Home >> Photography Theory And Practice >> Non Actinic Lighting Dark Room Lamps to Shutters 127 >> Photo Mechanical Processes in Brief_P1

Photo-Mechanical Processes in Brief 911

printing, line, plates, originals, ink, plate and blocks

Page: 1 2 3 4 5

PHOTO-MECHANICAL PROCESSES IN BRIEF 911. Generalities. The various processes which are used on a commercial scale at the present time for the photo-mechanical reproduction of drawings in line or wash, paintings and photo graphic originals may be classified into three main groups— Typographic or letterpress processes—among which are line photo-etching (i.e. photo-zinco), for the reproduction of originals in black and white without other tones ; and half-tone, for the reproduction of all full-tone originals.

In the plates thus obtained the printing ele ments are in one and the same plane projecting above the base.

Surface processes—in which are included collotype and photo-lithography. In collotype the printing plate is a layer of gelatine on a suitable flexible or rigid support ; iii photo-lithography, printing is no longer done from stone but from a sheet of aluminium or zinc.

In the printing plates thus obtained, the image is usually in the same plane as the background, or at a slight depth below this background.

Intaglio processes—of which the sole survivor, at any rate on the commercial scale, is screen photogravure, in which printing is done from flat plates, or, in the rotary form of the process, from cylinders. In photogravure plates or cylin ders the printing elements form minute cavities. Where the cavities are deepest the impression will be darkest.

Of these processes, the only ones applicable in the ordinary course to the illustration of letterpress set up in type are photo-zinco and half-tone. All such relief printing blocks may be reproduced in any required numbers by electrotyping ; line blocks and half-tones of very coarse screen, as used in daily newspapers, may be reproduced in any required numbers by stereotyping.' In the absence of special arrangements, blocks become the property of the customer of the photo-engraver and may be used for a large number of different publications.

The collotype process, by which for many years almost all the view postcards printed on the Continent have been produced, is the most suitable for small editions of prints or inset plates. For an edition of 500 copies, prices compare favourably with those by other pro cesses, provided that a sufficient number of originals are dealt with at one time to employ the machine at full capacity.

Photo-lithography has the advantage of allowing printing to be done on all kinds of paper, including those of common quality or pronounced texture. It is the process to be

preferred for the reproduction of line originals in a limited edition. Under favourable condi tions it may be used for full-tone originals, but only when a fairly large edition is required.

Rotary photogravure needs to be used for an edition of about ro,000 copies of an original in order to compete in price, and it is equally necessary that the whole surface of the etched cylinder should be filled with the reproductions. Smaller editions are sometimes printed from flat plates, but at a much greater expenditure of labour.

As a general rule, line and half-tone blocks are made by firms other than those that use them (photo-engravers and letterpress printers respectively). On the other hand, plates for collotype, gravure, and usually photo-litho printing, are used by the firms who prepare them. it is the exception for these plates to be kept, and they never become the property of the customer.

912. Line Etching. The production of a line block (i.e. zinco) comprises numerous opera tions, which are undertaken by workmen belonging to at least three branches of the trade, viz. — (a) Making of the negatives by the wet collodion process ; sensitizing the metal (zinc, except for very special purposes) with bichrom ated albumen ; assembling the stripped negatives on glass ; exposure to light ; inking the whole exposed plate ; washing out in cold water ; drying.

(b) Application to the image in greasy ink of resin powder of low melting-point ; uniting ink and varnish by moderate heat ; varnishing the back and edges of the metal plate, and also parts of the front surface not occupied bv subjects ; if required, transfer of tints, in line or stipple, on to certain parts ; treatment of the metal with a solution which causes it to repel greasy ink ; rolling-up with highly fusible ink, followed by etching for a very short time in nitric acid. From this stage there is repeated the series of operations : rolling, dusting with resin, heating (during which the ink flows down the sloping sides of the unetched lines), and etching until depth enough for printing is obtained in the etched parts. Finally the plate is cleaned.

Page: 1 2 3 4 5