Page:EB1911 - Volume 22.djvu/428

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413
PROCESS


negative by the lithographic press was introduced and patented by Sir Joseph Swan and his son, Donald Cameron-Swan. The sunk surfaces are rendered receptive of lithographic ink while the surface of the plate itself is kept damp with water or glycerin and water, and remains clean and free from ink when the plate is rolled.

The monotype is not a new, but a revival of a somewhat old, method of reproducing on paper a painting by an artist. The Monotypm design is executed on at plate by means of brushes, fingers or other tools, with paint or printer's ink. On the completion of the painting, paper is laid upon it, and plate and paper are together passed through a press, when the ink or colour is transferred to the paper. One impression only is possible, hence the name of the process. A method has been devised by Sir Hubert von Herkomer for dusting the painting while still wet with a fine metallic powder, which gives a tooth to and renders the surface sympathetic to a copper deposit when it is placed in the galvanic bath, by which means an electrotype of the painting, with its varying relief surfaces, is obtained, and forms a plate from which numerous impressions can be taken.

The very lar e number of impressions it is often required to get from the etched surface of a block has made it necessary to devise E, mm means for 'preserving the original block, and to prepare typ" and work rom duplicates, which can be renewed when necessa?. For t is process the original is coated with a film of the nest plumbago (black lead) powder before being” placed face to face with a bed of soft fine wax, into which it is pressed. The plumbago prevents adhesion and facilitates the withdrawal of the block after Contact with the wax. The wax mould which is thus obtained is suspended in a galvanic bath of sulphate of copper. On passing a current of electricity through the liquid to the mould, the copper at once begins to de osit itself in metallic form lover the face of the wax mould, and) in a short time the deposit becomes thick enough, either by itself or when backed up with other metal, to be used as a block in the place of the original. The very fine nature of process blocks, and the necessity of obtaining perfect impressions from them, has led to the introduction of gutta-percha instead of wax as the medium for making a mould. It is melted and poured in a liquid state upon the block, and when cold can be removed without the risk attending the use of wax, which is apt to give way in the course of the separation of the block from the mould. Gutta-percha is much more tenacious, and being somewhat flexible, does not break and tear, as wax is liable to do. The whole rocess requires the greatest care in its manipulation. Steelifacing is resorted to where long numbers have to be printed from photogravure plates. The finest film of steel is deposited by Stub an electric battery over the whole face of the plate, fum! which it hardens and protects. This steel face in time begins to wear, through the constant pressure and rubbing incidental to the process of printing, and the copper begins to show through it. As soon as this happens the plate is placed in an acid bath, in which the steel film disappears. The plate itself being still intact, can be re-steeled for further work. The changes which have taken place in the form of illustrations have necessarily been accompanied by changes in the machinery cb” smby which they are printed. Almost all the changes Muhfse and improvements have been initiated in the United ry' States of America. The vital change made in the interest of process block-printing is what is technically known as “hard packing." Before the introduction of process blocks the “ blanket " played an important part in all printing machines. It was a soft woollen sheet, which came between the plate or cylinder and the type and blocks, and modified the force of the contact between them. Owing to the increased fineness of the texture of the process block as compared with the wood engraving, it was found that the blanket was too coarse and soft a material, and that it interfered with the clearness and fineness of the printed result. Blankets of finer material were tried, withfim-Eroved results; but at last the blanket was entirely superseded y a glazed board, the machinery was more accurately construct, and the hard, finely-polished steel cylinder, without any intervening substance save the thin glazed board and the sheet of paper to be printed, was brou ht in contact with the type and blocks. The old soft blanket kept the cylinder or the fiat press in Contact with the type, in spite of the weak construction of 'much printing machinery.' The new method of work made no allowance for such construction; and the new machinery, to meet the new conditions, had to be very perfect in manufacture. About the old machines there was a lack of solidity, which allowed vibration. Modern work demands absolute rigidity in the machine; and a chief characteristic of the best modern printing machinery is strength and solidity, admitting of precision of impression. Another change has been in the nature and treatment of the printing paper. 4 I 3

Most elaborate methods were adopted for the moistening of the substance of paper before use. Most paper was printed on whilst damp, but damp aper had to disappear with the soft blanket, and a clay-surfaced or highly-calendered paper was introduced with a glazed face in harmony with the polished steel cylinder which pressed it against the type and blocks. It is essential to this paper that it be dry when used; to ensure the best results with it the paper should be kept some weeks or months before use, so that it may be absolutely dry, or seasoned. If printed on too soon, the clay surface tears away when in contact with the “ tacky " ink; and instead of the ink being deposited on the paper, bits of the paper surface are left on the forme, and white spots appear in the impression. The bits of paper surface so deposited on the forme get inked as they pass under the rollers, and impress black spots on the sheets that come after. New and unseasoned paper accounts for much bad printing, and this form of badness is due to the change in material due to the necessities of modern process work.

3. Planographic processes are such as are printed from a flat surface neither raised above the surrounding ground like a wood engraving or type letter, nor sunk below the ground like an etching or steel engraving. Lithography (q.'v.) with its fiat stone or plate may be taken as the type.

Woodbury type is a development rather than an invention by Walter Woodbury. By an old nature-printing process leaves and other things which lent themselves to the treatment were by extreme pressure forced into a flat surface of soft metal, and the mould so formed was used as a printing surface to reproduce the forms of the impressed object. Woodbury found that a film of bi chroma ted gelatin exposed to the action of light under a negative and the unaffected parts washed away gave him a relief image which was so hardened by the action of light aided by other hardening agents, that it could with no injury to the film itself-which could be used many times to make fresh moulds —be forced by hydraulic pressure into a thin fiat plate of lead or type metal, and that the mould so formed could be used in a similar way to the mould formed in the old nature printing process. But a Woodbury type print is rather a cast from the shallow mould than a print in the true sense. It is obtained by filling the mould with a warm solution of coloured gelatin and pressing on it a piece of hard surfaced paper. The pressure forces the solution away from the highest parts of this mould which come in actual contact with the paper, so that none of it is left between them and the surface of the paper which in these parts remains uncoloured. These are the high lights of the print. The pressure forces the colouring matter into the hollows of the mould, and this amount is graduated according to the depth of the hollows. The coloured gelatin gradually cools and hardens and adheres to the paper which on its removal from the mould retains a delicate cast of the impressed subject. The variety of light andshade is the result of the varying depth of the hollows and the consequent variation of the amount of colouring matter taken up by the impressed paper. The white paper is an important element in the result, the light reflected from it through this coloured gelatin varying according to the thickness of the gelatin film. A drawback to the use of the Woodbury type for book illustration is that every print has to be trimmed and mounted, and of course it cannot be printed with type. Stannotype is a variation upon Woodbury type. It is an attempt to do away with the need of the hydraulic press for the making of the mould. A film of bi chroma ted gelatin is exposed to the action of light under a positive instead of a negative and the unaffected parts washed away, by which means a mould is obtained corresponding exactly to that obtained in metal by pressure from a film exposed to light under a negative. This mould was covered by a coating of tin foil to give it the necessary metal surface, and good results were obtained from it, but for some reason it has never come much into use.

Collotype or photo type is a process in which the film of isinglass, gelatin or gum, treated with bichromate of potash with the addif tion of alum or some other hardening substance, becomes an actual printing surface inked with an ordinary roller and printed by an ordinary machine. A strong tough film made up of a first coating of a simple gelatinous nature covered by a second film of the sensitive bi chroma ted gelatin is spread upon glass and allowed to dry. Exposed to light under a reversed negative