Читать книгу Photogravure - Henry R. Blaney - Страница 10

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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 albumen 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 glass 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 Carbutt transparency plate.

Mr. Horace Wilmer says: "The class 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 glass, of course after allowing in focusing for the thickness of the glass 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 glass plate to be employed, but it will be obvious that with the ordinary dry plate all the imperfections of the glass, 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 glass 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 glass, as is often the case, for by backing it with a piece of plate-glass perfect contact is ensured everywhere.

We come now to the method of stripping the film from the glass. If the negative is made by the collodion process the matter is a simple one. The glass 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 passes through the negative, and releases it from the glass. 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 glass, 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 potassium 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 formulæ may be employed for the manufacture of the organic substance:—