THE COLLOTYPE PROCESS IN PRACTICE.
That theory and practice are two very different things holds good in photography especially, and perhaps in no other branch of our art have so many theoretical formulæ been promulgated as in the collotype or Lichtdruck process. As our readers are aware, we have had an opportunity of seeing collotype printing in operation in several European establishments of note, and have, from time to time, published in these columns our experiences. But requests still come to us so frequently for information on the process that we have deemed it well to make a practical summary for the benefit of those who are working--or desire to work--the method.
The formulæ and manipulations here set down are those of Löwy, Albert, Allgeyer, and Obernetter, four of the best authorities on the subject, and we can assure our readers there is nothing described but what is actually practiced.
Glass Plate for the Printing Block.--Herr Albert, of Munich, uses patent plate of nearly half an inch in thickness, as most of his work is printed upon the Schnell press (machine press). Herr Obernetter, of Vienna, since he only employs the slower and more careful hand press, prefers plate glass of ordinary thickness as being handier in manipulation and better adapted to the common printing-frame.
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.