Included below is the full text of an article which was published in The Penrose Annual: Review of the GraphicArts. (R.B. Fishenden Ed. London, Lund and Humphries & Co. Ltd.) Volume 40, 1938. The Penrose Annual was a British publication dedicated to the art of commercial and artistic printing. This article is the only source of information which could be found on the printing process used for almost all of Vassos' "illustrated" books.
Presumably all the modern lithographic shops, the world over, have become more or less familiar with the dot-etching method for colour correction, and have welcomed it. There has always been appreciation of what the photo-engraver could do by fine etching, because the lithographer could not retain all the photographic detail with his older methods; now it has been found that etching dots on gelatine dry plates is as practical as etching metal plates, and the lithographer feels that his industry leads all the printing processes. This is perhaps the attitude of most of the better shops in America that have discarded ground-glass retouching methods, and have adopted dot etching and deep-etched plates, which provide better reproduction and longer press runs. In spite of this progress, however, there are still difficulties in dot-etching methods, and not all the assurance there should be in the making of deep-etch plates. It is not easy for the average lithographic craftsman to visualize the value of dot sizes; difficult to get the help that is needed; almost impossible to take on photo-engravers, who may be clever as four-colour etchers, but who do not easily understand the subtleties of lithography; sales departments continue to groan over costs.
To be sure, dot etching is relatively new in lithography, is, in fact, semi-revolutionary, and all its possibilities have not yet been exhausted. Etching began with ferricyanide and hypo used, according to Murray, with certain precise manipulation and particular graphs, which, if carefully followed, enabled the etcher to determine exact values in dot sizes, but the scientific nature of this method was baffling to the average man, and it did not meet with wide favour. This has now been superseded by cyanide and iodine, with greater reliance on the etcher's judgment as to dot values, and possibly with greater success. Yet it is evident this is not the last word; in fact, one company has patented a device for electrically etching the dots, (1) so far confined to the one establishment, although it is not known that the device has any great advantage over ordinary chemical etching. It can be said, therefore, regardless of the general acceptance of dot etching, we still need something that is more mechanical, that will minimize the possibility of human error. A new method has been introduced which promises to give the advantages we need.
Remarkable advances have been made in photography; greater colour sensitivity, infra-red plates, and the new Kodachrome film. Separation negatives are now better than ever, so that offset colour work compares favourably with alternative processes.
The most interesting development now attracting attention in America is the Knudsen colour method, for which many advantages are claimed. The inventor, Hugo Knudsen, is an American citizen of Danish birth who is an ingenious investigator with a long and varied experience. His newest method for colour reproduction is radically different from anything in present use, and while radicalism fosters doubt, yet, because of Mr. Knudsen's reputation and ability, his process is gaining favour.
Some months ago, Mr. Knudsen told me what he had accomplished, and he was so convincing that plans were made for several plates to be printed in the National Lithographer. Colour copy, regular photographs, and wash drawings were supplied, and in a short time proofs were submitted that were carefully compared with the originals by several practical lithographers, who considered the results exceptionally good. The remarkable factor was the very short time required for the work. Mr. Knudsen stated that the total time for the colour plates was only a day, for work which by ordinary dot-etching methods would require four days as a minimum. Mr. Knudsen says that dot etching is not a part of his process, and the sheet carried four pages of illustration, high-light half-tones, duotone, and the four-colour job. My comments accompanied the illustrations, and were enthusiastic, but later events seem to justify what was said.
The Knudsen process has been successfully installed in a few plants in the United States, and quite recently in one of the largest plants in Chicago; it should be of general interest to all lithographers if these early successes are maintained.
The process is worked under licence, and it is natural that full information is not available, but the following details have been disclosed to me. In the first place, the basis is the Knudsen screens, which are not ruled screens on glass, but celluloid screens that are used in contact in a special vacuum holder; simple to work without any consideration for screen distance or special diaphragms. It is claimed, and the results appear to prove, that the screens give a better range of tone than is possible with any ruled screen. For colour work there are four different rulings at the proper angles, and, for the high-light reproductions, another screen to fill that particular need, with still another especially adapted to the reproduction of pencil drawings. In this connexion the inventor states: "While it is true that pencil drawings may be made with my regular screens and holders, I have found, over a period of years, that pencil drawings require a very skillful handling in order to hold true the finest pencil lines and smudges barely discernible from the white paper, and that this difficult operation required too much time; although, as I say, pencil drawings can be made without any handwork with my regular screens, the care and skill required always left me dissatisfied with the method. I have now constructed a new screen and holder which accomplishes the reproduction of pencil drawings with ease and surety, and with such truthfulness that the reproduction cannot be told apart from the originals." This is a broad claim, but the plate made from a pencil drawing for the National Lithographer was certainly quite satisfactory.
For colour work there is a series of tone scales and a special apparatus called a Densitycheck: a new comparison meter for control and surety of results. Evidently there is no masking of the separation negatives, but these must carry a correct rendering of the particular colour, which is again checked and verified with the Densitycheck apparatus. With the photographic methods there is also a special and simple plate method that makes failure exceptional, which is more than can be said of the general run of deep-etch plate methods. Altogether, the process seems to be complete in itself, and if time bears out what is now being done commercially by firms of high repute, then dot etching and other present means for lithographic plate-making are likely to be discarded.
The basis is the flexible celluloid screen that can be washed, that is used in contact in a vacuum holder, that does not require screen distance or diaphragm control, and needs no understanding of ruled glass-screen technique. Can it be that the ruled glass screen is no longer necessary in lithography? From a careful examination of many press sheets, in one and more colours, printed from Knudsen plates, it seems that this may be the final outcome. In the reproductions copy values are retained without special effort, and give a quality which is difficult to attain with ruled screens.
In the four illustrations accompanying this article, typical results by the Knudsen process are shown. The plates have been produced photographically, without any corrective handwork, retouching, dot etching, or other means ordinarily used.
Two plates are in full colour, and the first shows to advantage the perfect rendering of high-light detail, which is frequently lacking unless extra plates are added. The fourth plate is different in character, being a reproduction of a water-colour drawing.
The second subject shows what the process can do in picking up fine pencil lines in high-light technique, and the third demonstrates the perfect rendering of tones, and particularly the unusually crisp detail portrayed.
1. See "Electrical Dot Etching," J. S. Mertle, Penrose Annual, Vol. xxxviii, 1936, p.117.