Part 2 (2/2)
During this part of the operation great care must be taken that the transfer ink does not burn through overheating, as this would partially destroy its greasy nature and leave it hard and brittle. The transfer impression would suffer in consequence, and, though to all appearance perfect on the paper, it would be weak and ineffective when applied to the lithographic stone. Such an error of judgment is not at all unusual, and should therefore be the more carefully guarded against. It frequently occurs without the knowledge of the operator, owing, it may be, to his over-anxiety to complete his work in as short a time as possible.
The plate must now be cleaned, _i.e._ the surplus ink and sc.u.m must all be removed. This may be done before the plate is quite cool, and after a little experience it will be possible to accomplish the cleansing process without in any way disturbing the ink in the lines of the engraving. The rag used for cleaning must be tightly folded into the form of a pad and kept free from creases. After final cleansing and polis.h.i.+ng with whiting the plate is ready for an impression. The transfer paper requires damping until it is quite limp, when it is brought into contact with the inked plate and subjected to a very heavy pressure. The backing is a woollen blanket, preferably of fine texture; this ensures perfect contact between the plate and the paper. The plate is now very slightly warmed to dry the transfer paper, which is allowed to peel off; this it does very readily if, after a little while, the corners and edges are but slightly eased.
Oil of tar will effectually remove any accretions of copperplate transfer ink which may have hardened in the lines of the engraving.
It may be useful also to know that it is possible to use a small lithographic press in place of a copperplate press, a.s.suming, of course, that a sufficiently heavy and uniform pressure can be guaranteed. This is not altogether an innovation, yet it is not a familiar notion.
CHAPTER V
THE LITHOGRAPHIC PRESS
Mechanical Principles--Constructive Details--Sc.r.a.per-- Tympan--Practical Suggestions--Elastic Bedding.
It is not a little surprising to find that the mechanical principle of the lithographic press in general use to-day is almost identical with that which the pioneers of the craft employed so successfully. This is an interesting fact which either reflects much credit upon the ingenuity of the early lithographic printers or points to an unreasonable conservatism on the part of the present-day craftsmen. A discussion of this phase of the question would be of doubtful interest, for the practical printer has long been accustomed to regard it simply as a convenient appliance for the production of a heavy and readily adjustable pressure.
A brief examination will prove to what extent these requirements are fulfilled by the modern lithographic press (Fig. 5).
The simplicity of its construction suggests a first point for favourable criticism. In fact, its general mechanical arrangements are so exceedingly simple that the merest tyro might readily understand their principles and purpose.
The adjustability of the pressure by means of the screw D (Fig. 8) is both effective and necessary, owing to the constantly varying thickness of the lithographic stones.
The pressure of the boxwood sc.r.a.per B on the surface of the stone is perfectly rigid, and yet, owing to the intervention of the tympan C, is sufficiently elastic to ensure the closest possible contact. Figs. 6 and 7 show one or two constructive details by which the hand lever A and the cam motion E bring up the cylinder F to the bottom of the carriage or bed of the press, Fig. 8.
[Ill.u.s.tration: FIG. 5.]
It is in this position that the movement of the carriage gives the necessary pressure required to pull an impression. The shaft H runs across the press and operates a similar cam to E on the opposite side.
These two cams raise the bra.s.s block G and give the requisite support to the cylinder F when the pressure is applied.
These are the chief characteristics of the lithographic press, and as such they require not a little attention and intelligent manipulation.
It is practically impossible to secure a steady and uniform pressure unless the sc.r.a.per and tympan are carefully adjusted.
[Ill.u.s.tration: FIGS. 6 and 7.]
The former must be perfectly true with its V-shaped edge nicely rounded, and the latter tightly stretched on the frame C so that it will not sag or bulge when pressure is applied and the sc.r.a.per pa.s.ses over it. To reduce the enormous friction caused by this pressure the back of the tympan is usually dressed with a mixture of tallow and plumbago, a dressing which requires frequent renewal. The plumbago possesses but little body, and its salutary effect soon pa.s.ses away. To prevent this and to increase its adhesiveness it is sometimes mixed with a little gum. A mineral black which is found in large quant.i.ties in the west of England is even more effective than plumbago for this purpose. It forms a strong and flexible dressing for the leather, is peculiarly adhesive and provides an efficient lubricant.
It is a decided advantage to have two tympans in use, one for small stones and another for the larger sizes. It is obviously unwise to pull a number of impressions from small stones with a large tympan, for if this practice is persisted in the tympan leather not only loses its shape, but becomes perceptibly thinner on such parts as may have been most subjected to pressure.
For similar reasons it is advisable to have a number of boxwood sc.r.a.pers of different sizes. The ”dents” produced by a small stone on a large sc.r.a.per can only be removed by planing.
In lithographic press work some form of elastic bedding placed underneath the stone will not only materially a.s.sist the pressure, but will also minimise the risk of breakages. In fact, the pressure is frequently so keen and of such a direct character as to render this arrangement little short of a necessity. Extra thick linoleum will serve this purpose admirably, and a zinc covering for this bedding will complete the equipment of the lithographic press.
[Ill.u.s.tration: FIG. 8.]
The operations directly a.s.sociated with lithographic press work are of sufficient importance to warrant a full description of each, and will form the nucleus of the following chapter.
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