PRINTING PROCESSES.
111. RELIEF PRINTING*
The relief printing surface is similar in principle to that of a rubber stamp. The surface has depressions in it, to be sure, so that it looks like a surface for printing engravings, but that is as far as the similarity goes, for the ink is not a fluid one and does not get down into the crevices. It adheres to the elevated surfaces only. Woodcuts
A specially prepared wood, usually boxwood from Turkey, valuable for its evenness of grain and ability to take a high polish, is cut into convenient lengths. Next, one side, across the grain, is highly polished and buffed with zinc oxide, known to the artist as "Chinese white." The required sketch or drawing is traced on thin paper, reversed, and placed in intimate contact with this polished surface, and the sketch traced through the paper onto the wood by means of a sharp-pointed stylus. The most difficultfeat now follows in cutting away the remainder of the surface, leaving the design in relief, as the printing surface. This is usually done by a machine. In the printing of woodcuts, the block is inked and that ink is transferred directly to paper by pressure. This is especially true in art work where a very limited edition is wanted, for printing a few hundred copies by this process might ruin the wooden surface. Metal duplicates of this wooden surface are therefore made as electrotypes, by a process discussed later. In colored woodcuts, always associated in the home with etchings, separate blocks are made for each color desired, and each printed in perfect register over the same surface of the paper. This is of course analogous to a method of color printing certain cloths by hand.
Printing from Type Hand Setting of Type.-The principle of this is so simple that we need only mention that pieces of metal containing the characters in relief on the end, are assembled in their proper sequence by hand, clamped in a frame and inked with a typical printing ink. The ink adhering to the elevations on the type i s transferred to paper by pressure. The type is essentially a lead-tin-antimony alloy, the antimony adding hardness to the alloy and also causing it to expand on cooling. This means that when the type was originally formed by pouring the molten alloy into molds, each character would have well-defined and sharp edges. Some common formulas are:
* This is the last of
a series of artides which has been appearing in THISJOURNAL.
For Part I, see 7, 102&36 (May. 1930); Part 11, 7, 1263-76 (June. 1930). 1632
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(1) (2) (3)
Pb 60 70 77.5
(4)
82
Sb 30 18 16 15
Sn 10 10 6.5 3
Cu 2
Linotype.-This machine, perfected by Mergenthaler, is the oldest type-setting machine, hut still is one of the most popular. It casts the metal in lines of type, called "slugs," rather than in the individual characters. The operator works a keyboard like that of a typewriter. The pressing of a key, however, releases a brass matrix carrying that letter on the edge. The matrices, sufficientto make a single line of type, are carried mechanically in the order keyed, to a place where the operator can inspect the line for errors, rearranging the matrices by hand, if necessary. Then these matrices are inserted into a mold, into which the molten type metal is forced, also mechanically. The metal in the mold solidifies almost instantly, forming the slug, which then passes between knives which accurately cut it down to the desired thickness. In the meanwhile, the matrices, by very ingenious devices are returned to their origin, to be used over and over again. It is not uncommon to find three different lines of type heing made on the same machine a t the same time. The matrices for one line may be collecting a t the same time that the slug of the preceding lime is being molded, while matrices for a still earlier line are heing redistributed. The slugs, after being used, are thrown into the heated melting pot, to be used over and over again. The composition of linotype metal is usually practically the eutectic for these (1) or (2)
Pb 84.5
Sn 13.5
82
13
Sb
2 5
three components giving an alloy of melting point of 240°C. Monotype.-This process needs two machines, one of them heing a perforating machine, also worked by a keyboard, but which permits the operator to perforate a roll of paper, about 5 inches in width, in such a way that the position on the roll indicates the letter type. That roll is of course analogous to the perforated roll used in player pianos. This roll is later rnn through the second machine, where, due to the air pressure through the perforations, molten type metal is forced into a particular spot in a mold, again a matrix of the letter to be typed. The metal instantly solidifies, leaving the desired character in relief on its base, making the type in other words. Then the machine arranges these in proper sequence
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C o m m r r i i n l mwmuing Publishi"g
co.
FIGURE 12.-So~E SPECIAL FEATURES OF THE MONOTYPE SYSTEM The spool of paper ribbon, the matrix case, a matrix as used for casting display letters, etc., and monotype cast type.
with reference to the copy and thus a line of individual characters is made. (See F i p r e 12.) Since the line of type is not a single slug as in the linotype process the operator can correct the line after the type has been made. The metal used here is identical with linotype metal. A Newer Process.-Without going into detail, I merely want to mention that even the mechanical printing processes have gone "talkie" too, for there is now a machine in which the keyboard has been done away with, and the operator merely spells the words into the receiver and the machine delivers the type. Duplicates Very frequently the type set by hand or by machine, or the metal cuts, are not intended to he used on the press a t all. Since type metal is a relatively soft metal, the sharp edges of the printing surface would wear away on a long run and the lines of a cut widen, distorting the image. That would mean, in the case of type, that the copy would have to be reset and proof-read again. Rather than do this, duplicates of the original are usuaUy made either as electrotypes or stereotypes. Since innumerable duplicates can be made from one form, these can be printed on different presses simultaneously. In the second place proof will have to be read only on the original. Thirdly, these will he facsimile duplicates, not only as to
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subject matter, but even as to exad spacing of letters, etc. These processes work quite well with any type of relief printing surface. Electrotypes.-For relatively coarse work wax is used as the duplicating medium, while for very fine work, resulting from the proper use of a fine halftone screen, lead is employed. The wax, usually beeswax or ozokerite, is poured into molds and permitted to solidify in slabs of the proper thickness. This solid wax is placed in intimate contact with the type, clamped in a form and both placed in a press operated by hand, steam, electric, or hydraulic power. (SeeFigure 13.) The wax form is then removed and given a very thin film of graphite or "black lead" as i t is called in the in. . dustry. This is to make the mold a conductor of electricity, for it is to be made cathode in an acidified copper sulfate bath, the anode being a bar of pure copper. The reaction is, of course, the well-known one used in the purification of blister copper and in other forms of copper plating. The copper a t the anode is dissolved a t the same time that an equal quantity is plated out on the graphite surface of the mold. This plated film must be a tightly adhering, fine-grained, bright copper surface, which means that proper control of the concentra-
to eight hours for its formation, the cathode is removed and hot water is poured on the mold, permitting the copper deposit to be safely removed. Of course this thin copper film is not strong enough to he put on the press, so it is backedby more metal. The customary procedure is to place a sheet of tin foil on the reverse of the form and then heat sufficiently to cause the tin to alloy with the copper. Then molten type metal is poured on the tin and allowed to cool, after which i t is shaved down to the desired thickness and mounted on wood, thus forming the duplicate printing surface. On account of the fact that the copper printing surface is harder than the original type metal, longer runs can be made. (See Figure 14.) Lead Molds.-Where lead is used in making the duplicates, especially in very fine work, a sheet of lead about 1.5mm. thick is placed on the type
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or cut and enormous pressures applied. Then copper is electroplated, as mentioned in the wax process, except that, of course, the use of the graphite conducting film is unnecessary. This copper deposit is also backed with type metal. Nickel Types.-The wax or lead form is given a Rash coat of copper and the plating continued in a nickel bath. These duplicates are less affected by certain kinds of ink and wear better on the press, it is claimed. The sulfide of certain pigments, like ultramarine blue and vermilion, might tarnish a copper plate, but probably would not injure the plate seriously. Aluminotypes.-These are made by making a mold in plaster of Paris, and after it has set filling with a bard aluminum-copper alloy. This form need not be backed for it is made thick enough and strong enough to be put on the press after being shaved down to the necessary thickness. These
are made only in special work and hardly ever when the work is the result of a halftone screen finer than 133 lines per inch. Stereotypes.-The stereotype process is a relatively inexpensive method of making duplicate plates, doing very satisfactory work with type or coarse halftones. The matrix is made of papier mdcki or paper pulp, or it is built up of layers of wet tissue paper cemented together with a special paste. This wet paper is placed on the metal surface to be dnplicated, covered with a blanket and pressure applied by beating with a stiff backed brush or by carefully rolling. Then the matrix with its metal form is heated, dried, and the paper carefully removed. In the reverse of this form is poured the molten type metal, which when solidified becomes the printing surface. This process has a very decided advantage in newspaper work, where paper matrices can be sent by mail from one city to another with a minimum of postage.
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Photomechanical Processes Z i c L i e Cuts.-Let us see how the illustration of the halftone screen was reproduced. The original was a (Figure 8, p. 1269 of June JOURNAL) pen and ink drawing, in black on white, with no shades of grey or any other colors. This is the type of material reproduced by zinc cuts. The original drawing, four or five times the diameter of the printed one. was photographed in such a way that the image on the photographic plate was the exact size wanted on the printed page. This photographic plate, usually a wet plate, as mentioned earlier, was developed in the usual manner and the silver-gelatin film stripped off the glass plate and reversed on to another piece of glass. In the particular case that we are discussing this reversing was unnecessary, for there is no top or bottom, no right or left to the sketch, but it would have been absolutely necessary in case some printed matter was involved. Unless it was reversed by a prism in the camera it would have to be done mechanically to keep the words from reading from right to left. A zinc plate, about 1.5 mm. thick, is carefully cleaned and coated on one side with the bichromate-glue solution which is then allowed to dry. The image side of the glass plate is put in immediate contact with the sensitive side of the zinc plate and exposed to light. As is true in other hichromate processes, the light hardens the hydrogel, making i t insoluble. This takes place only through the cleared areas of the negative. If the zinc plate is now placed in water, the unexposed parts will wash away, leaving only the hardened film adhering in the areas which were exposed to light. The plate is then heated, which carbonizes the hardened gelatin, making a still better "acid-resist." Then the plate is given a flat etch with nitric acid, washed, and dried. Very carefully, "Dragons blood," a reddish resin formed by certain palms, is brushed over the plate, the brush always being moved in the same direction. The plate is rotated 90' and given a second brushing in the same way. Upon heating, this resin adheres to the sides of the pits formed during the first etch and protects the walls from further etching. This is the procedure used previous to each "bite." Of course during this etching process the back and the edges of the plate are "stopped out" with an acid-proof "resist." The same is applied to large areas to be etched, for they are stopped out and the zinc is removed later, mechanically, by cutting away the surface or "routing out" the zinc as the technical expression has it. This mechanical cutting away of the zinc is merely to save time and etching solution. The plate is then dried and "routed," having large areas cut away by a bit on a spindle spinning ten to twenty thousand revolutions per minute. After being hacked with wood it is now ready for the press.
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Although zinc is generally used, copper is sometimes substituted, for it permits of nicer work and finer lines, although it etches more slowly and costs more. Needless to say, from this zinc cut, many duplicate plates can be made as electrotypes or stereotypes. Color Plates.Suppose we want to reproduce the halftone screen (Figure 8) in two colors, say red and yellow. The original sketch would still be drawn in black and white, but the printer would he told that the job was to consist in printing small yellow squares on a red background. The original sketch would be photographed as mentioned previously. Then two separate exposures would be made on separate sensitized zinc plates, and the portions not to be printed with that particular ink stopped out before etching. Or two separate drawings would be made exactly to scale, one showing the part to be printed red and the other yellow, and each photographed and used for the proper ink. Printing consecutively from one plate with one colored ink and then with the other, on the same area and in perfect register, would give us the two-color print. This process would of course give only solid colon, with no shades or tones, if i t were not for a relatively simple machine which breaks up large areas of printing surface into small dots. This is called the "Ben Day" machine after its inventor. "Ben Day" Screen.-This screen is used in a great many printing operations, where it is used for the same purpose. Each screen consists of a transparent film having a smooth upper surface but having the reverse engraved in different designs, such as small dots regularly placed, small dots unsymmetrically placed, herringbone lines, fine parallel lines, etc. The bottom side is carefully inked and by means of a stylus or roller that ink is transferred to the exact area on the metal plate. Dragons blood is added to the wet ink and baked on the surface of the metal, forming in certain areas a coating capable of resisting the action of the etching solution. The plate now has large areas broken up into smaller printing surfaces, which, even if printed with red ink, will look pink due to admixture with the white of the paper. Sometimes the "Ben Day" screen ink is applied directly to the original sketch, previous to the design being photographed, accomplishing the same end results. An example of color print made with such saeens is the comic section of the Sunday pap&, although from an artistic viewpoint the finished print leaves a great deal to be desired, due to inaccuracy of register. See how many different saeens you can recognize on a single sheet. Halftones.-The subject, this time almost anything, is photographed using a halftone screen similar to the one mentioned under lithography (not the rotogravure one) in the camera. Again a wet plate is used, and after proper exposure the plate is developed, backed by collodion, and stripped
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FIGURE 15.-Tns PRINTING SURFACE OF A SMALL HAI.FToNE(Lower Left) AS IT APPEAR WHEN GREATLY ENLARGED (Left),AND A PRINT(Right) FROM THE ENLARGED PLATE
WOULD
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from the glass. This reversed image is put on another piece of glass. This plate is put in contact with the bichromate-gelatin side of a polished copper plate and exposed to light. After washing to remove the soluble colloid, the surface is etched in a ferric chloride solution, where the unprotected copper goes into solution. Of course the back and the edges of the plate have been painted with asphaltum to prevent an unwanted etching there. "Dragons blood" is used as mentioned previously. The halftone screen accomplishes the result mentioned earlier, for on inspection of the halftone negative it is noticed that it is not a continuoustone negative, but one actually made up of millions of small areas of varying diameters. The copper halftone cut made from this negative will show these small areas as printing surfaces of differentdimensions. (See Figure 15.) In case work on many presses is desired, duplicate plates can be made by the methods mentioned earlier. Color Halftone Process Work.-Here the process is exactly like the one outlined under color photolithography (pp. 1271-3 of June JOURNAL) up to the place where the image is put on the plate. Each halftone separation negative is used in making a halftone copper cut as outlined above. Of course there is a different copper cut for each colored ink to be used. So the ink is printed, and when it is dry, another colored ink is added from another plate, always in perfect register. Some presses have facilities for heating the paper and ink after each printing to increase the rate of drying, so that four colors can be printed on the same press. If the screen used in making the halftoue was very fine, only about 15,000 copies can be made, while for coarse or medium screen or line, a run of 100,000 copies is not uncommon. The common screens employed have 133 to 150 lines per inch, and down. As was mentioned, nickeltypes give longer runs, while chromium-plated electrotypes may give as many as one-half million good impressions. Of course stereotypes and electrotypes of these halftone cuts can be made. Such is certainly necessary in the printing of enormous editions. Recently Time stated these facts as to the Saturday Eoening P o ~ t : ~ There were 2,977,500 copies. Each copy weighed 1 lb., 14 oz. Its 272 pages, containing 275.000 words of editorial matter comprised 295L/ssq. ft., enough to paper the ceiling of a room 2Q/n ft. X 12 ft. An average reader (225 words per minute) would take 20 hr., 20 min., to pe-use it. Sixty 45-ton presses, working night and day At an average of $9000 per page, the advertising shifts, printed it in three weeks. revenue was approximately $1,512,000. The issue consumed 3000 tans of paper, 60 tons of ink.
..
Ink Again let me refer you to that excellent book on "Printing Ink" by W i b ~ r gwhere ,~ you will find very fine discussions as to the composition of Issue of Dec. 7, 1929. 'Harper & Bros., New York City, 1926, 199 pp.
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inks, their particular properties, uses, etc. It also contains a valuable bibliography on ink and printing processes in general. Additional References Additional works which can be read with a great deal of profit are: SIGMUND LEHNER,"Ink Manufacture." Scott, Greenwood & Son, London, 1914. DAVIDN. CARVALHO. "Forty Centuries of Ink," Banks Law Pub. Co., New York City. 1904. NORMAN ~ N D E R W O O D . "Chemistry and Technology of Printing Inks," D. Van Nostrsnd Ca.. New York City, 1915. C. A. MITCHELLAND T. C. HEPWORTH, "Inks, Their Composition and Manufacture," Chas. Griffin & Co., London, 1924.
