Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/724

700 LITHOGRAPHY and being much more portable than lithographic stones, and are easily cut into convenient sizes. They are grained in the same manner as lithographic stones, a muller of zinc being used instead of one of stone. Drawings on zinc, whether in chalk or ink, are executed on a grained surface. Zinc plates are subject to oxidation, and care must be taken to dry them off quickly after graining. The drawing is done precisely in the same way as on stone ; the etching solution is applied with a flat camel-hair brush. It con sists of a decoction of nut-galls ; a solution of gum and phosphoric acid is sometimes added. During printing the plate is screwed for support to a block of beech or other hard wood. As neither crayon nor ink penetrates the zinc as they do the stone, the adhesion of the ink forming the drawing is less thorough than in lithography, and greater precautions have to be taken to prevent accidents in printing. Chromo-Lithograph//. Great advances have been made in recent years in this branch of the art, notably in the reproduction of works of an artistic character. Its simplest form is the tint, in several gradations of one colour, printed over drawings in chalk or line ; in its more elaborate forms it includes imitations of water-colour drawings, decorative and ornamental designs, &c. The term " chromo-litho- graphy " is usually applied only to the more elaborate kinds of colour printing. All lithographs in two or more colours are printed from two or more stones. It is therefore necessary to employ some method to get a correct repetition of the subject on the first stone made upon the others, and to be able in printing to place the sheet so correctly in position that the second and each succeeding printing shall fall exactly into its place upon the first. Much of the success of the work depends upon this, and various modes of " registering," by "lay," by needles, by fixed points, <fcc., are employed. The first drawing is generally in outline. It is called the keystone, and provision is made in it for "registering," according to the particular method adopted by the artist. It is used only to take as many impressions on other stones as are required for the several colours, and as a moans of getting each colour in its exact place. In work of an artistic character it is omitted in printing. For ordinary colour printing the stone is polished ; when gradation of colour is required the stone is grained, but in a somewhat coarser way than for chalk drawing. It will be sufficient here to describe the production of drawings with two tints. The principal drawing is done upon a grained stone in chalk, and should be very bold, more like a sketch on tinted paper, the middle and finer tints being left out. The stone is then etched, and two impressions are taken, so that when each of these is put upon a roughly-grained stone, and passed through the press, counter impressions will be found upon the stones, revealing the drawing quite distinctly. After having cut in the outlines with a sharp- pointed graver, or steel needle, the artist covers those parts on the two stones which are not to appear in the one or the other colour, as well as the margin of the two stones, with a brush containing acid and gum. The stones are then warmed, and a composition containing the same ingredients, as soft chalk, with double the quantity of soap, and three times the quantity of tallow, is rubbed over it with a bit of coarse flannel, until it is of a dark greyish-brown colour. From having been previously cut in, the outline comes out very distinctly. The artist can now produce an effect similar to crayon sketches which have been washed in with two separate colours. Those portions which have been rubbed in, and which appear dark greyish-brown, form the middle tint, and the scraper may be used to reduce the colour of the tint where the gradation of colour is desired, the darkest portions bsins laid in with lithncmmTnV ii laid in with lithographic ink, and the blending together done with chalk, brush, pen, and scraper, so as to produce in many places the effect of shadings of one colour over another. When the work is of a very elaborate or complicated nature, the order in which the colours should succeed each other in printing is of much, importance, and requires to be very carefully considered. In highly finished chromo-lithographs, fifteen or more printings are frequently necessary. Difficulties sometimes arise from the paper stretching, either from the moisture on the surface of the stone or from the action of the press. Oleography differs from chromo-lithography only in name, and is a mere vulgar attempt to imitate oil painting. The finished print is mounted on canvas, sized, and varnished. The loaded colours and rough textures, if there happen to be such in the original, are suggested by embossing, with what result it is hardly needful to say. Instruments, Tools, and Apparatus used in Printing. Litho graphic presses are of a great variety of construction, and we can only glance at the chief points in their mechanism. The scraper is a wedge-formed plate of boxwood, fixed to the bottom of the platten ; its edge is covered with a piece of leather, and properly adjusted. The table on which the stone with the paper for receiving the im pression is placed, and on which the tympan is brought down, is, by means of a handle or wheel, brought upon the metallic moving roller and under the scraper ; the pressure is applied with a lever, and continued from one end of the stone to the other ; when it has passed through the press, the lever is lifted, the moving table brought to its original place, and the printed sheet removed. The lithographic steam-press began to be generally used about 1867, and has quite revolutionized the lithographic trade. The rollers for printing may be made of different lengths, from 6 to 24 inches long, and 3^ to 5 inches in diameter. They are made of wood of the alder and lime tree, with wooden handles to project and hold by ; the roller is then covered with several complete turns of flannel, well stretched and fastened by sewing at the extremities near the handles ; the whole is then covered with calf-skin, sewed with great care, so as to fit tightly, and laced near the handles. For printing chalk, tints, and colours, skins of different prepara tions are required. Hollow metallic rollers, covered with ilannel and calf-skin, are in use on some parts of the Continent, where they are preferred to rollers made of wood. The best varnishes for making the printing-ink are boiled from old linseed oil, of different degrees of strength thin, middle, strong, and very strong varnish ; for printing with gold-leaf, bronze, and dusting colours, the very strong varnish is required. For writings, maps, and music, common calcined lamp-black is ground with the varnish ; some hard blue is added to improve the colour, and to make the ink dry in a short time ; for chalk ink, Paris black is used instead of the common lamp-black. For tinting and colour-printing, colourless or bleached varni.sh must be used, as otherwise the purity of the colours will suffer. Printing. After the stones containing the writings or drawings in chalk and those used for tinting or colour-printing are retched, and the preparation has become dry, the stones may be put into the press and properly fixed. To prevent a stone containing drawings of any value from breaking, it should, if thin, be backed to another stone, or, what is better, to a slab of Aberdeen granite. This is done by a mixture of plaster of Paris and cold water, of the consistency of syrup, a pretty thick coating of which should be spread evenly and quickly on the slab ; the stone containing the drawing is placed into this layer, and moved in all directions until the plaster of Paris becomes quite hard, which will take place in a very short time. After the printing is accomplished, the stone can be quite easily removed from the slab, by using a chisel, and by giving some side strokes with a wooden hammer. The old dry plaster of Paris is now removed, and the slab is again fit for use. Everything being ready, the gum is entirely washed away with a soft Turkey sponge and water ; the writing or drawing is then ob literated by taking a clean sponge and oil of turpentine, to which maybe added, at pleasure, a few drops of sweet oil, after which the stone must be cleaned with water. An entirely clean sponge (or for writings, a bit of soft canvas manufactured for the purpose) is now required for the printing. The stone is slightly wetted with this sponge ; the printing roller, charged already with the proper ink, is passed repeatedly over the whole stone, and the writing or draw ing will begin to reappear ; the roller is again worked on the ink- slab, the stone wetted anew with the sponge, and again rolled over, until the writing or drawing appears in full strength. A sheet of damped paper (dry paper may be used for writings and drawings in line) is put on the stone, the tympan is let down, the scraper brought to its proper place, the pressure effected, and, by means of the handle, cross, or wheel, according to the style of press, the table with the stone is slowly and equally drawn through to nearly