Practical Lithography Part 6

Remove the part which requires altering with a mixture of equal parts etching solution and turpentine on a small piece of felt or flannel, and rinse well with water. Pour some special re-preparing solution in a saucer, and apply this solution with a camel-hair brush to the parts where the work has been removed; after the solution has remained for a few seconds rinse well with water. This operation is to be repeated several times till the surface looks clean, no work being visible; and after the plate is dried with clean white blotting paper it is ready to receive the additional work. When the alteration has been made, the rolling up should be done with the b.i.+.c.hromate solution, as per general directions.

[Ill.u.s.tration: FIG. 22.]

[Ill.u.s.tration: FIG. 23.]

"Where the alterations involve additional work only without erasures roll up the design first with a good, strong ink, and dust over with French chalk. The special re-preparing solution is used in the same manner as acetic or citric acid is employed on a litho-stone. For this purpose we recommend that the special solution should be diluted with an equal quant.i.ty of clean water, and the solution applied with a camel-hair brush. It should remain on for a few seconds, and then be quickly rinsed with clean water. This operation may with advantage be repeated two or three times for securing a clean surface for the additional work. When the plate has been thoroughly dried with clean blotting paper it is ready to receive the additional work. The rolling up should be done with the b.i.+.c.hromate solution, as per general directions."

Machine printing from zinc or aluminium plates requires but a slight modification of well-known methods; and although it may in some respects present entirely new features, the making ready of work in which several printings are employed is simplified, and consequently much time is saved. In the ordinary type of litho-printing machine the printing bed (Fig. 22) is levelled in the machine for the first printing, and, if this is carefully arranged, no alteration will be required throughout the whole series; for, in changing from one colour to another, the printing plate can be slipped from the bed and another subst.i.tuted in a very few minutes. The most scrupulous care should be exercised at all times to prevent dirt or grit of any kind insinuating itself between the surface of the printing bed and the back of the plate.

It is not in connection with the flat-bed printing machine, however, that the most decided progress is likely to be made in surface printing from metal plates. "The change which is already foreshadowed in the printing mechanism of to-day is shown by the growing demand for the rotary in place of the slow and tedious movements of the flat-bed press."

The mechanical principle of the rotary machine (Fig. 23) at once suggests an absolute precision of movement which it is scarcely possible to guarantee in the flat-bed press. There is no appreciable lift in the gearing of the cylinders when the impression is made, and practically no risk whatever of _slogger_, such as that described in Chap. VII. page 38. The uniform velocity of the cylinders, which is to a great extent due to the points already indicated, considerably reduces friction, produces perfect registration, and enables the machine to be worked at a high rate of speed. The machine shown on page 63 presents many novel and essentially practical features. The side elevation (Fig. 24) gives a fairly accurate idea of its general mechanical principles.

[Ill.u.s.tration: FIG. 24.]

CHAPTER XII

TIN-PLATE PRINTING

Its Evolution--Transfer and Direct Transfer Printing--The Coated Paper--Reversed Designs--Sequence of Printing-- Printing Inks--Purity of Tone--Drying.

Tin-plate printing or decoration is probably the most remarkable development of modern lithography. From a most unpretentious and unpromising beginning its evolution has been an unbroken record of phenomenal progress. Owing to its intrinsic merits and peculiarly a.s.sertive character it has created an enormous demand for its own productions.

Difficulties innumerable were encountered and surmounted, whilst a whole host of trade prejudices had to be removed before anything approaching a commercial success could be claimed. The original idea was to transfer a printed design from a suitably prepared paper to the metal plates in much the same manner as the children of to-day use transfer prints for decorative and other purposes. Though this method is now to some extent superseded, it is by no means obsolete. It is extensively used for certain cla.s.ses of work, and so long as the present type of direct tin-printing machine is used this transfer method of metal decoration will more or less be retained.

It is difficult, and in some cases impossible, to print unusually large or exceptionally thick metal plates by the direct process. Hence the value of Transfer Printing as a useful auxiliary process which can be resorted to without the expense of special machinery. The ordinary lithographic paper-printing machine, without any modification whatever, will fulfil every requirement.

Almost any paper which has been coated with a starchy composition will be suitable for Transfer Printing. Though not absolutely essential, it is decidedly an advantage to use a paper which is somewhat porous, not too hard. A brief description of the principles involved will probably lead to a clearer understanding of the points already indicated.

The print is made on the starchy composition with which the paper is coated, and never actually penetrates to the paper itself. This composition, being readily soluble in water, can be transferred from its paper base to any other surface, and will carry with it any print or impression previously made thereupon. In theory, then, this process is exceedingly simple, nor is it likely to present any insuperable difficulties in practice.

One of the most important features of Transfer Printing, which affects not only the colour sequence but the draughtsmans.h.i.+p, is the inversion of the design when printed. To the printer who is accustomed to chromo work on paper, this matter will probably offer many awkward situations.

The control over colour effects is somewhat restricted, and consequently not only care and skill, but a certain amount of special training and actual experience, will be necessary for the production of really successful work. In theory the outline forme should be printed first, but in practice it is frequently advantageous to introduce some of the transparent tints as first printings. In the first place, such prints rarely affect the design to any great extent as far as register or fit is concerned. Consequently, any distortion of the paper from atmospheric or other causes has no very serious consequences, and such distortion, _i.e._ expansion or contraction, would be most likely to occur during the initial printings. There is yet another and equally important reason for this suggested modification of the colour sequence. The light, tacky tints considerably improve the printing surface offered by the mucilaginous coating of the paper, and the stronger inks can be printed on it with a full body of colour yet without any tendency to smash and smear. The lighter tints, then, are printed first, and these are followed by the opaque colours, reds and yellows being last. The peculiar feature of such an arrangement is that, after the first few printings, the design becomes partly obscured, and the relative value of different colours, together with the effect of their super-position, are frequently questions of skilful calculation rather than visual appreciation. Thus successful manipulation is therefore more or less the result of careful observation and wide experience. In certain cla.s.ses of work it may, of course, be absolutely essential to ascertain the effects of colours as they are printed. This can be done by transferring a progressive print to a sheet of dull enamel surface paper, using gum water as a transferring medium. In all work where specific colour effects are required it will be advisable to make such test transfers again and again at different stages of the work.

Concerning the printing inks themselves but little need be said. They must be of good quality, varnishable, and of intense colouring power.

Purity of tone is requisite, not only under ordinary circ.u.mstances, but under what may be regarded as exceptional conditions. The print, when subsequently transferred to the metal plate, will be placed in a stove having a temperature of 140 Fahr. Few colours remain quite unaffected by this stoving, but with carefully selected printing inks the effect is scarcely perceptible. It can, moreover, usually be antic.i.p.ated, and to some extent minimised, either by judicious super-position or skilful preparation of the colours themselves before printing. The following example will suggest a number of expedients for the preservation of tone of colour in tin-plate decoration.

Pale blues will frequently become of a decidedly greenish _hue_ when varnished or stoved, and this may be modified by an exaggerated intensity of colour, even to hardness, when printing. At the same time, it must be remembered that "blues" are seldom, if ever, affected in _tone_. Pinks of the lighter shades, on the other hand, may suffer both in _hue_ and _tone_. "Reds" may be mixed with a more liberal amount of "lake" than they apparently need, and then backed up with solid ma.s.ses of "yellow" and "flesh." These will restore the bright vermilion hue, and likewise give a greater amount of density or covering power to the colour. Black, in the same manner, may be intensified by an underlay of blue. As far as purity and density are concerned, the "white" printing is by far the most important. The primary function of this printing in tin-plate decoration is the formation of an opaque ground on the metal plate--equivalent to the white paper. Without it the colours of a design would lose their brilliancy and effect, for their purity and density would be affected by the l.u.s.trous sheen of the bright metal plate.

It is a printing, then, of some, importance, and the most scrupulous care should be exercised during its manipulation. In consequence of the exceptional density required it is usual to repeat the "white" printing either two or three times. The first printing especially should be made with a smooth, well-mixed ink, which should be worked with the intention of securing a perfectly flat and solid impression rather than a heavy body of colour. The requisite amount of density can be obtained by the second or third impressions.

A slight tinge of "Oriental blue" given to the "white" will improve its appearance and counteract the discoloration produced by the varnis.h.i.+ng and stoving.

Another point with which the average printer is not familiar is the unusually rapid drying of each colour. The printing inks must dry on the surface, and not even in the slightest degree be allowed to permeate the paper. This is, in fact, the _crux_ of the whole matter; and, a.s.suming that the coating of the paper is suitable and perfect, the drying cannot be otherwise than on the surface. Dryers, preferably liquid, must be mixed with the printing inks in liberal quant.i.ties. Here, again, experience alone can be relied upon for guidance. Printing inks have their peculiar characteristics, and atmospheric conditions are ever varying. It would therefore be unwise to indicate any hard-and-fast lines upon which to work. There is at least one danger accompanying the use of dryers in any form which must not be overlooked. Colours may dry too hard and present for the succeeding printings a surface similar to a varnished sheet, which will offer no _grip_ to the printing ink, and upon which it will probably run, and, consequently, produce a defective impression. This is by no means an infrequent occurrence. Experience alone can prevent it, but the remedy is as simple as it is effective.

Briskly rub each printed sheet with a pad of soft rag and a little magnesia or French chalk. Magnesia is best. It is an almost impalpable powder, and has no ill effect even upon the most delicate tints. This rubbing will produce a slightly matt surface on the hard glossy ink, and on this a good solid impression may be made without further trouble.

The printed sheets should be exposed to the air for a few hours to accelerate drying. This may be done in any convenient manner, either by laying them out in frames or hanging them up. A "set off" in Transfer Printing should be sedulously avoided; not that the soiling of the back of a sheet is of any moment, but because the matter set off on the back of one sheet must be pulled off the face of another, and that very often in liberal proportions.

CHAPTER XIII

TIN-PLATE PRINTING--_continued_

Direct Tin Printing--The Machine--Peculiarities of Impression--Cylinder Covering--Colour Sequence--Printing Inks--Drying Racks--Air Drying and Stoving.

"Direct tin printing" is not, as the term might suggest, a _direct_ impression of the printing forme on the metal plate. It is in reality a transfer printing process in which the transferring medium is a cylinder with an elastic covering. This additional cylinder is one of the chief characteristics of a direct tin-printing machine, and, apart from a few accessory movements, it is in this respect alone that it differs from an ordinary lithographic printing machine. Fig. 25 gives a sufficiently clear idea of this distinguis.h.i.+ng feature. A detailed description of its mechanical principles is unnecessary, but the following points will prove helpful.

The impression is made from the printing forme upon the lower cylinder during the first half of its revolution, and transferred to the metal plate, which is carried by the upper cylinder as the revolution is completed. The reciprocal movement of these two cylinders is therefore a matter which demands the keenest possible attention. Their pitch must be absolutely accurate, the pressure of one against the other nicely adjusted, and the brake arrangement on both such as to ensure a perfectly steady revolution.

The lower cylinder is covered with a three-ply rubber blanket, which provides a sufficiently elastic impression surface.

[Ill.u.s.tration: 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 slight enlargement of the impression, and _vice versa_; 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 diminis.h.i.+ng the speed, would of course effect such a change. The effect, as just indicated, is, however, only perceptible when a very p.r.o.nounced 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 l.u.s.tre. The surface, again, is hard and excessively smooth, or, as it is sometimes expressively described, without _tooth_. 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 is seldom 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 quant.i.ty 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 subst.i.tutes 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 after printing, so that the air may freely circulate about them, and thus a.s.sist 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.

[Ill.u.s.tration: 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 in Fig.

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. 27A and 27B).

[Ill.u.s.tration: FIG. 27A.]

The question of air-drying _versus_ stoving has ever been a contentious one, and admits of considerable diversity of opinion. Air-drying _is_ preferable where convenient. It is much more economical than stoving, of course; but, on the other 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.

[Ill.u.s.tration: FIG. 27B.]

Some knowledge of their respective disadvantage should be acquired, however, in order to avoid, or at least minimise, them.