Exposure of the Printing Block under the Negative.--The exposure is very rapid. Any one conversant with photolithographic work will understand this. At any rate, every photographer knows that bichromated gelatine is much more rapid than the chloride of silver he generally has to do with.
There is no other way of measuring the exposure than by the photometer or personal experience, and the latter is by far the best.
After leaving the printing frame, the plate is immersed in cold water. Here it remains at discretion for half an hour, or an hour; the purpose, of course, being to wash out the soluble bichromate. It is when the print comes out of this bath that judgment is passed upon it. An experienced eye tells at once what it is fit for. If it is yellow, the yellowness must be of the slightest; indeed, Herr Furkl (the manager of Herr Löwy's Lichtdruck department) will not admit that a good plate is yellow at all. A yellow tint means that it will take up too much ink when the roller is passed over it. The plates of Herr Obernetter, however, are rather more yellow than Herr Löwy's--certainly only a tinge, but still yellow; and Herr Obernetter's work proves, at any rate, that the yellowish tinge is by no means inseparable from good results.
The washed and dried plate should appear like a design of ground and polished glass. The ground glass appearance is given by the grain. If there are pure high-lights (almost transparent) and opalescent shadows, the plate is a good one.
Printing from the Block.--We have now a printing-block ready for the press. If it is to be printed by machinery--that is to say, upon a Schnell press--the surface is etched; if it has to be more carefully handled in a hand press, etching is rarely resorted to; it is moistened only with glycerine and water. To etch a plate for a Schnell press, it is placed upon a leveling stand, and the following solution is poured upon it:
Glycerine............................. 150 parts.
Ammonia................................ 50 "
Nitrate of potash (saltpeter).......... 5 "
Water.................................. 25 "
Another equally good formula, recommended by Allgeyer, who managed Herr Albert's Lichtdruck printing for some years, is:
Glycerine............................. 500 parts.
Water................................. 500 "
Chloride of sodium (common salt)...... 15 "
In lieu of common salt, 15 parts of hyposulphite of soda, or other hygroscopic salt, such as chloride of calcium, may be employed.
The etching fluid is permitted to remain upon the image for half an hour. During this time, by gently moving the finger to and fro over the surface, the swelling or relief of the image can be distinctly felt. The plate is not washed, but the etching fluid simply poured off, so that the film remains impregnated with the glycerine and water; at the most, a piece of bibulous paper is used to absorb any superfluous quantity of the etching fluid. After etching, the plate is taken straight to the printing press. The inking up and printing are done very much as in lithography. If it requires a practiced hand to produce a good lithographic print, it stands to reason that in dealing with a gelatine printing block, instead of a stone, skill and practice are more necessary still. Therefore at this point the photographer should hand over the work to the lithographer, or rather the Lichtdruck printer. It is only by coaxing judiciously, with roller and sponge, that a good printing block can be obtained, and no amount of teaching theoretically can beget a good printer. To appreciate how skillful a printer must be, it is only necessary to see the imperfect proofs that first result, and to watch how these are gradually improved by dint of rolling, rubbing, etching, cleaning, etc. In all Lichldruck establishments, two kinds of rollers are used, viz., of leather and glue. In some establishments, too, they employ two kinds of ink; but Herr Löwy manages to secure delicacy and vigor at the same time by using one ink, but rolling up with two kinds of roller.