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 quantity 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 bichromate 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 substituted 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.