Metal Plates—Preparation—Manipulation—Descriptive Details—Machine Printing—The Printing Bed—Rotary Printing Machine.
Metal plate, as a substitute for stone, is now such an important factor in lithography that the printer who wishes to consider himself thoroughly efficient must possess a fairly comprehensive and practical knowledge of its manipulation and possibilities.
The prejudice which has hitherto checked the progress of this branch of lithography was not altogether of an unreasonable character. The plates themselves were far from reliable, and the difficulties resulting therefrom were a fruitful source of trouble and expense.
Metal, as a printing surface, is even yet a comparatively new factor in lithography, and the majority of printers have been working with lithographic stones from their apprenticeship till the present time. It is not surprising, therefore, that in relation to the use of stones almost every possible contingency has been provided for, but with metal plates a little fresh knowledge must necessarily be acquired before the workman can claim the same familiarity of manipulation which he may feel towards the parent process. This is, in fact, the point upon which the whole question usually turns. Good work can be produced from zinc and aluminium plates,—of that there is not the slightest doubt,—and it is equally certain that the advantages offered by their use are of a substantially practical character. They can be handled with ease and with absolutely no fear of breaking. They are much less costly than stone, and require less storage room.
A grain of a finer and sharper texture can be imparted to metal than is the case with stone, and what is even of greater importance, the character of such a grain remains unaffected for a considerable time. Surface inequalities are rarely met with in metal printing surfaces, and consequently uniform pressure is to a certain extent guaranteed. In photo-lithography it is possible to make a print from a negative direct on to the plate (Chap. XVII. [page 100]). This ensures an original of exceptional clearness and strength, especially in half-tone subjects.
Although an ordinary zinc plate, which has been carefully polished to free it from every trace of grease, can be used for lithographic printing, the best results are obtained from plates which have been specially prepared. A slight de-polishing with pumice sand and a piece of felt may impart the requisite “tooth” to the face of the plate, or the following method may be adopted:—Clean the plate with pumice sand and felt, and immediately immerse it in a hot bath containing:—
- 18 oz. Water.
- ¼ oz. Alum.
- 1 dram Nitric acid.
Keep this liquid in constant motion over the face of the plate until it assumes an even, silvery-grey appearance, and then wash it thoroughly with a plentiful supply of clean water. Dry at once, and quickly.
These plates can also be sand-grained by specially constructed machinery, or a variety of grains and stipple can be imparted to their surface by etching or sand blast.
A novel yet practical idea, which has met with considerable success, is to electrolytically prepare the surface of the plates. The value of this preparation has been amply demonstrated by its extensive adoption and successful use. Another distinctly progressive feature is a deposit of alumina on the zinc, which for printing purposes gives it all the advantages of an aluminium plate.