Photogravure.

by Henry R. Blaney.

INTRODUCTION.

About the year 1820 Niceph.o.r.e Niepce made the discovery that bitumen, under certain conditions, was sensitive to light. He dissolved it in oil of lavender, and spread a thin layer of the solution thus obtained upon stone. This he exposed under a drawing (making the paper transparent by waxing), and after sufficient exposure, oil of lavender was poured on.

Those portions of the bitumen which had been exposed to the action of the light had become insoluble, and so remained while the lines which had been protected by the drawing were dissolved away. By treating the stone with an acid these lines were bitten or eroded, and could be printed from. Niepce afterward employed metal plates instead of the stone.

Here we have the foundation for a number of printing processes of the present day, including photogravure.

For many years, however, progress in processes for intaglio printing was very slow. In 1852 Talbot introduced a process termed photoglyphy, and in 1854 Paul Pretsch, of Vienna, patented a process which he termed photogalvanography. In 1870 the late Walter B. Woodbury, inventor of the Woodburytype process, suggested to M. Rousselon, of M. M. Goupil & Co.,[A] a process which he had discovered, and which he describes[B]

as follows:

"The method, as perhaps many of your readers know, is based on the fact that some pigments used in carbon printing have an unpleasant habit of granulating when mixed with gelatine and bichromate, destructive to their use in carbon printing and Woodburytype, but bearing the essence of success in an engraving process where grain is necessary. The origin of this method was simply owing to my getting some bad reliefs, in which this effect was first noticed. Out of this arose the photo-engraving process which, as I said before, is now claimed as the invention of a Frenchman. But I am digressing.

"This relief, possessing a suitable grain, could, by hydraulic pressure, be made to transfer its minutest details to metal without any loss to fineness, so giving a plate possessing all the properties of a mezzotint. The methods. .h.i.therto used of electrotyping would have proved useless, as all detail would have been lost. The same thing applies to the new method I am now about to bring before your readers. The latter process of getting the grain transferred to a hard metal remains the same; but the novelty is in the method of producing the grained plate.

To those who have practiced the process of enameling, as used by Geymet and Alker, and others, my description will be better understood.

"I first coat a thin, polished steel plate (zinc will answer) with a very thin coating of gum, glucose, and bichromate as used for enameling.

This I dry rapidly, and, while still warm and desiccated, expose under a gla.s.s positive. On removal from the frame after exposure the plate is made to take up a slight amount of moisture by breathing on it.

"During this stage I brush or dust over it any hard powder, such as emery, powdered gla.s.s, etc, but these I keep of different degrees of fineness or coa.r.s.eness. No. 1, is of a coa.r.s.e quality, and is used first; No. 2 is finer; and No. 3 is of the finest grain obtainable.

These are obtained by pa.s.sing through muslin of different degrees of fineness. Having in the first stage of moisture used the No. 1, or coa.r.s.est, powder, after a time No. 2 is dusted over and adheres to the middle tints, while the very finest tones, which have almost lost their sticky qualities by the exposure to light, are treated to No. 3.

"Now we possess a granular picture having all the true qualities required in a photo-engraved plate, or, rather, such as will give a reverse in metal having these qualities. The steel or zinc plate is then to be exposed to light to completely harden the mixture all over, and is then treated exactly as in my other engraving process; that is, pressed into soft metal by hydraulic pressure, electrotyped, and then the surface is aciercised or coated with steel. The dark parts are thus represented by a coa.r.s.e grain, the middle tints by a medium grain, and the finest shades by the most infinitesimal particles, thus meeting all requirements necessary to a successful photo engraving process."

This process was taken up by a Frenchman and claimed by him as his own invention. The chief difficulty with it was that the plates before being perfect require the work of a skillful engraver, sometimes for weeks.

They were therefore very costly, six dollars per square inch being charged for the making of the plate alone.

Klic's process, 1886, was the next important improvement in photogravure or intaglio printing, and since then many other processes and improvements have been introduced by Obernetter, Waterhouse, Colls, Zuccato, Sawyer and others.

In the following chapters Mr. H. R. Blaney gives a working description of the process as practiced to-day by many of the leading firms in this and other countries. This originally appeared in the columns of THE PHOTOGRAPHIC TIMES, but I have made many additions that I have imagined may be of value to the student. A dividing line will be found between Mr. Blaney's writings and my own additions.

THE EDITOR.

[A] Now Boussod, Valadon et Cie.

[B] _British Journal Almanac_, 1874.

CHAPTER I.

THE NEGATIVE.

Any negative may be used for photogravure, that is, taken from nature, or from a painting or engraving, provided it is reversed, and, in the case of paintings, should, in addition, be on an orthochromatic plate.

The negative should be soft and brilliant, well exposed, and not hard or under-exposed. A reversed negative is always necessary if the print from the copper plate is required to be similar in regard to right and left, or if no other means are to be taken, to reverse the image upon the copper plate. Professionals use stripping plates especially made for this purpose for small work, or the reversed negative may be made in the copying camera. A fairly good reversed negative can be made by contact in the printing frame from an alb.u.men print from the original negative, the print made transparent with white wax by being placed on a piece of warm, clean metal and the wax rubbed over the face. To have the negative reversed, the print should first be placed, face out, against the gla.s.s of the printing frame, with its back against the sensitive surface of the transparency plate, the back closed in and exposed to a large lamp for about five seconds. Every care must be taken that you use the best of negatives, carefully retouched if necessary, as the professional photographic etchers have informed me that (from their standpoint) the success of the whole process depends on the quality of the original negative and the care taken in artistic retouching.

It will often happen in commercial photogravure work that plates have to be made from all kinds of original negatives. In cases where these are flat from over-exposure it is well to make a carbon transparency; intensifying the image with a strong solution of permanganate of potash, and from this make a fresh negative upon a slow or Carb.u.t.t transparency plate.

Mr. Horace Wilmer says: "The cla.s.s of negative most suitable is such as gives a good result by any of the printing processes. A bright sparkling negative will always give a good plate, but I do not find that any satisfactory results can be got from a soft flat negative. The negative should be as perfect as possible. It is absolutely useless to work from a faulty negative. Contrasts on it may be increased by retouching. Such contrasts are desirable because the tendency of the etching is to reduce them somewhat."

Perhaps the simplest way of obtaining a reversed negative is by placing the dry plate in the slide film inside and exposing through the gla.s.s, of course after allowing in focusing for the thickness of the gla.s.s plate. With the wet-collodion process, usually the method employed by large photomechanical printers, this method can be used because it is a simple matter to carefully examine the gla.s.s plate to be employed, but it will be obvious that with the ordinary dry plate all the imperfections of the gla.s.s, such as dirt, scratches, air-bubbles, etc., will be clearly reproduced in the image.

Another method largely employed to produce reversed negatives direct, is by means of a mirror or prism placed either before or behind the lens.

The prism is the more convenient, but if large sizes are used it becomes a costly piece of apparatus. The mirror, which should be a plane of gla.s.s silvered on its surface, is a less expensive affair. By either of these means the reversed negatives can be made direct without suffering the least in quality.

With celluloid or other flexible films, printing can, of course, be done from either side. Practical men, however, say that, except with the very thinnest films, there is an undoubted loss of sharpness in the grain when these films are reversed and with some mechanical processes.

Against this, however, it may be said that better contact can be obtained in printing than if the film were upon a piece of uneven gla.s.s, as is often the case, for by backing it with a piece of plate-gla.s.s perfect contact is ensured everywhere.

We come now to the method of stripping the film from the gla.s.s. If the negative is made by the collodion process the matter is a simple one.

The gla.s.s is treated with French chalk previous to collodionizing. After the negative is made and dried it is laid on a leveling stand and a solution of gelatine poured on it. When dry, it is readily stripped by running a knife all round. With ordinary dry-plates the method usually recommended is to immerse them in dilute hydrofluoric acid. The difficulty often experienced here is in the lateral expansion of the film. This will largely depend upon the plate, or rather the quality of the gelatine used. There are, however, two methods of securing the films to some medium unaffected by moisture, and so prevent expansion or distortion. The first is that recommended by Mr. A. Pumphrey and the second by Mr. H. J. Burton, modified descriptions of which are given in a recent number of _The British Journal of Photography_. If the negative is varnished this is removed. A thin film of gelatine is moistened in a dilute solution of hydrofluoric acid, one part of acid to sixty of water. This gelatine film is secured on paper by a coating of india-rubber.

The action of the dilute acid is to soften the gelatine, making it very adhesive. It can, in this state, be readily attached to the negative by squeegeeing. The acid in the film pa.s.ses through the negative, and releases it from the gla.s.s. It can then be lifted off and pinned to a flat surface to dry. The paper can afterward be stripped off, when dry, by moistening the back with a little benzole to dissolve the india-rubber. In this manner we get the stripped negative in exactly the same size as when on the gla.s.s, to which it can be restored at any time desired.

Burton, in his method, employs collodion in place of paper as the support. The negative is first coated with a thick collodion, and this is allowed ten minutes or so to set. It is then immersed in plain water until the film loses all appearance of greasiness. A few drops of hydrofluoric acid are added to the water, and the dish gently rocked.

The film will soon detach itself, when the plate should be at once rinsed. Another plate previously coated with gelatine, and dried, is placed in the dish, and the released film, after reversing, is floated upon it, the two removed together, and allowed to dry.

So far we have only treated upon reverse negatives, either obtained at once or reversed afterward. It often happens, however, that we have an ordinary negative, which is required to be reversed. This negative may be a valuable one, and the risk involved in stripping it be too great.

Another simple method of obtaining a reversed negative is by means of the powder process. Although this process is an old one, it appears to be but little known, for what reason we have never been able to define.

It is by no means difficult, and by its means a negative can be obtained direct from a negative without the intermediate positive transparency.

The principle of the process is this: An organic tacky substance is sensitized with pota.s.sium dichromate, and exposed under a reversed positive to the action of light. All those parts acted upon become hard, the stickiness disappearing according to the strength of the light action, while those parts protected by the darker parts of the positive retain their adhesiveness. If a colored powder be dusted over, it will be understood that it will adhere to the sticky parts only, forming a visible image, the same being a reproduction of the positive printed from. The process is very useful for the production of lantern slides and transparencies, or for the reproduction of negatives. Any of the following formulae may be employed for the manufacture of the organic substance:--

SOLUTION A.

Gum arabic 25 grammes Grape sugar 60 grammes Purified honey 15 grammes Alcohol, 40 deg 15 c.c.

Water 60 c.c.

SOLUTION B.

Saturated solution of ammonium dichromate.

Two solutions to be mixed together before using in proportions 15 A, 25 B, 50 water.

WOODBURY'S FORMULA.