This is a photo-mechanical process for placing on a planographic printing surface a reproduction of an original design without the intervention of handwork by the litho graphic artist.
Various copying methods are employed, the simplest being a process known as Vandyke, a method whereby line drawings and tracings on ordinary paper can be reproduced without the aid of a camera. The drawing is used as the negative for photographically printing the image on to a metal plate coated with a solution containing fish-glue, ammonium bichromate and ammonia. This has to be filtered, care being taken that the plate which is to be coated is chemically clean. When the coating is dry the plate is placed in a photographic printing frame and the copy placed in contact with the prepared plate. This is exposed to light for a pre-determined time. It is then removed from the
and the plate washed in running water, after which it is dyed with a methyl violet solution, when a transposition of the copy appears upon the plate : the bare parts of the metal repre sent the black lines of the original. The plate is then dried and inked over with a special transfer ink, when it is allowed to stand for some time and then developed in a weak solution of sul phuric acid. The film, acted upon by light, is easily removed, leaving the design outlined on the plate, which is printed in the ordinary way.
Another copying process now in general use is known as Typon. This process is extremely useful and especially adapted to the reprinting of old books. In this form of reproduction a specially constructed frame with a glass top and lights placed be neath is employed. On the glass, a yellow filter is placed, then a sheet of "typon paper," of special manufacture, with the emulsion side uppermost, the page or design being placed face downwards on to the paper. A sheet of rubber is stretched over this and clamped securely in a frame, the air being extracted until a close contact is obtained between the original and the photographic paper. The lights at the bottom are switched on and the necessary exposure given. The photographic record of the original is obtained by means of reflected light. The paper is developed and fixed, and
when dry becomes the negative from which a design is photo graphically printed on to the printing machine plate.
These are usually supplied to printers who do not possess photographic equipment by firms specializing in the work. The method of production is as follows. A sheet of suitable paper is coated with gelatine, sensitized by potassium bichromate. This is dried in a dark room and is then ready for exposure through a negative taken from the original to be reproduced. After being rolled up with transfer ink until it is covered evenly all over the surface, it is placed into water, then carefully rubbed over with a piece of cotton-wool, those parts which the light has not affected dissolving and washing away, leaving the design in a greasy ink upon the sheet of coated paper, which is then ready for transferring to the printing surface.
This is the modern method of placing a design on a printing plate. A negative is obtained and when developed and dried is photographically printed on to a lithographic plate coated with albumen and ammonium bi chromate. The negative and plate are placed in a printing down frame, the air being extracted by a pump and the plate and negative exposed to light for a certain period of time. The plate is then rolled up solidly with printing ink and placed under running water. A piece of cotton-wool is rubbed over the surface and where the light has not passed through the negative such parts of the coating are washed away, leaving the design on the plate, which after rolling up with ink, etching and gumming is ready for the press. Remarkable progress has recently been made in preparing photo-litho plates, especially where a number of duplicates have to be placed on the same plate.
A machine has been invented which will project on to a sensitized glass plate a photographic record of a design from a master negative. This machine automatically and accurately projects on to the plate any number of duplicates of the original with the pre-determined space between each. When developed, this large negative is printed down on to a machine printing plate.