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.
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.
Herr Löwy's formula is as follows:
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.
Drying the Sensitive Film.--Much depends upon the drying. A water bath with gas burner underneath is used for heating, and a slate slab, perfectly level, receives the glass plate. The drying chamber is kept at an even temperature of 50° C.
The object to be attained is a fine grain throughout the surface of the gelatine, and unless this grain is satisfactory the finished printing block never will be. If the gelatine film be too thick, then the grain will be coarse; or, again, if the temperature in drying be too high, there will be no grain at all. The drying is complete in two or three hours, and should not take longer.
The Negative to be Printed from.--The sensitive film being upon the surface of a thick glass plate, it is necessary that the cliché or negative employed should be upon patent plate, or not upon glass at all, so as to insure perfect contact. Best of all, is to employ a stripped negative, in which case absolute contact is insured in printing. It is only in these circumstances that the most perfect impression can be secured. If the negative is otherwise satisfactory, and only requires stripping, it must be upon a leveling stand, and fluid gelatine of a tolerable consistence is poured over it. When dry, a pen-knife is run around the margin, and the film leaves the glass without any trouble.
Herr Obernetter says that many of the negatives he receives have to be reproduced before they can be transformed into Lichtdruck plates, and he employs either the wet collodion process or the graphite method, according to circumstances. If the copy is desired to be softer than the original, collodion is employed; if vigor be desired, graphite is used, and here is his formula:
Dextrine. 62 grains.
Ordinary white sugar. 77 "
Bichromate of ammonia. 30.8 "
Water. 3.21 ounces.
Glycerine. 2 to 8 drops.
The film is dried at a temperature of 130° to 140° F. in about ten minutes, and while still warm is printed under a negative in diffused light for a period of five to fifteen minutes. In a well-timed print the image is slightly visible; the plate is again warmed a little above atmospheric temperature in a darkened room, and then fine levigated graphite is applied with a fine dusting brush, a sheet of white paper being held underneath to judge of the effect. Breathing upon the film renders it more capable of attracting the powder. When the desired vigor has been attained, the superfluous powder is dusted off, and the plate coated with normal collodion. Afterward the film is cut through at the margins of the plate by means of a sharp knife, and put into water. In a little while--from two to five minutes--the collodion, with the image, will be detached from the glass; the film is at once turned over in the water, and brought out upon the glass plate. Under a soft jet of water any air-bubbles that may exist between the collodion and the glass are removed, and then a solution of gum arabic (two grammes of gum dissolved in one hundred grammes of water) is poured over, and the film is allowed to dry spontaneously.
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.
Collotype printing is not merely done by hand presses, but is also done by machinery. At Herr Albert's a gas engine of six-horse power is employed to drive the machines, and each machine requires the attention of a skilled mechanic and a girl. The press is very like the lithographic quick press. Upon a big steel bed lies the little collotype block. The glass printing block, with its brownish film of gelatine, moves horizontally to and fro, and, as it does so, passes under half a dozen rollers, which not only supply ink, but disperse it. Some of the rollers are of leather and others of glue, and, whenever the printing block retires from underneath them, an ink slab takes the place of the block, and imparts more ink to the rollers; sometimes as many as eight rollers are used, for the difficulty of machine printing is to apply the ink as delicately and equally as possible. It is necessary at intervals to damp the block, and when the printer in charge finds this to be the case, he stops the press, and applies a little glycerine and water with a cloth or sponge; then a leather roller is passed over to remove superfluous moisture, and the press is again started.
Herr Obernetter relies upon the Star or Stern press--a small lithographic press--one man sufficing to manage it, who turns a wheel with large spokes, reminding one of the steering wheel of a ship. The Lichtdruck plate, gelatine film upward, is laid upon a sheet of plate glass by way of a bed, the plate having first been treated with a solution of glycerine and water; it is then inked up as previously described, except that Herr Obernetter uses two kinds of ink--a thick one and a thin--applied by two rollers of glue. In the first place, a moist sponge is rubbed over the surface; then a soft roller covered with wash-leather, and of the appearance of crêpe, is passed over two or three times to remove surplus moisture; then a roller charged with thick ink is put on, and then another with thin is applied. It takes fully five minutes to sponge and roll up a plate, the rolling being done gently and firmly. A sheet of paper is now laid upon the plate, the tympan is lowered, and the scraper adjusted with due pressure; a revolution of the wheel completes the printing, the well-known scraping action of the lithographic press being used in the operation.
FIG. 1.--ORDINARY NAPHTHA LIGHTER OF
MR. LOISEAU.
Some Lichtdruck prints are printed upon thick plate-paper, and are ready for binding without further ado, these being for book illustrations. Other pictures, that are to pass muster among silver photographs, are, on the other hand, printed upon fine thin paper, and then sized by dipping in a thin solution of gelatine; after drying, they are further dipped in a solution of shellac and spirit.--Photo. News.