Negatives for Collotype must be reversed, unless they are on celluloid film. These films also are far preferable for the purpose, than those made on ordinary dry plates, the latter being far from flat, and therefore liable either to break under pressure or to give an imperfect image, from insufficiently close contact with the collotype plate.
Sheets of best plate glass, at least S in. thick, are first ground truly level, and then coated with a sizing consisting of— Stale Beer . . . . . to parts Syrupy Silicate of Soda . . . . . i part and dried in an oven at 120 Fahr., after which they are allowed to cool, rinsed under the tap, and left in a rack to drain. The sensitive film is composed as follows : Hard Gelatine . . . . . . 3 oz.
Potassium Bichromate . . . . 30o gr.
Ammonia (strong) . . . . . . to min.
Water . . . 1 5 oz.
The gelatine is soaked for an hour in the water, and then dissolved gradually under heat, after which the other in gredients are stirred in. Immediately before coating, about an ounce of methylated spirit is added for every ounce of gelatine solution about to be used, and the whole is strained through fine muslin. The glass plates must be scrupulously level and warm to the touch. Thirty to fifty minims may be allowed for every square inch of glass. After coating, the plate will dry in about a quarter of an hour in a drying oven, in which the temperature must not exceed 115'. The films will keep for about ten days if protected from moisture and light.
Previous to exposure in a pressure-frame it is usual to mask off the edges, either with tinfoil or black paper. Progress of exposure maybe judged, either by means of an actinometer or by examination of the image through the glass at the back of the printing-frame. When complete, most workers advise that the plate should be laid face downwards on a piece of black paper, with the back exposed to the light, for a minute or two. This will harden the back of the film, and so insure its adherence to the glass during the subsequent operations.
Development takes place in several changes of water, until all the bichromate salt has been removed, when the film may be dried and put away in store for months, if necessary. When copies are to be taken, the collotype
plate is soaked in cold water and then "etched" in a solution composed either of Glycerine . . . . . 20 parts Water . . . 20 „ Ammonia . . . . . . r part Common Salt . . i „ Or of Glycerine • • . . . . i part Water . . . . . . z parts With a few drops of oxgall.
After being allowed to act for about half an hour, by which time the film will have been thoroughly penetrated, the superfluous glycerine is dabbed off; the picture inked in with lithographic ink, and proofs taken on an Albion or other press. A lithographic machine is often adaptable for collotype, but an extra set of rollers and a mask-frame for the margins are essential ; a special collotype machine is the best in the long run. Printing requires care and experience, and the plates have to be damped with fresh etching solutions after each 70 to wo impressions, according to the quality of paper used. If the prints are glazed with white water varnish or a label varnish, and then dried in moderate heat, they will closely resemble ordinary P.O.P., or albumen paper, according to the colour of the varnish employed. Only a limited number of copies can be obtained from each film before it wears out.
The above may serve as an outline of collotype methods, but in the hands of individual firms the process has been brought to the very highest perfection. A simplified process for the use of amateurs, substituting gelatine ready coated on parchment sheets for the heavy glass plates, was intro duced a few years ago. We have not heard any news about this lately. Collotype prints of- good quality are worthy of all praise ; inferior work is not remunerative, as it can not compete with photo-lithography, produced at a quarter the trouble and expense.
Among the leading works on photo-engraving, etc., may be mentioned The Half-tone Process, by Julius Verfusser, and Photo-Mechanical Processes, by W. T. Wilkinson.