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Collotype.—Apparatus.

BEFORE commencing any practical work it will, of course, be necessary to procure various utensils and material not usually found in the photographic studio. All these should be procured from reliable dealers and of the best quality, for the evil consequences of false economy will make themselves felt in endless failures. The best to be obtained are none too good for collotype. In the practice of photography the whole success depends on a series of apparent trifles, and the same may be said to hold good, but in a far greater degree, in this process, which is one in which the difficulties can scarcely be overestimated.

The photographer will most likely be already in possession of many pieces of apparatus he may utilise—for instance, dishes. The best and dearest are those of china; but for many—in fact, most—collotype purposes, those of tin or zinc may be used.

For warming or cooking the gelatine solutions tin vessels are the handiest, as they easily conduct the heat and are unbreakable. Although the chromated gelatine may remain in them for a short period without harm, it is not advisable to allow it to do so for any length of time, but to remove the solution and wash the vessel thoroughly with hot water, and at once carefully dry, otherwise they will soon corrode, and contaminate the gelatine solutions. The best utensils to use are wide-mouthed shallow jugs, as they are easily kept clean, and in them the chromated gelatine solution keeps well, and with their use no fear of decomposition need be entertained.

Filtering the gelatine solutions is a somewhat troublesome matter, and should be effected at a high temperature and as quickly as possible. The simplest method is to procure a piece of perfectly clean flannel of suitable size, thoroughly moisten it, and insert into a brass ring, which is provided on the outside with small barbel hooks, to which the flannel is fixed, as in the retinaculum of the chemist. The ring is provided with a clip and handle, by the former of which it may be attached to a vessel of almost any size, and the latter is a convenience in holding it over plates to which the gelatine has to be applied. A careful filtering is obviously essential to the production of clean plates. Many complicated filtering appliances have been devised for gelatine and other solutions difficult of filtration, as, for instance, those of albumen or gum. Baron Szretter describes in the “Photographische Correspondenz,” 1878, an apparatus constructed by him. It consists of two vessels, an upper and a lower one, which by means of longer or shorter tubes communicate with each other in accordance to the stronger or weaker pressure required by the liquid to be filtered. Soldered round the upper rim of the lower vessel is a ring of sheet brass, about two to three cm. in width; over this ring the filter paper is placed, which again is covered with a piece of strong felt enveloped in flannel. To prevent the liquid escaping round the sides of the ring a strong iron ring is applied, which by means of a screw presses against the felt so that no space exists between the ring and the paper. To prevent the pressure of the liquid forcing the felt out of position, and so tearing the paper, a metal wire gauge is used to keep the felt in place. The liquid placed in the upper vessel passes through the tube into the lower vessel through the paper and felt layer. When it is necessary to warm the solution to be filtered, as in the case of gelatine, the whole apparatus is covered with an outer covering, and on the other side a pipe is applied for the purpose of effecting a circulation of the heated liquid, which is thus kept constantly rising through the one pipe and returning through the other.[F] Heat is sustained at an even temperature during the whole operation by means of a small lamp. A simple method of filtering such solutions is to pass them through purified sheep’s wool, or spun glass, a quantity of which is placed in the tube of the funnel. The whole apparatus may be placed in a warm oven during the process, or the drying box may be utilised for the purpose.

Fig. 1.

[F] The above description not perhaps being perfectly clear, and having been unable to obtain any further description, I give a sketch (Fig. 1.) of an apparatus found to answer well for the filtration of gelatine solutions and other viscid bodies. A is a glass flask, B a rubber stopper, C a funnel containing a filtering medium of washleather, D a tube attached to a three-way cock E; one hand manipulates the latter, while the other works the piston F of a syringe G.—Trans.

Printing Frames of different sizes will be found to hand in the photographic studio, and may be utilised without alteration for printing the collotype plates, if they are deep and strong enough to bear the necessary pressure, which is usually applied through the medium of springs; these are better removed, and wooden wedges inserted in their stead between the cross-bars and the loose wooden back of the frames, as by these means far more pressure may be applied. By lifting the one half of the hinged back of the printing frame an examination by transmitted light of the collotype plate may be made and an experienced operator will in this manner judge the exposure of the plate.

The Actinometer is, however, recommended, particularly for a beginner, as it greatly aids in forming a correct idea of the exposure.

The Drying Box is of great importance to the successful working of the process. The opinions of the various practitioners with regard to the temperature at which the drying of the plates should be effected differ as widely as upon the advisability or otherwise of admitting a current of air through the box during the operation. The drying should be completed as rapidly as possible from the commencement of the operation, care being taken that the heat never exceeds 50°C. Many plate-makers simply dry the plates in an open apartment—of course, only illuminated by a non-actinic light—simply placing the plates on a horizontal surface, which may be maintained at the temperature indicated by a water bath, a lithographic stone, or merely a cast-iron plate arranged in a suitable manner for heating from below. This method of drying is open to many objections: the surface of the plate is seldom free from dust, and the gelatine coating is too liable to irregularities from draughts admitted to the apartment during 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 have long been in use, 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.

The Scrapers should be of either pear tree, lime, maple, or preferably box, with a blunt bottom edge, which should be planed true, and finally finished by rubbing on a sheet of glass-paper placed upon a perfectly flat surface. They must be adjusted with even more precision than is necessary in lithography, as owing to the difference in the printing surface, the same pressure cannot with safety be applied. With plates or stones of large size it will be found that the pressure of the scraper will be unequal, greater pressure being exerted in the centre than upon the edges of the printing surface. Many printers, to counteract this tendency, slightly scrape away or hollow out by means of a piece of glass the centre of the scraper. With sufficient pressure and plenty of elastic packing between the printing paper and the tympan, this slight difference may be easily overcome. Several scrapers should be provided of various lengths, so that one of suitable size may be always available to suit the particular work in hand. It should always be slightly longer than the width of the picture, but shorter than the width of the stone or plate. By pressing the edge of the scraper to the surface of the latter and looking towards a window no light should be visible between the two surfaces. If the edges of the scraper are carefully rounded, it may be covered with a strip of wash-leather,[G] smooth side outwards, and secured by means of tacks, by which arrangement the pull will be easier, the impression more even, and the tympan and scraper will last longer. The accompanying sketch represents one of Poirier’s special collotype presses, constructed entirely of iron. The details are so clearly shown in the engraving that after the description already given it will be unnecessary to point out further details.

[G] This is necessary only with metal tympans.

White Margins to Pictures.—To facilitate the production of pictures with white margins, instead of using a mask of paper either oiled or treated with a varnish, an arrangement has been introduced, as represented in fig. 7, the design of which is due to Mr. Raph. Schlegel. The drawing is not perspectively correct, but is made to show as clearly as possible the arrangement and manner of working. The scraper—not shown in the illustration—works on the tympan O. A, carriage or bed of the press, carrying the printing plate. B, a slide of light wood. This prevents the frame D and the printing paper M coming in contact with the plate before the impression is actually being taken by the scraper. The wooden slide bridges over the plate, and may be 15 to 25 millimetres higher than the plate itself. When the rolling up or inking of the plate has taken place the slide is pushed so far forwards as to allow only just sufficient room for the scraper to rest upon the plate. The action of the scraper during the taking of the impression causes the slide to work out, and allows the paper to come in contact with the print layer. E E are adjustable buttons to regulate the traverse of the carriage when printing. D is a second frame necessary to carry the strips of thin zinc, Ef Ef, Eg Eg, which entirely protect the edges of the paper to be printed upon, and so produce an impression with a white margin. These strips may be almost instantly adjusted to give any required size. The frame D must be so much smaller than the frame L as to be easily covered by the latter, allowing for the projecting screws and nuts of the clamps f. The zinc strips should be reduced to a feather edge on the sides next the picture, and should have secured to them strips of fine paper (foreign letter-paper), which should extend over the edges about three millimetres, to prevent damage to the gelatine layer. f f are clamps for the zinc strips Ef. They are put through the slots, and the tension adjusted by the nuts. The zinc strips are perforated by two holes at each end, and secured to the clamps by means of screws. Eg are also zinc strips; g clamps provided with two holes, in each of which a hook is inserted. Corresponding with these a second pair of hooks are inserted in holes in the frame I, the hooks being connected together by the strong indiarubber bands h. This gives to the strips Eg a certain amount of elastic tension. The clamps K are also adjustable in slots, and similar to f; L, frame for the reception of the tympan; M, large clamp for stretching the elastic sheet O; N, zinc tympan, greased to facilitate its movement under the scraper, it also prevents grease penetrating the sheet O; P, a bracket on bed A, against which the tympan frame rests when raised and thrown back. The arrangement is used as follows: When the plate has been satisfactorily inked or rolled in, the slide or bridge B is adjusted over the plate, leaving a strip of about three mm. uncovered to allow of adjustment of the scraper. Lower the frame D in which the zinc strips have been adjusted to the size of picture; the printing paper is placed over this slide, and covered with a few sheets of paper backing. The tympan frame L, together with the sheeting and greased zinc plate, are lowered upon the work, the scraper adjusted, and an impression taken.

Fig. 7.