COLOUR PRINTING. The introduction of colour print ing is obscure, and the claims of the Chinese and Japanese cannot be discounted, for it is probable that these peoples were practising the methods of colour block printing, which they follow to-day and for which they are famed throughout the world, long before the inception of printing from movable types. The use of coloured inks was not common with the early printers, but, strangely enough, the very first books printed at Mainz in the sth century were decorated with initials in colour. The "42-line Bible," be lieved to be the first book printed from movable type, contained initials in red, some printed and others drawn in the spaces left for the purpose; whilst the later works produced at Mainz had initials in both red and blue, the Psalter of 1457 being the most famous of these. Colour printing, therefore, may be said to have commenced with the birth of the typographic printer's art. The desire to rubricate the new method of producing books in the early days is not to be wondered at when it is recalled that print ing took the place of the beautifully decorated written books pro duced in the scriptoriums—works of art which call for the ad miration of the world as evidence of unstinted patience and skill given to what was often a life's work. In 1486, the famous school master printer of St. Albans produced a bock which contained woodcuts of armorial designs printed in red, blue and brown, but strange to relate, when yellow was introduced it was put in by hand. This was the first specimen of English printing (and the last for over 200 years) in which coloured inks were introduced.
Early in the i6th century a new method of colour printing from wood blocks, called chiaroscuro, was introduced. This consisted of a black outline to which tints were added. There is uncertainty about the inventor of this process, both Germany and Italy claim ing the credit for its invention. It found great favour on the Con tinent. About this period, what might be called stencilling was introduced. This was used principally in the production of playing cards. About the year 163o the method of obtaining prints from copper-plates, into which the design was incised (intaglio) was practised. There are a number of colour prints in the British Museum produced by this process, the method entailing the rubbing of coloured inks into hollows cut below the surface, and, when the plate was completely inked, passing it through a press, thereby obtaining an impression on a sheet of paper. The i8th century saw great developments in the art of colour printing. New methods were invented, and it was at this period that three colour mezzotint prints were produced by a German of French extraction, named Le Blon, he being the first to apply the prin ciples of the three primary colours (without, of course, the aid of photography). His method was to dissect the original paint ing into its primary colour and tints, and make a separate mezzo tint plate for each colour ; these he printed in the respective coloured inks one on top of the other and, when the three im pressions were registered into each other, they produced a result approximating the original picture. Le Blon's process was largely practised by others after his death in Paris, in 1741. In 1718 an Englishman named Kirkall worked a process which had as its key a mezzotint plate, the colours being added by means of wood blocks. He also reverted to the original method of preparing an intaglio plate, dabbing the colours on to the plate by hand. This method of colour printing is still used for producing costly prints for art dealers. About the end of the i8th century a large number of mezzotints, stipples and line engravings in colour were produced, the best of which now command fabulous prices.
Early in the r9th century, chiaroscuro printing was revived by William Savage, and some of his prints contained as many as 3o colours. At this time, Senefelder's invention of lithography was applied to colour reproduction, when the picture was printed in flat tints. It was not until midway through the century that the superimposing of colour was a ttempted, and became known as chromo-lithography. In 1834, the first colour prints by George Baxter were published. The method employed in producing this class of colour work, now known as Baxter prints, was by the use of an intaglio key plate which was printed in black or brown, to which was added a number of colours printed from wood or metal blocks. Colour printing from woodcuts and line blocks was carried on until the introduction of what is now known as the three-colour process—a method in daily use by letterpress printers. In the latter half of the i gth century chromo-lithography began to make rapid strides, and collotype, a beautiful process, was developed to meet the demands of art dealers for a high standard of colour printing. (See COLLOTYPE.) Mainly due to the application of photography and the advance of mechanical science, great developments have taken place in colour printing within the last half century.
Numerous operations take place before the plate is mounted on wood to make it type high to meet the printer's requirements. The outline is known as a key-plate, and colours are added as required, either by photographic means or by the offset method. Tints of the colour in which the block is to be printed are pro duced by means of stipple, or lines made on the plate before etching. Many of the delightful colour illustrations in Kate Greenaway's, Walter Crane's and Randolph Caldecott's books are printed from woodcuts or line blocks.
In making colour plates the ruled screen, which breaks up and provides the necessary reticulation required for a printing surface, is turned round so that the dots are at a different angle for each colour. When the screen negatives are developed they are photographically printed on to a piece of metal, usually copper, and etched in a similar manner to that employed in making line blocks. Before a set of colour blocks is in a condition to be sent to the printer, however, a vast amount of artistic and pains taking work has to be done by etchers who endeavour, by means of repeated immersions in the etching bath, to obtain results in the finished print approximating to the original painting. To obtain the best results from half-tone blocks, a paper with a glossy surface, known as art paper, must be used. Objection, however, is frequently made to the glare of this coated paper but experiments are being made to make blocks which will print satis factorily on a matt (i.e., non-glossy) paper and recent results give reason to hope that this will materialize in the near future.
Platen Machines.—Those generally known as "heavy" platens are most suitable for colour printing. The forme or plate is securely fixed in an iron frame called a chase, and placed on the "bed" in a perpendicular position, the inking rollers passing over the face of the forme, thus inking the surface. The ink, which is of a buttery consistency, is stored in a trough, and can be reg ulated according to the supply required. A sheet of paper is placed (usually by hand) on to the guides fixed on to the platen, which is flat and moves up with a rocking motion to get the im pression from the inked plate. The platen then rolls back, and the printer removes the printed sheet with his left hand and places t on a board fixed to the machine, at the same time placing another sheet in position on the platen as it begins to make its upward movement. Sheets of about 22-iin. by i tin. can be printed on this type of machine, but great skill is required to place the sheet accurately into the guides, which if not done, will prevent the subsequent colours from registering, i.e., fitting each other so that the finished print is sharp and clean, and the colour values of the original are faithfully reproduced.
Cylinder Machines.—These are used for large sheets and a heavy class of work. In this instance, the formes or plates are placed in a horizontal position and fixed securely on the bed of the machine which moves to and fro. During the travel the plates are inked by a series of rollers held in brackets. These receive their supply of ink from a slab tc which the ink has been trans ferred from a duct, a trough which can be regulated to supply the amount of ink necessary. Working in conjunction with the bed on which the forme is placed is a large cylinder, secured in brackets, which rotates as the bed makes its to-and-fro move ment, carrying with it a sheet of paper which has been placed in lays. This is taken by means of a "gripper," which automatically takes the sheet round the cylinder, and thus obtains the im pression from the plates. The sheets are then mechanically de posited on to a board, where they are piled up one on top of the other until they are removed by the printer. On this class of machine, sheets ranging from 2 21in. by i 7 2 in. to 6oin. by 4oin., and sometimes larger, are printed. There are special machines for printing two, three or four colours before the sheet is de livered, but these are not used for the best class of work, although fair commercial results are obtained by this method, generally known as multi-colour printing.
By whichever type of machine the work is printed, a great deal of work requiring care and skill is necessary before the actual printing of the sheet commences. The blocks have to be made type high, and this is done by the skilful fixing of pieces of paper below the wood mount so that the printing plate will present an even surface to the inking rollers in order that the plate may be correctly inked. This is called underlaying. An other operation, called overlaying, is necessary. Briefly, this means the cutting out and pasting together of pieces of paper and fixing them on the machine in such a way that the requisite amount of pressure will be brought to bear upon the different tones contained in the plate. Thus, the solids would be repre sented by the thickest parts of the overlay, the middle tones correspondingly less, and the high lights just sufficient to make the impression firm and no more. The treatment of the ink also requires attention, in order that each colour should print clearly on the top of the previous one. Care is likewise required to protect the sheets from the effects of changing temperature; while the adjustments of the machine have to receive special attention in order to ensure that each of the colours will register, otherwise the work would be spoiled.
Intaglio Printing.—The intaglio method of producing prints is theoretically an ideal one since it produces the tones of an original in a similar way to that in which many artists obtain "weight" in a picture—by applying varying amounts of colour. In other words, solids in an intaglio print have a greater quantity of ink than the middle tones and high-lights, the latter being practically plain paper. The method applicable to this process may be divided into two sections : hand work and mechanical. The first includes engravings, etchings, aquatints, mezzotints and photogravure. Each process has a distinctive characteristic and special method of preparing the plate, which it is necessary to understand when dealing with colour prints commanding high prices.
Line Engraving.—The method of preparing plates for line engraving is as follows : a carefully prepared piece of copper is coated with a varnish on to which the design is drawn or trans ferred. The drawing is then gone over line by line with a liner or graver which cuts furrows into the metal plate. Various shapes and sizes of gravers are used according to the character of the line required, the incisions varying in depth and width according to the nature of the drawing. The greatest care, therefore, must be taken to preserve the surface of the plate from damage, since the slightest scratch will print.
Stipple Engraving.—In this method the plate is obtained by etching the copper or zinc sheet, which is covered with a wax acid resist, the work being produced en the plate by means of dots of various sizes and forms made by needles. These remove the wax coating and leave the metal bare. The plate is placed in a bath of diluted nitric acid and the hare metal is etched away. The parts which will appear as white in the print are protected by the wax. After necessary touching up has been done the plate is ready for printing.
Etchings.—As the term suggests, etchings are obtained from plates on which the work has been bitten below the surface. The method of making the plate is somewhat similar to that followed in stipple engraving. A selected plate is coated with an acid resisting preparation—a composition of wax, the design being drawn upon this with a steel point which removes the wax and leaves the metal bare. A mordant (i.e., acid) is then applied and etches away the metal where the steel point has removed the wax. Repeated application of the acid is required, the darker tones re quiring more than the lighter. Aquatints. A plate for aquatint printing is covered evenly with powdered resin or bitumen. This is done by placing the plate in a box where resin or bitumen is agitated. The dust falls on to the plate which is then heated, melting the grains of powder which run together, and leaving the surface partially protected. This forms an acid-resisting ground. The subject to be reproduced is either drawn or transferred to the ground. The edges and back of the plate are stopped out with some acid-resisting liquid and after the first etch the parts of the picture which are to be white are stopped out, and the plate is again placed in the acid bath. This is repeated until all the tones are treated. The plate is then removed from the bath, washed and dried. Aquatint is often used along with etching, the subject being first etched, and the lines protected, before adding the aquatint grounds and proceeding with the additional etchings.
Mezzotints.—This is probably the ideal intaglio process for producing colour prints. It is not an etching process but one which is entirely dependent upon the skill of the artist. A flat and highly polished sheet of copper is roughened all over with what is called a rocking tool. This is from tin. to Sin. broad and made with varying degrees of fine teeth. The tool is rocked back wards and forwards in every direction over the face of the plate until the plate is a mass of sharp teeth. An impression taken from the plate at this stage would yield a solid black velvety print. The subject is then drawn or traced on to the "rocked" plate. A tool called a scraper is employed to get a surface with different gradations of tones, from high lights to solids. This is done by expertly scraping away the grain or tooth from the plate in a greater or less degree, according to the strength of the tone re quired. When the grain is completely removed the high lights will appear white in the print, the deepest tones being represented by the natural grain obtained by the rocking tool.
Photogravure.—The making of a photogravure plate is some what similar to that adopted in aquatint, where the copper-plate receives a ground of resin or bitumen, but, instead of drawing the design on the ground, a special photographic print is em ployed. What is known as a carbon tissue is made, i.e., a mixture of gelatin and carbon or pigment being made and a thin film of this spread on a piece of paper. When this is dry it is treated with potassium bichromate, which after drying is exposed to light under a reversed photographic positive of the subject. Gelatin thus treated with bichromate and dried becomes sensitive to light, and acquires the property of becoming insoluble in hot water wherever light has acted upon it; thus the layer of gelatin is penetrated by the light and rendered insoluble to varying depths according to the proportional strength of the light passing through the positive. This printed carbon tissue is soaked in cold water and pressed upon the prepared plate. Warm water is then applied, and as the gelatin not affected by light remains soluble, it dis solves, the paper support coming away at the same time, leaving a tone mould or relief of the subject affixed to the plate. This, along with the resin ground, constitutes the acid resist. The plate is then subjected to a series of etchings by a suitable mordant.
Intaglio Plates.—The printing of intaglio plates is indeed an art, calling for great skill, extreme patience, a high artistic sense and much experience. The general principle in inking a plate is to rub the colour all over it with what is called a "dabber," i.e., a piece of blanket made into a pad. Care must be taken to get the ink into the deepest hollows as well as into the lightest tones. During this work the plate is kept warm by placing it on a stove. The surplus ink is then carefully wiped off the surface of the plate by cloths varying in degrees of fineness. Discretion must be used by the printer in wiping the plate, some parts re quiring more than others. When producing colour prints the colours are painted on the plate by small dabbers or paper stumps, i.e., a piece of suitable paper rolled in the shape of a pencil and dipped into the special colour required. This is then dabbed into the tones in the plate in accordance with the scheme of the original picture which the printer follows. When all the colours are filled in, the plate is wiped and polished, the finishing touch being given by the palm of the hand. It is then ready for the im pression to be taken. This is done by placing the inked plate on the bed of what is known as a copper-plate press. A piece of special damped paper is placed upon the face of the plate, which is then backed with a soft packing and the impression obtained by running the plate and paper under a steel cylinder. Extreme care has to be taken in removing the print from the plate. It is by these intaglio processes that the high-priced prints sold by art dealers are produced. Usually, about 20 colours are used in a colour print, and frequently it takes at least a day to obtain a print.
Machine Photogravure.—Thanks to the progress of me chanical science, within recent years it has been made possible to apply the principles of photogravure (intaglio) to every day use, more particularly in the production of magazines and supple ments to newspapers. This method of printing has made greater advances in America and on the Continent than in Great Britain, although excellent work is done in England where probably the best examples of colour work are produced. This method of re production masquerades under many titles, i.e., gravure, machine photogravure, rotogravure, intaglio printing and sometimes photo gravure. The method of preparing the printing surface is similar to that of photogravure; a photographic print is taken on a film of sensitized gelatin spread over a sheet of paper and mixed with a coloured pigment. To provide the necessary reticulation, in place of the resinous ground employed in hand photogravure, a photographic copy of a ruled screen is incorporated on the carbon tissue along with the design and then transferred to the copper-plate or cylinder, the latter being generally used. The plate is then mechanically etched and sent to the press room for printing.
Machines.—The machines used are of two types. One is the flat-bed machine, where the plate is flat and placed on a bed over which a cylinder carrying a sheet of paper travels, thus obtaining the impression. The inking and cleaning of the plate are done mechanically. The other variety is the rotary machine where the paper receives its impression when travelling between two cylin ders, one of which bears the work and the other provides the necessary impression. This style is in most general use. The etched cylinder runs in a bath of volatile ink, and as it rotates a fine and delicately adjusted knife, called a "doctor," scrapes the surplus ink off the surface. This falls back into the trough, leav ing the cylinder clean. The paper is either fed in by hand or sup plied from a reel or web, this passing between the printing and pressure cylinders. Frequently both sides of the paper are printed before it is automatically cut up into sheets. Colour reproductions by this method require a distinct cylinder for each colour, the separation of colours being obtained in the same manner as that in the making of three-colour blocks. Great difficulty, however, is experienced due to register troubles in obtaining results equal to those by other printing processes. Experiments are continually being made, and no doubt in time colour reproductions will be come part of the ordinary day's work of the gravure printer, in stead of being confined to one or two firms who have made colour work a speciality.
Planographic Printing.—This, as the term denotes, means surface printing, such as lithography, and possibly collotype might be included.
Lithography.—The basic principle of this process is the antip athy of water to grease. A design is drawn or transferred on to a level stone or a sheet of metal which must be free from grease. This must also be water-absorbing. Upon a dry surface the lithographic artist either draws or paints the design in an ink of a greasy nature, with a pen or brush. The stone is then prepared for printing by gumming, rolling up with printing ink. , and etching; the effect being that, when the printing surface is damped over with water and a roller charged with printing ink passed over the stone, the design which has rejected the water will accept the ink from the roller, while the clean parts of the stone which have absorbed the water repel the ink. A sheet of paper is then brought into contact with the stone and nm through a press, thus an impression of the design is transferred to the sheet. The first attempts at colour printing by this process were obtained by printing flat tints. According to the number of colours in the picture, offsets were transferred on to as many stones as the number of colours required, and the parts of the design which had to be printed in a particular colour were painted in, following the practice of the engravers who cut woodcuts for colour printing. The next development was to print colours on the top of each other. This was done by stippling, and demanded great skill and patience. When the original picture was handed over to the lithographic artist he would first make an intricate tracing of the original, detailing every variation in form and colour; after drawing on the stone an elaborate key to scale, he proceeded to paint in the parts which had to be solid, and the graduated tints by making dots with a pen in circular formation, varying the sizes and spaces between them according to the tones and tints of colour he desired to reproduce. Thus, in each of the stones which represented a separate printing, he would in corporate a particular colour and its component tints. When the drawing of the various colours was finished, which would some times take weeks, proofs were taken on a hand press and altera tions made where necessary. It was by this means that certain colours in their various hues would be obtained ; i.e., super-imposing blue on yellow produced a green; violet was obtained by printing blue on the top of red; and so on. The drawing was done with a black ink, but this did not affect the colour to be printed as it only provided the means by which the design is absorbed by the stone, each stone being printed separately in a different coloured ink. The colours were registered into each other by means of marks called "register marks." When artists use the lithographic process to reproduce their own work, they sometimes draw the design by freehand direct on the stone. Few lithographic artists are capable of doing this, the usual practice being to trace the original and reduce it to the size required by scaling. Another method is to use grained stones on which the artist works with crayons. It is by this means that chalk drawings are reproduced. Stippling is a tedious and laborious operation calling for great proficiency and many years of training. Many chromo-lithographic prints had as many as 25 printings, and good lithographic prints, produced in the early part of the 19th century, now command high prices. Photo-Lithography.—As the demand for speed became greater mechanical means were adopted, and hand work has now largely been superseded by photographic methods, by which either a transfer taken from a specially prepared half-tone relief block is put on the printing surface, or what are known as photo-litho transfers are employed. The modern practice is to make screen colour negatives in the same way as those made for three-colour printing. The lithographic printing plate is coated with albumen and ammonium bichromate, the negative being placed in contact with it, and exposed to light. When sufficient exposure has been given, the plate is washed and the part of the sensitized coating which has been acted upon by light remains on the plate, and from this impressions are obtained. A different stone or plate is required for each colour and the sheet of paper passes through the press or machine for each printing. Reproducing pictures by this process has materially reduced the number of printings, seven or eight being the average number required.
Lithographic Machines.—The work is proved upon a press carrying the stone or plate on which, when damped and inked, a sheet of paper is placed opposite marks and backed with a few sheets of paper or other material. A hinged frame with a sheet of leather or metal called a tympan is then lowered on to the stone and run through under a scraper. Small editions can be printed on a press but when a large number of copies are re quired a machine, either flat-bed or rotary, is used.
Flat-bed.—By this method, the stone is placed on the carriage and levelled up to a certain height so that the proper inking may be given to the stone and the necessary pressure obtained to transmit an impression of the design from the stone to the paper. This is done by passing the sheet round a cylinder which is supported by brackets and runs in unison with the bed. At the crown of the cylinder there are boards to hold the paper, each sheet is laid into guides and by means of grippers, the sheet is car ried by the cylinder over the stone. Behind the cylinder an appa ratus is attached for automatically damping the stone as it passes to and fro, while in front of the cylinder there is a series of inking rollers which supply the necessary ink. The printed sheets are either taken off by hand or mechanically by "flyers" and deposited on to the delivery board.
Rotary.—This type of machine has a plate stretched round one cylinder, with apparatus for automatically damping and ink ing. The sheets are fed into the machine and taken round the impression cylinder by means of grippers, and thus the im pression of the design is transferred to the paper. This type of machine allows for higher speed than the flat-bed and is mainly used for poster printing. Both these methods are rapidly giving way to what is called "offset." By the offset method the sheet of paper does not come in direct contact with the plate upon which the design is drawn, but transfers an inked impression to a sheet of rubber stretched round an auxiliary cylinder, when, as the machine rotates, the paper comes into contact with the rubber cylinder from which it receives its impression. The chief feature of the offset principle is that paper with a rough surface, or even fabric, can be printed, often adding additional quality to the print. This process has made rapid strides, and has, without doubt, great possibilities as a colour printing process.
Collotype.—Collotype is claimed to be the ideal colour re productive process. Prints produced by this process command higher prices than those obtained by other processes, with the exception of hand intaglio methods. In this instance, no mechan ical reticulation such as that obtained by the use of a screen, is required. The necessary grain is provided by the natural drying of the gelatin and cannot be distinguished by the naked eye. Continuous tone negatives are used, and these are retouched, to provide the proper colour rendering, by workers with a keen appreciation of colour and its analysis. It is then photographically printed on a sensitized film of gelatin floated on a glass plate about tin. thick. It is then washed in running water, and the parts mostly affected by the light, which has passed through the negative, are hard and insoluble, whilst those which have only been partially affected are more or less soluble according to the intensity of light which has passed through the negative on to the sensitized gelatin film. The surface is, more or less, water absorbent, and consequently resists the ink in varying degrees. There is no mechanical printing process more capable of giving finer reproductions of coloured subjects. It reproduces the atmos phere of the original picture with remarkable fidelity, whilst its capacity for giving depth of colour and quality is unsurpassed by any other printing method. The average number of printings is seven or eight.