Herr Löwy, of Vienna, again, uses plate glass a quarter of an inch thick, as his productions range from the finest to the roughest.
_Preliminary Coating of the Glass Plate_.--Herr Albert's original plan was to apply a preliminary coating of bichromated gelatine to the thick glass plate, the film being exposed to light through the back of the glass, and thus rendered insoluble and tightly cemented to the surface; this film serving as a basis for the second sensitive coating, that was afterward impressed by the negative. This double treatment is now definitely abandoned in most Lichtdruck establishments, and, instead, a preliminary coating of soluble silicate and albumen dissolved in water is used.
Herr Löwy's method and formula are as follows: The glass plate is cleaned, and coated with--
Soluble glass. 3 parts. White of egg. 7 " Water. 9 to 10 "
The soluble glass must be free from caustic potash. The mixture, which must be used fresh, is carefully filtered, and spread evenly over the previously cleaned glass plate. The superfluous liquid is flowed off, and the film dried either spontaneously or by slightly warming. The film is generally dry in a few minutes, when it is rinsed with water, and again dried; at this stage the plate bears an open, porous film, slightly opalescent--so slight, however, as only to be observed by an experienced eye.
_Application of the Sensitive Film_.--We now come to the second stage of the process, the application of a film of bichromated gelatine to the plate.
Bichromate of potash. 16 grammes. Gelatine. 2½ ounces. Water. 20 to 22 "
According to the weather, the amount of water must be varied; but in any case the solution is a very fluid one. An ounce is about 35 grammes, as most of our readers know. A practical collotypist sees at a glance the quality of the prepared plate, without any preliminary testing. A good preliminary film is a glass that is transparent, yet slightly dull; the film is so thin, you can scarcely believe it is there. The plate is slightly warmed upon a slate slab, underneath which is a water bath; it is then flooded with the above mixture of bichromated gelatine, leaving only sufficient to make a very thin film. When coated, the plate is placed in the drying chamber.