture are embossed at right angles to them. Some of the characteristic effects which can be produced on this board by the use of the crayon and scraper are suggested by Nos. 6 and 7,Fig. 30. No. 8 gives a stipple which is both printed and embossed. No. 9 is a plain board upon which pen-and-scraper effects alone are produced. Drawings in pen and ink, on Nos. 10 and 11 patterns, may be effectively handled by a free and skilful use of the scraper. Embossed lines only are the peculiar features of these boards, but variety of texture can be obtained by scraping these lines into dots.
Scraper board sketches almost invariably represent a maximum of effect with a minimum of work, and for this reason alone such an adaptable and simple medium should soon win its way into general favour.
Their merit, however, is not confined to this one point. They provide almost unrivalled copy for photo reproduction, and can therefore be applied to a variety of purposes. Even a cursory glance at the scraper board sketch onpage 91will reveal many points of interest and value which a more careful scrutiny can scarcely fail to emphasise.
Small patches of scraper boards can be introduced into process drawings of any description, and brilliant results be secured thereby. In such a manner clouds, waves, foliage, and a variety of other effects can be introduced.
Shading mediums are already well known to lithographers, yet it is doubtful if their usefulness for the amplification of sketches or process drawings is fully appreciated. They offer almost endless combinations of texture and tint, and are therefore most useful and valuable accessories in the hands of a resourceful artist.
Scraper Board Work.Fig. 29.—Reproduction of a drawing made on Gilby & Hermann’s scraper boards.
Scraper Board Textures.Fig. 30.—Some scraper board textures.
Many artists favour crayon work on a grained paper, adoptinga broad sketchy treatment in liberal proportions, so that in the subsequent reduction the freedom of the original will be toned down just sufficiently to enhance the picturesque and artistic value. Drawings made on scraper boards, grained papers, or by the aid of shading mediums, are photographed and reproduced by the ordinary line method. In making sketches for “half-tone” photo-mechanical reproduction it must be remembered that, as far as photo-lithography is concerned, it is impossible to accentuate effects by what is known as “fine etching” when applied to photo-engraving. A negative for photo-process work of any description should be absolutely perfect in every respect. The whites of the copy in the negative should be of an absolutely opaque black, showing clean, sharp edges, with clear glass representing the lines, dots, etc. These are all-important factors, and their influence upon the reproduction of the original can scarcely be over-estimated.
A Copying Table—Exposure—Illumination—Photo-litho Transfers—The Paper—Printing—Developing—A Direct Process.
To discuss the respective merits of the “wet” collodion, collodion emulsion, and dry-plate processes lies beyond the province of this work, as does also a detailed description of the operations involved.
The processes are purely photographic, and have already been presented to the craft in various forms. It is, moreover, almost impossible to bring within the limits of a single chapter anything approaching to a comprehensive record of the multitudinous details upon which process photography is based. All that can be attempted is to take one or two outstanding features which suggest a few useful hints.
The original or copy for reproduction must be on the same optical plane as the sensitive plate in the camera,i.e.they must be absolutely parallel with each other. There are several ways of ensuring this. The most convenient method is to use a copying table and board similar toFig. 31. With such an arrangement as this direct copying can be attained through the lens, or the camera can be turned half-way round, and the image reversed by means of a mirror or prism attachment. The chief advantage of such an arrangement is, that the position of the camera can be altered at will without affecting the relative positions of the plate and copy.
Photographic Copying Board.Fig. 31.
Correct exposure and sufficient illumination of the copy are important factors in photographic reproductions of any kind, but they are of infinitely greater importance when applied to photo-process reproduction. One is, to a certain extent, dependent upon the other. The former must of necessity be controlled by the latter; yet no amount of exposure will compensate for defective illumination. Where artificial light is employed the advantage of using two lights is obviously great. Apart from the greater brilliancy and intensity of the light, the illumination of the copy is more evenly distributed. In scraper-board copies no shadows are thrown from the embossed dots or lines, and the granular texture of grained papers is almost entirely eliminated.
A bichromated, gelatine paper can be obtained by coating a hard writing paper of medium thickness with a gelatinous solution consisting of 1 oz. of gelatine and 1 oz. of water, and afterwards sensitising it with bichromate of potassium. It is advisable, however, to use the commercial varieties of coated paper, and to sensitise it as it is required.
The sensitising solution can be prepared by dissolving 1 oz. of bichromate of potassium in 20 fluid oz. of water. Add to this sufficient ammonia to give it a bright orange tinge. Keep this solution at a temperature of 60° Fahr., and float the paper on it for about one minute. Pin or clip the paper to a board or squeeze it to glass, and dry in a dark room.
Print this paper under the negative in a diffused light until the design appears in a rich golden-brown colour, when the exposure may be regarded as sufficient. The time allowed for such an exposure will, of course, vary according to the quality of the negative and the intensity of the light.
A development of the print may now be proceeded with in the following manner.
Thin down a little transfer ink with turpentine anddistribute it evenly on a composition roller. Roll up the print until it is completely covered with an exceedingly fine film of ink, after which allow the turpentine to evaporate. Immerse the transfer in tepid water for about 10 minutes, and then rub it gently with a piece of cotton lint previously soaked in water, until the superfluous ink is removed and the design stands out clean and sharp.
The transference of the print to stone can be accomplished in the usual lithographic manner. The chief points to be observed are, to allow sufficient time for printing, and to ink up the transfer with the thinnest possible film of ink.
A photo print can be made direct on the zinc or aluminium plate, and by a slight modification of the photo-engraving process it can be developed according to lithographic methods.
It is impossible to introduce an intermediate process without, in some manner at least, depreciating the quality of the work. In a direct photo print on zinc, or, in fact, on any suitable printing surface, the finer qualities of the work are much more likely to be retained than when a transfer print is made under the negative and afterwards transferred in the usual manner.
Coat a finely grained zinc plate with sensitised asphalt solution and expose it under a negative for about 4½ minutes in direct sunlight, and from 12 to 15 minutes in a diffused light. The action of light on the asphalt solution is to render it insoluble in turpentine, so that if a sufficiently exposed plate is immersed in pure turpentine the lines, etc., of the design, being of course represented by clear lines in the negative, will remain intact, while the surrounding portions will be dissolved and washed away. After development wash the plate freely in water, and dry it by fanning or with a pair of bellows. Let it stand for about 10 minutes and then slightly etch it with a very weak solution of nitricacid. Cover the work with strong, fresh gum, and dry it thoroughly and quickly. Remove the gum and “rub up” the design with black ink in the usual way. Wash, dry, and dust over with French chalk. The plate can then be prepared in the manner described in Chap. XI.page 61.
THE END