Photo “Glass” Printing.[N]
THERE remains to be mentioned another variation of the Collotype process known under the above vague title, the distinctive characteristic of which method of preparing the plate is that the negative is taken upon a sheet of glass sufficiently thick to stand the necessary pressure in printing when converted direct into a Collotype plate. After the completion and drying of the collodion negative it is coated upon the film side with the chromated gelatine solution, dried in the drying-box, laid face downwards upon a black velvet surface, and exposed through the back of the glass. The first publication of this method of preparing plates emanated from the State Printing Works at Berlin, and the following is an outline of the method there practised; it is scarcely necessary to state that it is only available for line subjects. The collodion used in the preparation of the negative should be one capable of producing clear, hard negatives entirely free from fog; probably some modification of the dark slide may be necessary to enable it to receive a plate of sufficient strength to stand printing from. The collodion negative being completed it is flooded with the chromated gelatine as applied to Collotype plates, dried, and exposed through the negative until the print layer has assumed a dark brown colour; it is well washed in cold water and damped before printing by an application of gum water containing a little ox-gall; this is done to facilitate the production of perfectly clear impressions. The same end may be attained by the application of dilute ammonia and glycerine. The author has experimented for several years in the same direction with the object of producing heliographic printing plates in copper, for which purpose this method of producing reliefs in gelatine is well suited. From the negative printing plate described, which is treated in all respects as a Collotype plate, impressions may be taken upon transfer paper, and transferred to stone or zinc. Before coating the negative with the chromated gelatine the plate should be allowed to dry. In consequence of printing from the back the lines will always be slightly wider than the original, to meet which difficulty Brandt has proposed putting the negative, when fixed and washed, but still wet, into a weak solution of chromated gelatine, which will, if a suitable collodion has been employed, entirely penetrate the latter. This variation appears worthy of commendation; it is, therefore, here given in the abstract:—“Having produced a negative in the usual manner, on a plate which has been lightly but carefully albumenised, and having fixed and washed the same, it is placed for five minutes in a solution prepared as follows: Allow 45 parts of gelatine to swell in water, and pour off the surplus, add 50 parts of distilled water, and apply sufficient heat to completely liquify the gelatine, adding a solution of 5·5 parts of bichromate of ammonium in 100 parts of water, and heat again to 60° RÉamur. With plates of large size the solution may be poured over the plate, and it may be necessary to repeat this operation several times to ensure an even and sufficient coating. Immediately previous to the application of the chromated layer the negative should be rapidly washed over with warm water. A sufficient quantity of the chromated solution must be allowed to remain upon the plate, and the latter warmed in a horizontal position over a spirit lamp. When steam arises the plate has to be held in a perpendicular position for a minute, restored to its horizontal state, and again warmed until the coating of the plate is quite dry, care being taken that at no time the plate is so heated that it becomes unpleasant when placed upon the hand. Place the plate face downwards upon a black support, print until the fine lines are plainly visible, when the plate may be washed and further treated as in Collotype.”
