APPENDIX.THE STEAM COLLOTYPE MACHINE.
THE STEAM COLLOTYPE MACHINE.
THE earliest steam machines for Collotype printing were made by Faber & Co., of Offenbach; they were, however, so complicated and delicate in construction that other machines have entirely supplanted them. The three principal varieties are those of Alauzet & Co., of Paris; Koch, of Leipzig; and Schmiers, Werner & Stein, each of which may be considered entirely successful; they possess features of great similarity, differing not in principle, but only in details of construction. Contrasting the work of the machine with that of the press, the printing is much more rapid, and the prints more uniform and regular, while the “life” of the plate is greatly lengthened; one capable of yielding 250 impressions on the scraper press would, in all probability, give 1,000 on the machine—a fact easily accounted for when it is considered that the one is produced by a scraping pressure while the other is made by the rotation of a cylinder. As will be seen by the illustrations, the machines bear a considerable resemblance to those used in lithography—they may, in fact, be used for lithographicprinting—with very greatly increased inking power. The machines are so constructed that the plate may be inked once, twice, or three times, each inking giving more vigour, but, of course, a corresponding decrease in the number of impressions yielded; the output may be relatively estimated at 1,500, 1,000, and 800 per day. It is, of course, an impossibility to teach machine printing by means of instructions such as may be conveyed in a work like the present, and the following remarks are added more with a view of giving a general idea of the nature of the necessary operations than with any idea of making the reader a competent machine minder.
The Preparation of the Platefor the machine is identical with its preparation for the hand-press, but greater uniformity, both in the negative, exposure, and etching, are demanded than by the hand-press. For the latter considerable latitude is permissible, as the intelligent printer may, by varying his rollers, his rate of rolling and other little dodges, produce perfect impressions from plates which would, under the entirely mechanical action of the machine, fail lamentably.
Adjustment of the Rollersis so effected that they are lifted about 2 mm. when passing over the inking slabs and the printing plate; if a greater lift is necessary to bring them upon the printing surface marks will be produced upon the plate, and if arranged for a less lift they will probably skid over the plate, damaging the printing surface. Theyshould also be adjusted perfectly parallel with the surface of the inking tables, or one side only may come in contact with the work.
Fixing the Plateto the bed of the machine is effected in several ways, but usually with small clips, as described for the hand-press. All adhering gelatine, etc., must be removed from the plate, and a thin sheet of white paper inserted between it and the bed of the press. The faces of the metal clips must be protected with strips of leather, cork, or some such elastic bodies, otherwise the pressure exerted would cause the glass to fly. A straight edge is placed across the machine, and the bed lowered by means of inclined planes attached to suitable gearing until the surface of the plate is well below the circumference of the cylinder, and from this position it is brought up to its proper level, raising each side equally, that the pressure may be perfectly uniform.
Packing the Cylinder.—The cylinders are so constructed that the iron surface does not come in actual contact with the printing paper. In fact, there is a deficiency between the actual and necessary diameter of the cylinder of a few mm., sometimes as much as 7 mm. This is compensated for by the addition to its surface of “packing,” which gives a more elastic pressure than the unyielding surface of iron. This packing is not applied to the entire surface of the cylinder, but is so devised that it exerts pressure upon the surface of the picture only. Asheet of smooth four-ply cardboard is attached to the cylinder by means of thin glue, this first sheet usually exceeding the size of the picture. To this is attached a second piece, cut of the exact size of the image, and attached by means of the glue, forming a raised block in exact register with the position of the picture on the printing plate; this is covered with a thickness or two of fine blotting-paper, and over the whole the sheeting of the cylinder is tightly stretched in the usual well-known manner. It is self-evident that the circumference of the cylinder must traverse at exactly the same speed as the bed of the machine, or there will be a horizontal rubbing or friction between the surface of the printing paper and that of the printing layer, which will result in impressions lacking in sharpness, and ultimate destruction of the plate.
Masking the Printsfor the production of white margins is usually performed by one of two methods, each having advantages and disadvantages which should be carefully considered before adopting either; both are in practical use. In one is used a light rectangular iron framework or “frisket,” conforming to the size of the cylinder, to which it is attached by hinges. This framework is opened and closed automatically, remaining open during the period the cylinder is resting and the inking is taking place. At this point the “layer-on” inserts the paper, it is seized by the grippers, and the masking frame closes down into suitable grooves in the circumferenceof the cylinder, such grooves being useful to prevent any lateral movement of the frame. To the latter are attached four movable strips of thin zinc, which may be adjusted by means of set screws to form an opening of any desired size. The printing paper being inserted in the grippers, the frame closes, holding the paper upon the cylinder, the strips of zinc at the same time protecting the edges of the paper from immediate contact with the printing layer, the cylinder revolves, and the printing is effected. In large sizes the zinc strips are necessarily of such strength that the edges would cut and damage the printing layer; it is therefore usual to adjust the strips to within about 5 mm. of the edges of the picture, and by attaching strips of varnished tissue paper to the zinc, reducing the aperture to the size of the picture in hand. In large sizes—where thin paper is used, or where narrow margins are desired—this system of masking is best, as the whole of the operation is entirely automatic. The layer-on may devote the whole of his attention to removing impressions and inserting fresh paper.
Another Systemof masking frequently made use of is one in which an aperture of suitable size is cut in a piece of cardboard, such aperture being, say, 2 mm. larger than the print. The portion removed is reduced in size by 2 mm. all round, leaving it exactly the size of the impression desired. This piece is utilised for packing by attaching to the cylinder as previously described. The cardboardmask, presenting the exact appearance of a sunk mount, is affixed by means of a bar to the cylinder, and when folded down upon the latter, the “cut out” used as packing exactly occupies the centre of the mask, leaving a marginal space all round of 2 mm. Before printing, the mask must be varnished with negative varnish, bitumen in benzol, or otherwise waterproofed, to prevent adhesion to the print layer. The mask has to be turned back by hand at the insertion of each sheet—a considerable disadvantage when compared with the automatic frame. On the other hand, its advantages are simplicity of preparation, and the possibility it allows of masking and printing several irregularly-disposed pictures on one sheet.
Damping the Plate.—The use of the cylinder pressure has further advantages than merely prolonging the life of the plate—the operation of damping is much less needed than on the hand-press. The printing paper does not come in contact with the plate before the instant of printing or remain upon it after the operation, but is held to the cylinder by the grippers and mask, and so carried over the inked plate, consequently little opportunity is afforded for absorption of the etching fluid. With chalk paper and a new plate damping may be necessary every twenty-five impressions, but as the plate becomes older fifty or even one hundred may be printed between the etchings. On plain paper of hard surface much longer numbers may be expected.
The Inking Tables, two in number, are provided one at either end of the machine, necessitating two sets of inking rollers—one of leather for applying the thick ink, and another of composition for supplying the “half-tone,” the latter set requiring such light application to the plate that no riders are required. There are also the usual distributing rollers, to which the ink is generally supplied by the printer as may be necessary, the quantity being so small that no actually satisfactory automatic duct has yet been devised. A proper adjustment of the consistency of the two inks to the requirements of the plate in hand is a matter of considerable delicacy, for should the ink supplied to the leather rollers be too thin it is immediately removed from the printing plate by the composition rollers; these again transfer it to the table devoted to the thin ink, and a general “wash up” is the necessary result. The ink first supplied to the leather rollers should be as stiff as possible, afterwards reducing as experience may dictate, while to the composition or tone rollers merely coloured varnish, or even varnish alone, is applied, as the latter often remove enough colour from the printing plate to sufficiently tint the varnish applied to their surface. The leather rollers should be cleaned daily by scraping, and the composition set washed, unless the weather be very cold or damp. In such cases they may be allowed to stand under ink, as they will in that condition be less susceptible to atmospheric changes.
Double Printingis sometimes resorted to for work of a high class, and when negatives too thin to produce good results otherwise are unavoidably used, such work is usually printed on chalk paper and without masking. The printing plate is usually inked up twice and an impression taken; without removing the impression from the cylinder the plate is again twice inked and a second impression pulled on the same piece of paper, the two images being exactly super-imposed. The great gain in contrast will be understood when it is considered that while the high lights of the picture gain absolutely nothing, the strength of the shadows is exactly doubled. The results produced in this way are simply astonishing when contrasted with silver prints from the same negatives.