Collotype and Photo-lithography - Julius Schnauss

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but chiefly for letterpress work and steam machines in general. A soft composition of a very elastic nature is quite useless for collotype; its substance must, on the contrary, be tolerably hard, and of a brown colour, as usually employed on the steam letterpress jobbing machines. For hand rollers for collotype it is preferable to have a strong iron frame with double handles, as used by letterpress printers. Such rollers are preferable to those of leather, and will answer perfectly for laying on the first stiff ink, and also for rolling up the half-tones. To clean they should on no account be scraped, but simply washed with turpentine, and on this account are much easier cleaned than leather rollers. Schaarwächter gives the following as an excellent composition for collotype rollers: Brown glue, three parts, steeped for two days in cold water. The pieces should then be dried in a cloth, and afterwards dissolved. When melted add, with constant stirring, one part of treacle, which has been previously boiled, allow the whole to boil, and skim. The composition is then run into a thin copper cylinder or mould, which must be polished smooth, clean, and well oiled; and in the centre of which has been placed the wooden cylinder or stock. After cooling, the roller is pushed out of the mould, and washed with alcohol or turpentine to remove any trace of greasiness due to oiling the mould, placed in a bath of tannic acid, washed with water, and dried. The composition rollers, as used in typographic printing, are made with glycerine instead of treacle, and may be used for collotype if not too soft.

      Indiarubber Rollers are more expensive, but possess special properties rendering them particularly suitable for photo-lithography. In construction a wooden stock with suitable handles is covered with very thick well-vulcanised indiarubber, which has a fine smooth surface, rendering it easy to clean by means of turpentine. If it becomes “tacky” on the surface it should be well rubbed with French chalk, or soap-stone, which will readily absorb the turpentine, but this coating must be thoroughly removed before again using the roller. An excellent suggestion appeared in one of the polytechnical periodicals, that the rollers should before use be coated with a solution of pure or bottle rubber in benzole or bisulphide of carbon after having been well washed with turpentine.

      These indiarubber rollers are not suited for the first inking of the plate, but are invaluable for rolling up the half-tints of the collotype plate. As they somewhat readily take colour off the plate they must be first rolled up in a thinner ink and a separate slab. According to “Lithographia” these indiarubber rollers may be cleaned with lye, or strong alcoholic spirit. If cleaned with turpentine it may be advantageously followed by rinsing with soap and water; by so doing the application of French chalk is unnecessary. These rollers are, by comparison with others, nearly indestructible, and therefore cheap, notwithstanding their high first cost.

       Fig. 5.

      Velvet Rollers have latterly come much into use, and are very successfully used for inking-up the transfers in photo-lithography. They may be also used in photo-xylography and photo-ceramic processes, and also for certain purposes in collotype. A necessary precaution is that the seam of the velvet should always be kept off the surface subjected to the rolling, and this may be carried out if the roller is, say, 10–½ centimetres in diameter, a length of 31·2 centimetres may then be rolled without using the seam. The sketch will explain the construction. The cylinder or stock is of pear-wood, and the handles of box; first covering serge, which before sewing on should be well shrunk in hot water; over this comes the velvet, the edges of which must not overlap, but must be drawn together as close as possible by means of thread. One advantage of this roller is that it is ready for immediate use; it must, after use, be carefully cleaned before being placed aside; this is best done by repeated rollings on a clean slab, and the judicious application of a little turpentine. Turpentine only may be used to thin ink for use with this roller.

      Drying Rollers.—In addition to those already enumerated, all of which are intended for the application of the ink to the plate, other rollers are required, similar in construction, but covered with wash-leather or cloth, and used for the purpose of absorbing superfluous moisture. For these it is necessary, or at least advisable, that the frames be so constructed that the rollers may be readily removed when by continued use they have become too damp. This method of drying the plates is far preferable to the customary one of wiping with a cloth, for even with the greatest care, if the printing should extend over hundreds of copies, the layer upon the plate will inevitably get scratched when wiping or rubbing is resorted to, especially if isinglass—which tends to soften the film—be present.

      Palette Knife or Spatula.—It is unnecessary to describe; they are used for mixing the ink and applying it to the slab and rollers, and for cleaning the latter. They may be either of hard wood or metal.

      The Ink Slab may be either of slate, a lithographic stone, or a sheet of strong zinc securely attached to a block of wood truly planed.

       The Press is an indispensable requisite for printing operations. There are small wooden hand-presses, especially constructed for collotype, which may be purchased at a very low price—£4 to £5; but the more economical way is to procure a small lithographic press constructed of iron. This will not only be found more durable, but it may, if occasion arise, be used either for lithographic or zincographic printing, as it will bear the necessary pressure, which is greatly in excess of that applied in collotype printing. The hand-press constructed by Sutter is a form that may be strongly recommended. If the output of prints is to be large, a steam machine will be necessary, the cost of which will be not less than £200 to £250. Fig. 6 represents a press which has been largely used for collotype. D is the bed or carriage upon which the plate is fixed in the manner described further on. By turning the crank handle C the bed is carried under the scraper, which is secured in the scraper-box A. The scraper is kept in position by the lever B. The scraper-box is represented as closed down in the illustration, but in actual practice it would not be in the position shown until the tympan-frame E had been lowered.

      The Tympan, of either leather or thin sheet zinc or brass, is marked F in the illustration. All iron portions of the press subject to friction should be frequently cleaned and carefully oiled with a good machine-oil.

       Fig. 6.

      White Margins to Pictures.—To

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