PFSNTING PROCESSES. I. DEPRESSED SURFACE PRINTING S A ~B. L AWNSON, UNIVERSITY OF CINCINNATI,CWCINNATI. OHIO Introduction This will introduce a brief series of articles on the various methods of reproducing illustrations and printed matter. In no case will the artistic merits of any particular piece of art be discussed as such. If etchings, for example, are to be considered, it will not be from the point of view of the contributions of etchings and etchers to the happiness of the owner or viewer, but from the standpoint of the chemical and mechanical manipulations involved in the process of making and printing etchings.
Zinc Line Etching
Copper
Half -Tone
Metal Lithographic Plate Rotogravure Printing Surface FIGURE 1.-MAGNIFIED VIEWS OF PRINTING %ATE SURFACES* We have several ways of approaching the general subject of printed reproductions. The order of topics might be chronological, alphabetical, or based on a scheme' depending on the type of printing surface. The last-named plan will be used here. The printing surface may be (1) an engraved or "intaglio" one,a depressedsurface into which ink flows and from which that ink is transferred onto paper or cloth. Etchings, dry-points, steel engravings, and rotogravure prints are good examples. There is (2) planography, printing from a practically plane surface, where the metal or stone is so sensitized that i t will take ink-only on certain areas, as in *Thesefour cuts by courtesy of Mr. Paul W. Dorst. Research Assistant, Lithographic Technical Foundation, University of Cincinnati. 1028
VOL.7, NO.5
PRINTING PROCESSES. I
1029
both stone and so-called metal lithography. Lastly, there is (3) printing from an elevated surface, "relief" printing as it is called, on which a relatively stiff ink adheres to elevations and which is transferred to paper. This third process consists in printing from zinc cuts, copper halftones, type, wood cuts, electrotypes, stereotypes, etc.'
Etchings The word etching is derived from the Dutch word "etzen" (to eat). Originally iron plates were used, but now, copper, aluminum, and zinc are quite common. These plates, usually 18 gage (1.2 rum. thick), are cleaned by rubbing with an abrasive, like snake-stone, and finally more carefully polished by some softer material, ordinarily charcoal. They are then covered with a " g r o u n d a n acid-resisting material which protects the plate from the etching solution. Naturally this ground must adhere very tightly to the plate, must not flow, chip, or read with the etching solntion. It is usually made by melting together wax, asphalt, and pitch. It may be applied cold, which means that the ground is dissolved in some solvent (an organic liquid such as chloroform) then poured on the metal and the solvent permitted to evaporate, or it may be applied "hot" as a semi-solid, warm paste and dabbed on uniformly. Next a film of carbon black is carefully deposited on the ground, usually by means of a smoky flame. This is merely to permit the etcher to see better the results of his work. The back of th; plate is also covered by some inexpensive resist such as shellac or pitch. The plate is now ready for the artist. He takes a sharp needle and scratches through the ground down to, but not into, the metal underneath, making the desired design. He may employ a mirror when viewing the plate, for his design must be a mirror image of what is wanted. When completed, the plate is placed in an etching solution, called technically a "mordant." , ~ of the mordants used were: Quoting L ~ m s d e nsome
For Iron NHICI
15th Century Formula
17th Century Formula
.Ma FeSOd White wine vinegar NH&l Vert de Griz Bay salt (sel common)
i
3 parts 4 02. 4 oz. 6 02.
Part I o f this series will deal with the "intaglio" surface, Part 1 1 6 t h planopraphy, and Part I11 with "relief" printing. 'See references at end of article, page 1036.
1030
JOURNAL OF CHEMICAL EDUCATION
MAY,1930
Hi501
mo3
1:: Alum
Dutch Mordant
Kc101 HNOs solution or HC1 Kc101 or FeCL Dil. HNOaor KNOs HCl
Common Copper Mordants Common Zinc Mordants
880 g. 100 g. 20 g.
+ +
These various solutions will attack the bare metal exposed by the removal of the ground with the etcher's needle. The nitric acid, hydrochloric acid, or sulfuric acid, due to the liberation of a gas by interaction with the metal, will make a relatively wide line, caused by undermining the ground, the gas of course blowing off particles of the protective ground. The Dutch mordant, acting with practically no effervescence, will give a sharp, fine line. After sufficient reaction has taken place, i. e., after the operator believes that the lines are deep and wide enough to hold the desired quantity of ink, the plate is removed from the etching solution, washed, and the excess ground removed by a solvent. If the plate is not to be uniformly etched, the etcher may, after partial etching and before removal of the ground, "stop out" certain areas with varnish and permit the remainder of the plate to be etched more deeply. In any case, the plate free from ground now has certain depressions where the metal has been removed by c h b i c a l reaction with the etching solution, according to the following equations:
+ Cu --t CuCI, + 2FeC1, or 8HN08 + 3Zn +3Zn(NOJn + 2NO + 4Hn0 2FeC5
The plate is then inked by means of a roller, or by dabbing, and the excess ink is removed with a soft cloth. The unetched portion of the plate is not wiped absolutely free from ink, but a very thin film is purposely left on i t to break the contrast between the ink and the paper on which it is to be printed. Next a dampened sheet of suitable paper is placed on the inked surface, then several blotters and blankets and all run through a press, similar in principle to a clothes wringer, the ink being transferred from the crevices in the plate to the paper. In case of very delicate etchings, only a dozen or so impressions can be made from a single plate: However, several hundred are not uncommon, especially if the zinc etching plate is given a flash coat of copper electrolytically and then surfaced with "steel" in the same way. A very thin film of plated chromium also increases the life of the plate enormously.
VOL. 7, No. 5
PRINTING PROCESSES. I
1031
Dry-Point (Kalte Nadel) As the name infers, no etching solution is needed, for a steel needle or diamond-pointed instrument is used actually to cut into the metal plate in making the design. Naturally the greater the pressure applied, the deeper will be the depression in the metal, and the darker will be the inked line formed, for the scratched surface is inked and printed as described for etchings. On inspection with a hand lens, it is noticed that the cross-section of the ink-holding depression is U-shaped in the case of an etched plate, but Vshaped in the dq-point plate. Furthermore, there will be microscopic burs of curled metal on the surface of the dq-point line, due to the "gouging out" effect of the harsh mechanical cutting instrument, which leaves somewhat ragged edges on the lines of the finished print. Since these burs define the line, and since they are naturally worn away during the printing process, fewer good prints can be made from a dry-point plate. One of the important uses of the dry-point procedure, however, is to correct an improperly etched plate. Once a plate is etched, additional lines can be added by dry-point, thus combining the results of these two processes on a single plate and print.
Soft Ground (Vernis Mou) Here we also use a plane, polished plate, covered this time, however, with a relatively soft ground, usually tallow. On top of this ground is placed a thin sheet of paper. Upon tracing a aesign, again of course as a mirror image of what is wanted, by means of a hard-pointed pencil, sufficient pressure is exerted to cause the ground directly underneath to adhere to the paper. When this paper is stripped from the plate, there remains the bare metal where the design was traced. The plate is then etched in the conventional manner and prints are taken from it as described above. Naturally the lines are a trifle wider, and some of the detail is lost by this procedure. Aquatint This is another etching process, but areas are etched rather than lines. The polished prepared plate is "spotted" with rosin by one of two methods. The rosin is dissolved in some volatile solvent and flowed on, or the rosin is dusted on by blowing the powdered material into a box in which a plate is placed horizontally. The deposit is made to adhere to the plate by warming, where each little particle melts and individually adheres to the plate. This ground differs from the tallow or wax grounds mentioned earlier, in that it is not a continuous one. In other words, a quantity of rosin insufficient to completely cover the whole area, has been used. That means that under a hand lens, small dots of the bare metal and of rosin can be seen over the entire surface of the plate.
1032
JOURNALOF CHEMICAL EDUCATION
MAY, 1930
Suppose the aquatint is to represent a block, capital E, with its shadow, the E to be in black and the shadow in grey, on a white paper. The E and its shadow are traced on the ground, and the rest of the plate is "stopped out" with acid-resistant ground. The plate is then dipped in the mordant. After an exposure of sufficient length of time to etch the E and its shadow, the plate is removed, washed, and now the shadow is "stopped out," as well as the background. Now when the plate is again etched, the block E will be the only area reacted upon by the etching solution. When this plate is removed from the solution, washed and cleaned, i t forms a printing surface holding two different thicknesses of ink, none for the background, more for the shadow, and the greatest depth for the E itself. The printed replica shows tones in place of Jines, etched areas in place of etched lines. The process can become as complicated as the artist chooses. It must be remembered that since the surface had all over i t small areas of mordant-resisting rosin, the surface of the etched plate is made of millions of little wells, the walls being the unetched plate. Bank Note and Steel Engraving This is essentially line and dot engraving, done from an especially treated iron plate. In the case of the lines, they are chiseled out by moving a small chisel forward. The shavings are hlpwn away, and the burs adhering on the edge of the chiseled line are remove: by a scraper. Generally the steel plate when finished is given a flash coat of copper electrolytically, and then chromium plated in the same way. Previous to the chromiumplating improvement, making tens of thousands of prints would decrease the depth of the engraved lines to the extent that the plate became worthless. However, surfacing the printing plate with a hard metal like chromium has increased the life of the individual plate manifold. In making steel hank-note engraving plates, usually a master plate is made and case hardened. Then a relief duplicate is made by impressing the original plate against a relatively soft steel plate, which in turn is also case hardened. From this second plate, many duplicates of the original can be made by similar impression on soft steel. Photogravure (Rotogravure) The light and shade in an etching process depend ordinarily on the depth of the etched surface, for the deeper i t is, the more ink is retained to be transferred to the paper. The etched depression must be slightly rough to retain the ink. In the photogravure process, the etched surface consists of millions of little wells, each capable of holding a different depth of ink. On printing from such a surface, the result looks like a continuous-
VOL, 7, No. 5
PRINTING PROCESSES. I
1033
tone photograph, rather than a newspaper halftone, for these wells are so close together that they can only be seen with a hand lens. The process is roughly as follows: carbon tissue, which is a paper having a surface coating of gelatin and a reddish brown pigment, is sensitized by the addition of potassium dichromate (K2Cr20,), to which ammonium hydroxide (NHIOH) has been added until the color changes from the orange of the dichromate to the yellow of the chromate. This tissue is exposed to light directly underneath a screen made of black squares and
clear lines, as illustrated in the sketch below. These clear lines are placed uniformly 150 to 175 per linear inch. Light going through the clear spaces, after long exposure, hardens the bichromate-gelatin directly underneath, forming relatively insoluble gelatin. The areas directly underneath the black squares are, of course, unaffected. The screen is now removed, and a continuous-tone positive substituted and the carbon tissue is again exposed to light, this time, however, for a relatively short time. The same thing happens, for where the light falls on the tissue, i t hardens the gelatin. This continuous-tone positive is
1034
JOURNAL OF CHEMICAL EDUCATION
MAY,1930
made by merely photographing the subject and, after development in the conventional manner, making a positive plate of this negative in the same way that one would make a positive print of any negative. The tissue now shows varying depths of insoluble gelatin depending on the opacity of the continuous-tone positive, plus a regular pattern of criss-crossed parallel lines of very highly tanned gelatin. This tissue is now ready to be placed on the specially prepared copper cylinder. This cylinder is usually made of a cast iron core on which copper has been plated electrolytically from a comer sulfate bath until about a 4 mm. thickness has been built up. Naturally the cylinder has been smoothed and cleaned in preparation for the exposed carbon tissue, which is to be firmly attached Sometimes the cleaned grained copper is given a thin coating of gelatin to act as an adhesive, while ordinarily the copper is merely cleaned by a dilute aqueous solution of salt and acetic acid. The exposed paper, face downward on the copper cylinder, is permitted to soak in warm water (30-40°C.) itntil the sensitive exposed film adheres to the copper and the paper can be removed with safety. There are now on the copper varying thicknesses of tanned gelatin, from 0.01 mm. in the - high-lights on the original positive down to the thinnest film on the bare pIGuREa , - ~ ~ f ~s~~~~~~ : TK& PAPERFROM THE COPPERCYLINDER metal, in case of the most opaque LEAVING THE TANNED GEUTIN (IN ON TAE cOPPZR, Right; sAxE portions of the screen and positive. SECTION O F CYLINDER ASTER ETCHING.See Figure 3. NOTICE THE WELLS OP DIFBRENT Areas not to be etched are now DEPTHS stopped out with bitumen or asphalt paint. The cylinder is then revolved in ferric chloride solutions of varying concentrations, starting with the most concentrated solution. This femc chloride must penetrate the film of hardened gelatin before i t can react with the copper underneath. Obviously i t reaches the copper first through the thinnest films and etches deepest there. Therefore the surface is pitted by millions of holes of varying depths. These are the future inkcontaining wells (see Rotogravure Printing Surface, Figure 1, page 1028). Notice that the cross-section of the well is large compared with the thickness of the intervening wall. &.
VOL.
7, NO. 5
PRINTING PROCESSES. I
The etching reaction a t first may be: PeCla Cu = FeCL
+
1035
+ CuCl
especially if the etching solution contains too little free hydrochloric acid. This reaction is not desired on account of the insolubility of the cuprous chloride. Too large a hydrogen-ion concentration is also inadvisable, due to its increasing the rate of penetration and etching too rapidly. The acidity is usually controlled by adding peptized ferric hydroxide. Naturally the temperature of the etching bath plays an important part in the rate of etching. A thirty-five per cent ferric chloride solution will etch to a depth of 1mm. in ten hours a t 15'C., while either a t a lower or higher temperature that figure is decreased. It will be noticed that not only must an accurate control of the hydrogen-ion concentration be made, but also of the temperature. After the etching is completed, taking into consideration the type of paper on which the material is to be printed, the size of the edition, etc., the cylinder is washed with water, then 5y0 sodium hydroxide, followed by water and a 5y0 hydrochloric acid solution, then rapidly dried. The cylinder is now ready for the press where i t comes in contact with a fluid ink. then with a flexible steel "doctor blade" which removes the superfluous ink from the surface, then with the paper on which the ink is to be transferred. Due to the fact that the printing is from a cylindrical surface, the process is a continuous one. Since we are not using a stiff ink, but a r a t h q fluid one, the ink flows together, more or less, when transferred to the paper, especially if the quantity of ink is large. That accounts for the f a d that, even under a hand lens, i t is difficult to recognize individual dots of ink in the darker portions of the print, while they are readily noticed in the high-lights. But rather than being a handicap, this flowing together actually is an advan tage, for i t removes the harshness and makes the finished product more like a continuous-tone photograph. An average run is about 175,000 impressions, after which the surface is reground, polished, and re-etched. Depending on the thickness of copper originally deposited, the cylinder is good for a t least twenty-five runs before additional copper need be plated on the core. In the case of colored rotogravure, a rather expensive process, but one giving very beautiful results, separate cylinders are made for each color desired, and the paper is printed successively with each color in perfect register. Since the theory of colored photogravure is similar to that employed in making colored lithographs and colored prints, it will be discussed in greater detail in future articles. Newer improvements in this field consist in chromium plating the finished cylinder to minimize the risk of scratching in handling, and in using thin copper plates in place of the heavy cylinder.
1036
JOURNAL OF CHEMICAL EDUCATION
MAY,1930
A very fine article covering the various theoretical phases of photogravure etching has been written by H. Mills Cartwright, published in "Proceedings of the Seventh International Congress of Photography," July, 1928, under the title "The Progress of Photogravure Etching."
Inks The drying of an ink takes place by oxidation, evaporation, and penetration into the paper. The paper used in a photogravure process will not permit of any great penetration because of its glazed surface, consequently most of the drying is accomplished by evaporation. To aid in this process alcohol is sometimes added and the printed paper warmed by a heated drying drum. The binder used to keep the pigment or lake on the paper is usually a natural hydrocarbon called ~ilsonite. However, i t is colored too dark to be used in the lighter colored inks, and so resins and gums are substituted. Obviously gritty particles must be absent for if they got in between the plate and the "doctor blade" they might scratch the printing surface. The ink used in most intaglio processes, except rotogravure, must have an oil, which on oxidation can bind the pigment to the paper. This is usually linseed oil, which has been previously treated with catalysts to aid in the oxidation. The ink must dry fast so that i t will not offset on to the next sheet. The coloring matter may be a lake, an earthy pigment, an artificially prepared insoluble colored substance, or eome form of carbon black.
Additional References There is a wealth of very valuable material in these books: H E ~ ~ ~ STRUCK, A N N "Die Kunst des Radierens," Paul Cassirer, Berlin, 1908. E ~ S S.TLUMSDEN,"The Art of Etching," J. P. Lippincott, Phila., 1925. C m n m s W. HACKELMAN, "Commercial Engraving and Printing," Commercial Engraving Publishing Company, Indianapolis, Indiana, second printing. revised. 1924.
Chemical Abstracts Now Covers 1500 Periodicals. According to E. J. Crane, in a recent issue of the News Edition of Industriel and Engineering Chemistry, there are over 1500 periodicals now appearing which publish a t least occasional articles reporting new information of chemical interest. In its efforts to cover the chemical literature of the world completely, Chemical Abstracts is continually seeking to learn of new scientific journals and to unearth obscure ones. The total number systematically examined has just reached the 1500 mark, and experience has shown that there is always another to be found in Japan or Russia or some other distant country and that one or two new ones will put in their appearance before this can be printed. These periodicals have been classfied: (1) by subjects or fields covered, and (2) by countries in which they are published. Tables showing the number and percentage in each class accompany the original article.
