Practical Lithography

by Alfred Seymour

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 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, 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 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.