Collotype and Photo-lithography. Julius Schnauss
the process. They are more frequently dried in specially-constructed boxes provided with screws for accurately levelling the plates, and through which only a small circulation of air takes place. These boxes are usually rectangular in shape, the upright sides being of wood and the bottom of sheet iron. The lid is an open framework covered with a close orange or black cotton material, the whole standing upon four iron legs over a spirit or gas flame. In the upper part of the box a thermometer is fixed, about the centre of either the side or lid, in such a position that it may be readily observed without the necessity of opening the box. Strong horizontal iron bars are placed across at about the centre, and are provided with thumbscrews, upon which may be placed a plate of glass bearing a circular spirit level, by which means the plates may be levelled with the greatest accuracy. The sheet-iron bottom of the box being heated unevenly, it becomes necessary to mitigate this inconvenience as far as possible, which is easily done by covering the plate to a depth of about half-an-inch with dry river sand, over which should be placed tissue paper to keep down any possible dust.
Fig. 2.
An Automatic Regulator of practical value is that devised by Ruegheimer. It consists of a glass tube, A, A1, A2. A1 is closed with an indiarubber stopper, through which passes a glass tube B, the lower end of which is cut off at an angle. It is attached to the gas supply pipe. The tube C is connected to the burners. To D is attached, by means of rubber tubing, a glass bulb F, which is placed inside the drying-box. G G is mercury, and H a rubber stopper by which the pressure on the mercury and quantity of air in F may be regulated. The action of the instrument is obvious. The gas passes down B, over the surface of the mercury and by the tube C to the burner. On the bulb F reaching a certain temperature, the mercury will allow just sufficient gas to pass from the tube B to maintain the box at a given heat. If it should fall, the mercury recedes from the aperture of the tube B, a larger quantity of gas passes to the burner, and the temperature is restored to a normal degree. If the air in the ball F expands to too great an extent, the mercury rises, and would eventually entirely close the aperture and cut off the gas supply, unless the tube B is provided with a small hole acting as a by-pass. The tube B may be moved up and down through the rubber stopper at A1 as a means of adjustment.
Fig. 3.
M. Thiel, of Paris, uses a very convenient drying-box, which, with his permission, is here explained. In a brick-lined receptacle under the laboratory floor lies the water-heating apparatus, which is constructed of sheet copper, and is capable of containing about four litres of water, utilised in the production of steam for heating the box. a, fig. 3, is the pipe passing through the wall into the drying-box; b is the gas supply pipe to the two atmospheric burners. Fig. 4 represents a plan of the drying-box; c c c c the perpendicular sides of the same, a a a gives a plan and position and arrangement of the earthenware heating pipes lying in a serpentine form at the bottom of the box, entirely covered with dry sand, and this again covered, as before described, with tissue paper. b is the outlet of the steam pipe. d d d d are movable horizontal iron bars with adjusting screws, on which the plates are levelled. The box is covered by a hinged lid, which is raised about a couple of inches during the drying to allow the air to circulate. The dimensions of the box will be determined by the size of the plates to be used, several of which may be placed side by side. Its height is about half a metre inside, and the plates are placed about its centre.
Fig. 4.
Glass Plates, to be used for printing from, must as a first consideration have their surfaces ground quite true and parallel. Since the introduction of special collotype presses requiring less pressure, the thickness is of less consequence than formerly, but for convenience of handling and to withstand the necessary pressure, in the printing frames, plates of at least four millimetres in thickness are desirable. Many use them from 8 to 10 millimetres in thickness; this, in the larger sizes, means a weight both inconvenient and difficult to handle. It is probably easier to work upon plain glass surfaces, and since means have been discovered of causing the chromated gelatine to adhere to polished glass with sufficient tenacity to produce several hundred impressions, the employment of ground glass plates is much more a matter of choice than formerly. The ground surface, however, assists the formation of a grain in the case of thin layers, and the operation of grinding serves to remove accidental scratches from the surface of the plates. As in practice these damages constantly arise, it will in the long run be found both desirable and economical to employ the ground plates.
Ink Rollers are also of great consequence in both collotype and lithographic operations. For printing from stone leather rollers have always been exclusively employed, and they are still used in some collotype establishments, more particularly where hand presses are yet worked.
The Leather Roller consists of a wooden cylinder or stock of about 21 to 42 cm. in length and 9 to 11 cm. in diameter, with handles at either end, usually turned in one piece with the cylinder. Boxwood handles are sometimes let into the ends of the cylinder, but although smoother to work, they not infrequently work loose. In using these rollers the handles do not come in direct contact with the hands, but are covered with a protection of stout leather, which not only protects the printer’s hands from heating, but enables him by a heavier or lighter grip of the handles to apply a heavier or lighter pressure of the roller—a point of great value in inking the plate.
The wooden stock of the roller is first covered with a double thickness of woollen material—flannel or Melton cloth—and over this is drawn the cover of calf-skin, flesh side outwards. The manipulation of the seam must be managed with extreme care, as any unevenness would render the roller useless. At both ends of the cylinder the leather projects, and is usually drawn tight with string or nailed on. There are two descriptions of leather rollers—smooth and coarse. The latter are only used to apply ink to the stone or plate, and then, with the smooth roller, the proper distribution of the ink is effected. For the latter purpose, in collotype, hard glue or indiarubber rollers are employed, being considered far preferable. When a leather roller is in good order, and its use has been thoroughly mastered, it is looked upon by many printers as indispensable. A serious disadvantage is that when new they are almost unusable for collotype, and after prolonged use the leather stretches, they become wrinkled, and this, of course, affects the even distribution of the ink. A new leather roller needs a special preparation or “breaking in” before it may be considered fit for use. It should be well saturated with a fatty matter, a very effectual way of doing which is to roll it well at intervals in a moderate supply of thin varnish as used for thinning printing ink, this process being persevered with so long as the roller continues capable of absorption. A better but more troublesome method is to roll the rollers twice daily in thin varnish, continuing the operation for a week, at the end of which time the roller may be well rolled upon the inking slab in stiff black printing ink, the superfluous ink being occasionally removed from both the roller and slab by scraping with a blunt knife, and a fresh supply of ink applied. This troublesome and tedious operation should be continued for a period extending over several weeks until it has become quite smooth and even, though it will still be found better to use it only for common work, or, if possible, on stone. To keep the roller in good condition it should, on each occasion after use, be carefully scraped with a clean, smooth-edged knife—not too sharp. Some practice in this operation is required to avoid cutting the leather. Great care should be taken to avoid this, the blade of the knife being held almost flat upon the roller during the process. The scraping should always be in one direction, to ensure which it is advisable to place a mark upon one of the handles.
Unless these precautions be observed the ink will dry in, and completely spoil the leather. If not to be used again for some time tallow should be carefully and thoroughly rubbed into the surface of the roller as a preservative, but before again using this coating must be removed either by means of a knife or washing with turpentine.
Glue Rollers