1911 Encyclopædia Britannica/Process (printing)
PROCESS, a general term now technically employed for the photo-mechanical processes by which illustrations are reproduced in printing. Until the last quarter of the 19th century reproductive processes, save as regards line reproduction, can hardly be said to have had an existence. Paintings, drawings, and engravings, which it was desired to put into form which by means of the printing-press could be multiplied indefinitely had to go through a process of interpretation by an engraver or draughtsman, who, on a metal plate, a block of wood or stone, gave a rendering of the original subject. The means at his disposal were lines and dots, which, varying in their thickness and proximity, expressed dark or light passages in the scheme of light and shade of the original. It will readily be understood how such interpretations would vary. An engraver with fine art instincts would produce a result as distinct in character as an engraving as was the original as a painting or drawing, and engravings were sought after as works of art, and treasured for their artistic qualities. But engraving of this kind took time. Years were devoted to the production of one steel- or copperplate, while wood engravers who were artists could only work on a block when in the mood; and for that mood the publisher had to wait, and he grew impatient and was willing to accept rapid interpretation of originals by men who could produce them under other than artistic conditions. But the pain of the artist at the bad rendering of his original was often great, so that he, not less than the publisher, though for another reason, hailed the attempts that were being made to reproduce his work mechanically without the intervention of the translator or interpreter. The ideal of an artist would naturally be a reproduction of his work in facsimile, which retained all, or as many as possible of, the individual characteristics of his work; and to give him this was the aim of the school of wood engravers which originated in the United States and made a last stand to maintain the position of their art in the field of book illustration. By a system of extremely fine work the American wood engravers were able to keep much closer to the tones of an original than had previously been possible; but the result was obtained at the sacrifice of the artistic rendering of the best old engravings, and was so mechanical in its character that when it had to compete with a real mechanical process the engraving could not hold its ground, the enormous difference in the cost of production being a factor of sufficient importance in itself to make it impossible for the engraving to retain the field. A similar development had been going on in the other branches of engraving. The line engraver and the etcher, to whom had been entrusted the interpreting of works of art first produced in other forms, found themselves faced by mechanical reproductions in plate form which, while preserving more of the character of the original work, were produced in much less time and at a greatly reduced cost. It has thus come about that the last quarter of the 19th century witnessed the dispossession of the hand engraver from the field of interpretative engraving, and the occupation of his position by the chemist and the mechanician.
The term “process,” which has come to be applied to all photo-mechanical reproductions, is a somewhat unfortunate one, inasmuch as it is descriptive of nothing. From time to time various names have been given to its varying forms, indicative either of the name of the inventor or of some peculiarity of method. Zincography, gillotype, photogravure, heliogravure, heliotype, photo type, albertype, are illustrations of the kind of name given often to very slightly varying applications of the same principle, but usage has come to apply the term “process” to any printing surface that is produced by chemical and mechanical means. The whole of these processes may be arranged under three heads: (1) relief; (2) intaglio; (3) planographic.
1. Relief Processes.—An engraving in relief is one in which the printing surface stands up above the surrounding ground. The history of the development of relief processes is really the history of photography (q.v.); for whilst attempts were made to obtain results without the aid of photography, by drawing upon plates with prepared chalk or ink, “rolling them up” with printer's ink and etching away the ground with acid, as in the case of zincography, the real progress of all process has been upon the lines of photography; and to Niepce and Daguerre may be attributed the origin of the modern mechanical and chemical processes.
Speaking broadly, all the modern “processes” are the outcome of a discovery by Mungo Ponton that a preparation of albumen or other colloid substance and bichromate of potash could be hardened and rendered insoluble and nonabsorbent in water by exposure to light, and that as a photographic negative permitted the passage through it of light in varying degrees of intensity, so a film of the preparation placed under a negative was liable to be hardened and rendered insoluble in degrees varying with the intensity of the light affecting it. This discovery governs the production of process blocks or plates of all kinds.
The methods of reproduction of pure line work differ greatly from those for the reproduction of originals in tone. As the Line Blocks. first necessity in securing a good result is the suitability of the original to be reproduced, it is desirable to make clear the character of a good original. This should be of one tone or degree of colour all through. It may be all grey; it is better that it be all black. It may not be black in parts only and grey in others. The lines of an original may be of any variety of thickness. It is necessary, therefore, for the draughtsman to see that he works with a good black ink, or ink that will tell as black when it is exposed to the photographic plate. Inks of a warm tone—that is, inclining to red or orange—yield better results than cold inks which incline to blue.
Most prepared liquid inks have a tendency to lose their blackness by exposure to the atmosphere on the removal of the cork from the bottle. The ideal ink is one freshly ground from a dry cake of colour when beginning work. Indian ink is good if well ground and kept sufficiently thick to assure the necessary blackness. It has the advantage of not washing up when colour in washes is passed over it, but it must be used freshly ground. The addition of a little Indian yellow, burnt sienna or sepia, gives a warmth of tone to it and renders it photographically more active. Bourgeois ink, prepared by Bourgeois of Paris, appears to be prepared with the admixture of some warm colour with the black base. It is a good ink for the purpose, and is prepared both in solid and liquid form. Lampblack gives good black lines; so does ivory black, which is warmer in tone than lampblack. Higgins' Indian ink or American drawing ink is an American ink made in liquid form which has the reputation of not fading by exposure. Stephens's Ebony Stain is a fine black medium which does not clog the pen; if it thickens and dries, it cracks off and does not corrode the pen.
Besides the pen a brush brought to a fine point is much preferred by some artists, as it yields a line less monotonous than that given by a pen, though the brush cannot be used so freely. The paper used should be smooth and as white as possible. A paper is made with a surface coating of white chalk, which admits of the use of a scraper to remove lines or to break them up.
It is not possible to lay down a rule for the amount of reduction to be made when photographing for the reproduction; the liner the drawing the less should be the reduction made; but experience is the only guide. Sometimes, where the lines are very fine and the drawing minute in character, an enlargement is desirable. Where drawings are reduced too much, there is a tendency for the spaces between the lines to fill up, and to give a coarse, heavy result. Faulty drawing is not lessened by reduction. On the contrary, the fault becomes more evident, so it is desirable to make all necessary corrections in the drawing.
The original drawing which has to be reproduced is photographed to the size of the required block. The negative taken is absolutely dense except where the lines of the drawing have affected it, and these are absolutely clear, admitting the unrestricted passage of light through them. A piece of planished copper or zinc is prepared or made sensitive to light by a preparation of albumen or gelatin and bichromate of potash spread upon its surface. The negative is laid upon the sensitized metal and placed in the light in the way an ordinary photograph is printed. The light passes through the transparent lines of the negative and hardens the bichromated film beneath them. Both negative and plate are then taken into a darkened room, where the metal plate is rolled with an inked roller, placed in a bath of cold water and allowed to soak until the albumen and bichromate becomes so softened everywhere, except where the light has hardened them, that they all wash away, and nothing is left but the hardened lines. The lines are dusted with asphalt, which by heat is melted on to them, and makes a ground which resists the action of acid. A coat of varnish is put over the back and edges of the plate, to protect them from the acid also, and only the spaces between the lines on the surface are left free to its action. The plate is then placed in a bath of dilute nitric acid, which eats away the metal where ever it is exposed; but it leaves the lines of the drawing, which are protected by the hardened film standing up above the eaten or etched surface; and these lines, which correspond to those of a wood engraving, are the printing surface of the plate. The plate is mounted on a wood or metal block, made type-high, and it can then be used along with type in the printing-press.
Various devices have been resorted to that effects of tone may be obtained by means of the simple line process. Grained papers with a surface of chalk, upon which are printed close-ruled lines crossing at right angles, or rows of dots give the papers a heavy, flat, “tone,” upon which a drawing can be made in pencil, chalk or ink, and gradations of tone introduced by means of scrapers, which remove partially or entirely the black ruled lines or dots, leaving, if desired, high lights of pure white. A drawing on such paper consists of lines or dots, a combination of the original lines or dots of the paper and those of the drawing itself, the scraper splitting up lines into dots or removing them altogether. The result is quite easily reproduced by the line process. Another method is by the use of what are known as Day's “shades,” or shading mediums. They are transparent films of gelatin which have upon them lines or dots in varying combination in relief, so that they can be inked up by a roller. When placed over a drawing, their transparency enables the operator to see exactly what passage he is dealing with, and he can by means of a burnisher impress the lines or dots of the shade upon any passage of the drawing; these lines or dots then become part of the drawing; and are reproduced in the usual way.
Pencil or chalk drawings upon simple white-grained paper, where the pencil or chalk passing over the ruts or hollows in the paper makes a mark on the top of the grain only, are also reproducible by the line process, but such drawings are apt to be unequal in colour and difficult to deal with. The difficulty led to the invention of a process by Henry Matheson, who, not having the capital to work it, joined the late Mr Dawson, senior, whose sons continued to work the process with Matheson under the name of the Swelled Gelatin Swelled Gelatin Process. Process. It is based upon the fact that gelatin, sensitized with bichromate of potash, swells when placed in water, and swells in proportion to the amount of light to which it has been exposed. A negative taken from a drawing which varies in tone, not being thoroughly black all through, varies in the quality of its transparent lines and dots; and when a piece of paper or glass coated with sensitized gelatin is exposed to the action of such a negative it is affected according to the amount of light the negative allows to pass. After making a print on such paper or glass, it is placed in a dish of water and the surface allowed to swell, which it does in varying degrees, the portion unaffected by the light absorbing most water and swelling most, the hardened lines of the drawing not swelling at all. This swelled print is then placed in a frame, and a preparation of plaster is poured upon it to make a mould of its surface. When this has set and the gelatin has been removed, this mould is filled with a preparation of wax, which sets in a few minutes sufficiently for it to be released from its plaster mould. Additional wax is built up when necessary upon the “whites,” as they are technically called—that is, the passages which represent what will be the hollows in the block—so that these may be as deep as possible; and this wax mould is electrotyped in copper. The lines and dots of this copper block, which when finished is backed up with metal and mounted, vary slightly in height, the result being that the slightly lower dots do not come so closely in contact with the inking roller or with the paper, and so produce when printed a grey impression corresponding to the greyness of the original drawing.
The drawback to the use of the process is that it is about three times as costly as the ordinary process. It is a method much used for the reproduction of line and stipple engravings, where fine dots and lines are apt to be printed in delicate tones. The finest results by this method are producible, however, by omitting the plaster mould and wax-cast stages, and by coating the sensitized gelatin with plumbago or other impalpable metal preparation which will enable it to receive a copper deposit to qualify it to take its place in the electrotype's bath, and so to get the needed thin coating of copper from the surface of the gelatin itself; but this needs to be done with the greatest care, and is still more costly.
A non-photographic process of obtaining line blocks in relief has been for a long time successfully worked by Messrs Dawson. A Typographic Etching. brass plate is coated with a film or ground of wax upon which a tracing of the drawing to be reproduced may be rubbed down. By means of an etching needle the lines of the drawings are incised upon the thin wax ground down to the surface of the brass plate. A pencil of wax and a pencil of hot metal are then used to produce a flow of melted wax which drops from the wax pencil upon the ridges of wax between the lines and builds them up until they are of sufficient height. The risk that this wax may run into the incised lines has to be carefully guarded against, but skilful treatment manages so that it stops at the edges and does not run over. In maps and diagrams where lettering or figures are necessary, type is impressed into the wax with a very neat and precise result. By this means a mould is formed, an electrotype from which gives a really good relief block which may be printed with type.
The invention of line processes only stimulated the efforts to find out some means whereby tones might be reproduced on Half-tone Processes. blocks or plates that could be printed along with type in the ordinary rapid printing-press. It is only possible to approximate to the printing of a flat or graduated tone by producing a broken or granulated surface which shall present a series of lines or dots that, when inked and impressed upon paper, shall by the variations of proximity and size give the impression of an unbroken tone. This necessitates the lines or dots being so small that the eye shall not at a glance appreciate the broken-up character of the surface of the block. Many efforts resulted in the production of what is known as the screen, which itself was only made possible by the invention of ruling machines of a delicacy previously unknown.
A screen is made by coating a sheet of glass—which must be flawless both as to body and surface—with a composition analogous to the ground used by an etcher to coat his plate before drawing upon it with his needle. The glass so coated is placed in an automatic ruling machine, of which the ruling point is a diamond, and which can be adjusted so as to rule any number of lines from 50 to 300 to the inch. The lines are ruled diagonally on the glass, and at mathematically equal distances from each other. The sheet of glass, after ruling, is treated with hydrofluoric acid, and the lines where the ground is cleared away by the diamond point are etched or bitten into it. The plate is cleaned up and an opaque dark pigment rubbed into the lines. Two such ruled sheets of glass are sealed together face to face with Canada balsam, with the diagonally ruled lines crossing each other at right angles, the result being a grating or screen containing innumerable little squares of clear glass through which the light can pass, which it cannot do through the ruled lines, which are filled by the opaque pigment.
To produce a half-tone block from a picture, a black and white drawing in tone, or a photograph, a negative is exposed in the camera in the usual way, with this screen quite close to it but not in contact; and the subject is photographed on to the negative through the screen, and what is termed a “screen negative” is the result. It is a photograph of so much of the original as could affect the negative through the little clear squares of the screen, and represents the tones of it by innumerable dots and lines, the size and proximity of which are regulated by the fineness or coarseness of the screen used.
In the early days zinc was the metal used for these half-tone blocks; but experience showed that though more difficult to etch to the necessary depth, the closer, denser texture of copper rendered plates of this metal much more suitable for the production of the best blocks, and zinc now is used only for inferior blocks. Whichever metal may be used, a sheet of it, most carefully planished, is sensitized with a coating of gelatin or fish-glue and bichromate of potash, dried and exposed under the screen negative to the action of light, as in the ordinary method of photographic printing. The action of the light hardens the gelatin film, the portion not so hardened being soluble by water. The plate with the gelatin picture in lines and dots is exposed to heat and the image is burnt in on the surface of the metal like an enamel, which enables the photographic picture to resist the subsequent etching. The plate is placed in a bath of iron perchloride and etched until sufficient depth is obtained. Wherever the surface of the plate is free from the lines and dots, it is bitten away by the perchloride, and the lines and dots are left in relief. This first biting in the bath produces a rather flat general impression of the original, and is termed “rough etching.” To produce finer results, and to bring out the contrasts of black and white necessary to a good reproduction, the block has to go through processes of stopping out and rebiting similar to those of etching an intaglio plate. This “fine etching” calls for the artistic taste and judgment of the craftsman; and with a good photograph to work from the final quality of a block will depend largely upon its treatment by the line etcher. A substitute for the acid bath has been found in an acid blast. The acid is driven in the form of a spray with some force on to the surface of the prepared plate, which it etches more rapidly and more effectively than the bath.
One risk to be guarded against is the under biting of the lines and dots which form the printing surface. As soon as the acid has eaten its way downwards past the protecting surface film, it will attack the sides of the upstanding dots as well as the ground that supports them, with the result that they become weakened and rendered liable to break off in the process of printing, as well as to make the obtaining of electrotypes from the blocks a matter of extreme difficulty, the underbitten points breaking or tearing away in the mould. To avoid this underbiting a fatty ground is laid over the surface of the block each time it is etched; by exposure to heat this ground is sufficiently melted to permit of its running down the sides of the upstanding points, and so giving them the required protection. The acid blast is less liable than the bath process to eat away the sides of the dots. This method of making tone relief blocks is most generally known as the “Meisenbach” process, from Meisenbach, of Munich, who was the first to make it commercially successful, but the history of its development is somewhat obscure. Fox Talbot as early as 1852 took out a patent for using a screen of crape or muslin; he also suggested dusting glass with a fine powder to produce a grain screen. All the early ruled screens were single line, and the credit is due of suggesting the shifting of the single line screen during the operation and, by reversing it, producing the effect of the double line, to Sir Joseph Swan, who patented the process in 1879. Meisenbach's patent for a similar method is dated 1882. The development of the screen was the important factor in the development of the process. The early screens were photographs of ruled plates and the great advance was made by Max Levy of Philadelphia, who made it possible by his ruling machines to produce screens of a fineness and clearness not previously practicable. It was F. E. Ives who, in 1886, introduced ruled screens placed face to face and sealed up so as to produce cross-lined screens.
The chief objection to this process is its inability to reproduce the extremes of expression employed by the artist in black and white; actual white is impossible, and delicate tones, such as are characteristic of skies, are destroyed by the cross-bar lines of the screen, which cover down all light passages and rob the reproduction of that brilliancy which characterized wood engraving. It is true that the addition of hand engraving can be resorted to in the case of the process block, and lights and other varieties of tone and form introduced, but this can only be done on blocks of very fine texture, and the cost of reproduction is greatly increased by the introduction of such handwork by the engraver.
The most important development of the half-tone process is in the direction of the reproduction of works in colour by means Three Colour Blocks. of relief blocks. The theories of colour (q.v.) in light and in pigments enter largely into this development. White or solar light is composed of rays of light of three distinct colours, red, green and violet, which are called the primary or fundamental colours because by their combination in various proportions all other tones of colour are produced, but they cannot themselves be produced by any combination of other coloured rays. The theory of pigmental colour differs from this in that the primary or foundation colours from which all others are produced, while being themselves unproducible by any admixture, are blue, red and yellow, and while the combination of the red, green and violet of the scientist produces white, the combination of the primaries of pigments in their full strength produces black.
Colour is the result of the absorption and reflection of the rays of light which strike upon a body. The rays which are reflected are those which affect the vision and produce the sense of colour. Should the object absorb all the rays it appears black, should it absorb none but reflect all it is white, and between these two extremes lie an infinite variety of tones. Filters have been made which absorb and refuse passage to certain coloured rays, while permitting the passage of others, e.g. a photographic filter of a certain colour will absorb and stop the passage of red and green rays, while permitting the passage through it of the violet. It will then be perceived how, when a picture or other coloured object is placed before a camera, with one of these filters between it and the exposed negative, the rays of light of the colour which can pass through the filter to the negative will be the only ones which can affect it, and that it is possible in this way to secure on three separate negatives a record, of the green, red and violet rays which are reflected from its coloured surface by any object placed before the camera.
These records are coloured photographs; they are simply ordinary negatives, records of colour values which may he translated into colour by the use of coloured inks. The principle governing the process is analysis or separation followed by recombination. Positives are made from these colour records, from which by means of the rule screens already described half-tone process blocks are made which, when printed one over the other in coloured inks, combine again the colours which were separated by the filtering process and give approximately a reproduction of the original in its true colours. The colour used with each block must have relation to the filter used in its production. It must represent a combination of the two colours stopped out by the filter when making the negative from which the block was made, that is to say, the colour used must be complementary to the colours stopped out. Certain subjects which are amenable to long exposures can be dealt with by what is known as the “direct process,” whereby the screen negative and the colour record are made by one operation on the same plate. By this means six of the fifteen otherwise necessary operations are saved, but the method is not always practicable.
As far back as 1861 the suggestion was made at the Royal Institution by Clerk Maxwell to reproduce objects in their natural colours by superimposing the three primary colours. Later Baron Ransomut, of Vienna, Mr Collen, a gentleman who taught drawing to Queen Victoria, and two Frenchmen, MM. Chas. Cros and Ducos du Hauron, carried on the idea and made experiments with the aid of photography, which were still further developed in Germany by Professor Husnik, of Prague, Dr Vogel, of Berlin, and others; but it was in America that the first three-colour blocks for letterpress printing were made, F. E. Ives, at Philadelphia, being their maker in 1881.
This three-colour relief process has made great advances in recent years. The first great practical difficulty which had to be overcome was to produce three screen blocks which could be printed one over the other. Were the screens of each block used at the same angle, the lines and dots would print on the top of one another; but a great deal of the colour result depends upon a considerable proportion of each colour being on the white paper. Artists know well that much purer and more brilliant results are produced by placing touches of colour side by side than one over another; small patches of red and blue, placed side by side, yield to the eye a purple of much greater purity and beauty than the same touches of colour worked one over the other. Consequently it was found necessary to turn the screen at a different angle for each block, so that the lines should not fall on each other but should cross each other; but the risk of this is that, used at certain angles, the crossing of the screen lines will produce what is known as the moiré antique result. Vogel took out a patent in Great Britain for the process, and he therein stated that the screens should be used at certain stated angles. He also proposed to use single-line screens, similar to those used by F. E. Ives at Philadelphia, instead of cross-line; but it has since been found that the cross- or double line screens can be used successfully; and that the angle at which they can be used is not a fixed one.
Filters are made in a dry or wet form. The dry filter is made by spreading a film of gelatin or collodion, tinted by an aniline colour, Colour Filters. upon a piece of glass. The wet filter is a cell or trough made of two sheets of glass, sealed all round and filled with water tinted with an aniline dye or colour. The accuracy of the tint of the colour-filter may be tested by the spectroscope, or by an instrument invented by Sir William Abney, and known as the Abney colour sensitometer. This is a theoretical test. The practical test is by photographing through them patches of blue, red, and yellow. If, for example, the filter for blue records the full strength of blue with the full strength of the colour of the negative, while giving slight or no record at all of the red and yellow, it is practically a true filter. It is possible to treat the negatives themselves so as to render them more sensitive to the special colour they are intended to record. Indeed Dr Albert, of Munich, has produced a collodion emulsion which is so sensitized that the various colour sensations are directly obtained without the interposition of a colour filter. Different makes of plates demand different colour-filters. (For colour-filter making see Ives, Photographic Journal, vol. xx. No. 11). The preparation of these colour-filters calls for great perfection of quality in the materials employed, and great accuracy in the using of them. The glass, whether for the dry or wet filter, must be absolutely flat as to its surface, and its two sides must be absolutely parallel. In the wet filter the glasses forming the sides of the cell or trough must be parallel to each other.
Coloured glass is sometimes used in combination with the tinted collodion, but there is no particular advantage in this, because two glasses are always used in the making of a filter, and each one may, if desired, be coated with different dyes and afterwards cemented to ether with Canada balsam.
The following dyes or their equivalents form a basis for nearly all three-colour filters:—
|For the red printing negative||Brilliant green.|
|” blue ””||Cochineal red.|
|” yellow ” ”||Methyl violet.|
The first dye named is the base colour in each case, the second is employed in small proportions to produce the required modification of tint.
The theory of the three-colour process is that the same three colours shall be used for the printing of every subject; and there is no doubt that if the filtration were perfect and the printing inks absolutely pure, the theory would work out fairly correctly in practice; but there is room for improvement in both these matters, and it is therefore often found desirable to print special subjects with special pigments, which makes it difficult to print several subjects together. Special care is called for on the part of the Need of Careful Printing. printer. There must be the most perfect register of the three subjects, otherwise a blurred effect results; there must be constant watchfulness to see that there is no excess of ink of any one colour, or the whole scheme of colour will be destroyed. This three-colour process has been a rather long time in establishing itself and nothing has so tended to retard it as bad printing. Good blocks have been obtainable, but in the hands of ordinary printers they have yielded but indifferent results. It is hardly to be expected that the untrained eye of the ordinary printer should be successful where the work requires the cultivated judgment of an artist. There is one other necessity for success in all tone relief work, and that is the use of the right quality of paper and ink. The blocks are so delicate they soon fill up if an excess of ink is used. Ink of a good quality can be used in much less quantity than common kinds, but it must be impressed upon paper that is sympathetic and will “bear out” the ink.
The best results can be obtained only with the use of what is known as “coated” paper. It is a paper which, after manufacture, is passed through a bath of a preparation of china clay, which by means of brushes is rubbed into the surface of the paper. When dry the surface takes a high polish, and is sensitive to the smallest amount of ink. The polish of this coated paper is objectionable to many readers of illustrated books, and the clay adds considerably to the weight. Paper makers are, however, supplying a dull surfaced highly calendered rag paper which is very good for artistic and scientific illustrations and obviates both the glossy surface and the supposed lack of permanency of chromo paper.
2. Intaglio Processes.—An intaglio engraving is one in which the printing surface is sunk below the surrounding portions of the plate; the lines or dots—pressed, cut or bitten into the surface—holding the ink which is to be impressed upon the paper when the original surface of the plate is wiped clean. The old-fashioned steel engraving may be taken as the type of an intaglio plate, in which the lines which printed were cut into the surface of the plate, instead of being left standing up in relief, as in the case of a wood engraving.
“Photogravure” is the name by which the many processes are generally known by means of which intaglio engravings are made mechanically, “heliogravure” being another name for the process, or special application of it. Photogravure reproduces the tones of photographs or drawings, and gives the nearest approach to a facsimile reproduction that has yet been arrived at. Gelatin bichromatized is the medium by means of which the photogravure plate is produced; but as the screen is not used in ordinary work, it is necessary to produce an ink-holding grain in some way upon the plate. This is done by allowing a cloud of bitumen dust, raised inside a box, to settle upon the surface of a copper plate; it is fixed by heat, which, though insufficient to melt it, is enough to attach the fine grains to the plate. Over this prepared surface is laid the film of bichromatized gelatin, upon which is printed the subject through a glass positive; the usual hardening process takes place by the action of light, followed by a washing out of the unhardened portions of the gelatin. The plate is exposed to the action of ferric chloride, which attacks it most strongly in the least exposed parts, but which cannot eat it away in broad flat masses of dark, even in the non-exposed portions, owing to the existence of the bitumen granulation, which ensures the keeping of a grained surface even in the darkest passages.
Photogravure is a costly process to employ for illustration. The plates have to be printed slowly, with much hand work, as in the case of etchings. It is the printing that makes its use expensive, rather than the making of the plates; and as each plate must be printed separately and on special paper, it cannot be employed with type, like relief blocks.
There is much uncertainty about the production of plates by the photogravure method; and although great improvements have been made in the process, it is often necessary to produce several plates before a satisfactory one is obtained. In all these reproductive processes the more artistic the workman the better the result; this is especially true of photogravure, in which the aim is to come very much nearer to the original work of the artist designer than in the less perfect processes.
The method of Rousillon, which was adopted by Goupil in the production of photogravure plates in the early days of the process, was to prepare the surface of the plate with a secret preparation of certain salts, which crystallized under the action of light, so that when exposed under the negative the surface was broken up by this crystallization more or less, according to the amount of light the negative permitted to reach it. The plate with its crystallized surface was then electrotypes, and the electrotype was the plate used for printing. It was a deposit process, as opposed to an etching process.
Photogravure plates are made also by the use of the grain screen, in which the reticulations of the screen take the place of the bitumen powder in producing a grain; it is the inversion of the method by means of which points and lines are produced in the relief block. It has not, however, come much into favour, probably owing to the greater coarseness of the grain and the consequent loss of softness in the tones. An application of this method has, however, been made in the development known as the Rembrandt intaglio process. It is a secret Rembrandt Intaglio Process. process; but the secret lies more in the press by which the plates are printed than in the plates themselves, which are intaglio plates made with a very fine screen and bent to a cylinder. The attempt to print photogravure plates by machinery was given up because the plates were so shallow they would not stand the wear and tear, and their life was too short and the results too indifferent; but the use of the grain screen renders possible stronger, deeper plates, that will stand harder wear. There is little doubt that the machine used is some form of the machine used to print wall-papers, in which there is a central cylinder engraved with the design, inked by rollers with which it comes in contact. The ink not only fills up the intaglio or sunk portion which has to print the design, but covers as well the whole surface of the plate. To clean this surface, leaving ink only in the sunk dots and lines, another metal cylinder is employed, ground and grooved somewhat like the shaft of the common steel of the dinner table used to sharpen knives, the grooved surface of which, passing over the engraved cylinder, scrapes clean its inked surface, leaving ink only in the sunk portions, which will, as the cylinder comes in contact with the paper, deposit itself and print the picture. The results produced by the Rembrandt intaglio process are softer and smoother than those given by photogravure, and they are free from the gritty qualities which occasionally characterize photogravure; but they lack the brilliancy and depth of the latter. The process on the whole is less costly to use, mainly because the printing is so much more rapid, and is turned out by a machine instead of by hand.
A method of printing intaglio plates made from a screen 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 Monotype. 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 large number of impressions it is often required to get from the etched surface of a block has made it necessary to devise Electrotypes. means for preserving the original block, and to prepare and work rom duplicates, which can be renewed when necessary. For this process the original is coated with a film of the finest 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 deposit itself in metallic form over 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 process requires the greatest care in its manipulation.
Steel-facing is resorted to where long numbers have to be printed from photogravure plates. The finest film of steel is deposited by Steel-facing. an electric battery over the whole face of the plate, 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 Changes in Machinery. by which they are printed. Almost all the changes and improvements have been initiated in the United 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, with improved results; but at last the blanket was entirely superseded by a glazed board, the machinery was more accurately constructed, 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 brought in contact with the type and blocks. The old soft blanket kept the cylinder or the flat 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. Most elaborate methods were adopted for the moistening of the substance of paper before use. Most paper was printed on whilst damp, but damp paper 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 flat 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 bichromated 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 flat 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 and shade 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 bichromated 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 phototype is a process in which the film of isinglass, gelatin or gum, treated with bichromate of potash with the addition 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 bichromated gelatin is spread upon glass and allowed to dry. Exposed to light under a reversed negative the unprotected parts are hardened in proportion to the amount of protection they receive from the negative. After exposure under the negative the back of the film is exposed to the action of sunlight through the glass at its back, so that the whole film may be rendered as hard and tough and durable as possible to stand the wear and tear of the process of printing. When in its place in the printing press the film must be kept moistened. The soft parts unacted upon by the light, but from which the bichromate has been since washed, will absorb moisture in proportion to the action the light has exercised upon it, the absolutely hard parts refusing moisture altogether. The film may now be inked with an ordinary inking roller, the ink being freely taken up by the hard and unmoistened passages and by the partly hardened in proportion to the amount of moisture they are capable of absorbing; as in lithography, the constant moistening of the printing surface is a necessity. Collotype is largely used for postcards. It may be printed in a lithographic or ordinary vertical press of the letterpress printer. Admirable colour results are obtained by this process.
Heliotype is a variation of the method of producing the film which is first spread as described upon waxed glass and then stripped from the glass when dry. After hardening the back of the film it is laid down upon a metal plate and firmly secured to it by the use of an india-rubber cement. It is remarkable the admirable results that are obtainable by so delicate a process. The films have not a long life; a few hundreds only can be printed from each, but the renewal of the film is a simple matter. The result is very like a photograph. The use of heliotype is, however, practically obsolete.
Photolithography.—Zinc or aluminium plates are now frequently used instead of the more cumbrous stones for all so called lithographic printing. These plates have the same affinity for fat ink as stone, the method of dealing with them being practically the same as with stones, and the description may be taken as applying to both. The stone itself may be rendered sensitive by coating it with a thin film of bichromated gelatin, exposing it under a reversed negative of the required subject and treating the hardened film as it is treated in the case of collotype. A better plan is to render sensitive a sheet of unsized or transfer paper which is exposed under a negative, moistened, and rolled with transfer ink, which is of a specially fatty nature, and adheres only to the parts hardened by exposure which are unaffected by the moistening and remain dry. This inked sheet is laid upon the stone and the two together are subjected to great pressure, passing through a lithographic press. After further moistening the sheet of transfer paper is peeled off, the stone leaving the inked drawing behind it. The usual methods of lithography are then followed, the stone is treated with a preparation of acid and gum, kept moist and printed from in the ordinary lithographic method. Lithography of all kinds can only deal with lines or solid blocks. Tints present difficulties which are best dealt with by other methods of reproduction, but attempts have been made to obtain tints lithographically by breaking up the solid surfaces of the gelatin print with a grain before rolling it with ink and transferring it to the stone.
One of the most successful of such attempts is known as the Ink Photo process, which is more or less of a secret process worked by Messrs Sprague. None of them, however, yield so sound a result as a good drawing made in line, as the grain has a tendency to fill up. Transfers may also be made on to zinc plates which will take the lithographic ink equally well with stones. The plates may be etched—as the inked surfaces resist the action of acid—and by this means a relief plate made, which when mounted on a block, type-high, may be printed typographically. It is known in this form as zincography.
Authorities.—Eugène Michel Chevreul, Considérations sur la reproduction par les procédés de M. Niepce de Saint Victor des images gravées dessinées on imprimées (Paris, 1847); Niepce de Saint Victor, Mémoire sur la gravure héliographique sur acier et sur verre (Batignolles, 1854); Niepce de Saint Victor, Traité pratique de la gravure héliographique sur acier et sur verre (Paris, 1856); Alexander de Courcy Scott, On Photo-Zincography and other Photographic Processes employed at the Ordnance Survey Office, Southampton (London, 1862); G. Field, Chromatography (London, 1885); C. Motteroz, Essai sur les gravures chimiques en relief (Paris, 1871); Dr H. Vogel and J. R. Sawyer, Das photographische Pigment Verfahren oder der Kohledruck (Berlin); W. von Bezold, Theory of Colours, 1876; (Boston, U.S.A., 1891); J. Husnik, Das Gesammtgebiet des Lichtdrucks (Vienna, 1880, and editions); Ceymet, Traité pratique de phototypie (Paris, 1883, and editions); W. T. Wilkinson, Photo-Engraving on Zinc and Copper, in Line and Half-Tone (London, 1886); Alexander Leslie, The Practical Instructor of Photo-Engraving and Zinc-Etching Processes (New York, and editions); E. Leitze, Modern Heliographic Processes (New York, 1899); W. T. Wilkinson, Photo-Engraving (London, and editions); Professor Church, Colour (London, 1891); W. de W. Abney, Colour Measurement and Mixture (London, 1891); R. Meldola, The Chemistry of Photography (London, 1891); Colonel Waterhouse, Practical Notes on the Preparation of Drawings for Photographic Reproduction (London, 1890); Carl Schraubstaedler, Photo-Engraving; a Practical Treatise on the Production of Printing Blocks by Modern Photographic Methods (St Louis, U.S.A., 1892); Dr H. Vogel, The Chemistry of Light (London, 1892); S. R. Koehler, Museum of Fine Arts: a Catalogue of an Exhibition illustrating Reproduction Methods down to the Latest Times (Boston, U.S.A., 1892); Jules Adeline, Les Arts de reproduction vulgarisés (Paris); Sir J. Norman Lockyer, Studies in Spectrum Analysis (London, 1894); H. D. Farquhar, The Grammar of Photo-Engraving, trans. from the German (London); C. G. Zander, Photo-Trichromatic Printing (Leicester, 1896); H. W. Singer and William Strang, Etching, Engraving, and other Methods of Printing Pictures (London, 1897); W. Gamble, “The History of the Half-tone Dot,” (The Photographic Journal, Feb. 20, 1897); T. D. Bolas and others, A Handbook of Photography in Colours (London, 1900); W. de W. Abney, Photography. Penrose's Process Annual contains each year a list of the latest works dealing with the development and progress of mechanical photo processes. (E. Ba.)