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SHOWING THE SEPARATE COLOURS EMPLOYED IN PHOTO-REPRODUCTION BY THE THREE-COLOUR PROCESS
The three primary colours are separated out by photography, each colour sensation is etched on copper, and when the blocks representing Yellow (A), Red (B), and
Blue (C). as illustrated above. are superimposed in the printing press, the result (D) is a reproduction of the original in all its combinations of colour

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, admixture of some warm colour with the black base. It is a good ink prepared by Bourgeois of Paris, appears to be prepared with the for the purpose, and is prepared both in solid and liquid form. Lampblack gives good black lines; so does ivory black, which is drawing ink is an American ink made in liquid form which has the warmer in tone than lampblack. Higgins' Indian ink or American 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.

the attempts that were being made to reproduce his work | ink, or ink that will tell as black when it is exposed to the photomechanically without the intervention of the translator or graphic plate. Inks of a warm tone-that is, inclining to red interpreter. The ideal of an artist would naturally be a repro- or orange-yield better results than cold inks which incline to duction of his work in facsimile, which retained all, or as many blue. 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, phototype, 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.

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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.

Line
Blocks.

The methods of reproduction of pure line work differ greatly from those for the reproduction of originals in tone. As the 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

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 finer 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 picce 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 whereever 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

Swelled Gelatin Process.

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 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 dors 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 electrotyper'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 brass plate is coated with a film or ground of wax upon Typographic which a tracing of the drawing to be reproduced may Etching. 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 blocks or plates that could be printed along with Half-tone type in the ordinary rapid printing-press. It is Processes. 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.

flawless both as to body and surface with a composition A screen is made by coating a sheet of glass-which must be 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 fine 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 underbiting 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 These records are coloured photographs; they are simply or tearing away in the mould. To avoid this underbiting a ordinary negatives, records of colour values which may be fatty ground is laid over the surface of the block each time it is translated into colour by the use of coloured inks. The principle etched; by exposure to heat this ground is sufficiently melted governing the process is analysis or separation followed by to permit of its running down the sides of the upstanding points, recombination. Positives are made from these colour records, and so giving them the required protection. The acid blast is from which by means of the rule screens already described less liable than the bath process to eat away the sides of the dots. half-tone process blocks are made which, when printed one over This method of making tone relief blocks is most generally the other in coloured inks, combine again the colours which were known as the "Meisenbach" process, from Meisenbach, of separated by the filtering process and give approximately a Munich, who was the first to make it commercially successful, reproduction of the original in its true colours. The colour but the history of its development is somewhat obscure. Fox used with each block must have a relation to the filter used in its Talbot as early as 1852 took out a patent for using a screen of production. It must represent a combination of the two colours crape or muslin; he also suggested dusting glass with a fine stopped out by the filter when making the negative from which powder to produce a grain screen. All the early ruled screens the block was made, that is to say, the colour used must be were single line, and the credit is due of suggesting the shifting complementary to the colours stopped out. Certain subjects of the single line screen during the operation and, by reversing it, which are amenable to long exposures can be dealt with by producing the effect of the double line, to Sir Joseph Swan, who what is known as the "direct process," whereby the screen patented the process in 1879. Meisenbach's patent for a similar negative and the colour record are made by one operation on method is dated 1882. The development of the screen was the the same plate. By this means six of the fifteen otherwise important factor in the development of the process. The necessary operations are saved, but the method is not always early screens were photographs of ruled plates and the great practicable. 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 of relief blocks. The theories of colour (q.v.) in Colour light and in pigments enter largely into this developBlocks. ment. 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 retuse 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.

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 doubleline screens can be used successfully; and that the angle at which they can be used is not a fixed one.

Colour

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, upon a piece of glass. The wet filter is a cell or trough made of two sheets of glass, sealed all round Filters. 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 em-
ployed, 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
together with Canada balsam.
The following dyes or their equivalents form a basis for nearly all
three-colour filters:-

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of tint.

Need of Careful Printing.

Naphthol green.

The first dye named is the base colour in each case, the second is
employed in small proportions to produce the required modification
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
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.

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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 electrotyped, 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 process; but the secret lies more in the press by which Rembrandt the plates are printed than in the plates themselves, Intaglio 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

Process.

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 2. Intaglio Processes. An intaglio engraving is one in which grain screen renders possible stronger, deeper plates, that will the printing surface is sunk below the surrounding portions of stand harder wear. There is little doubt that the machine used the plate; the lines or dots-pressed, cut or bitten into the surface is some form of the machine used to print wall-papers, in which -holding the ink which is to be impressed upon the paper when there is a central cylinder engraved with the design, inked by the original surface of the plate is wiped clean. The old-rollers with which it comes in contact. The ink not only fills fashioned steel engraving may be taken as the type of an intaglio up the intaglio or sunk portion which has to print the design, plate, in which the lines which printed were cut into the surface but covers as well the whole surface of the plate. To clean this of the plate, instead of being left standing up in relief, as in the surface, leaving ink only in the sunk dots and lines, another case of a wood engraving. 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.

"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 ncarest 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

A method of printing intaglio plates made from a screen

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