Photo-Lithography

The main characteristic of this process is that either the original from which the reproduction is to be made must be prepared in distinct lines, strokes or points, or that any half-tone which may be present in the photographic negative must be broken up on the stone or plate into lines or points.

The basis for the preparation of a photo-litho is as a rule a photographic negative on glass or gelatine, which, as the word negative implies, when examined by transmitted light, must have all its tones reversed, that is to say, the drawing or the lines, strokes or points which must in the print be black, must look transparent, whilst the other part of the negative which forms the groundwork must be covered or opaque.

If every photo-lithographic process was to be described in detail there would be a great many, but actually they may all be classified according to two principal methods:—

(a.) One, in which the stone or plate is itself coated with the light sensitive substance and exposed under a reversed negative, so that a reversed image is formed on the stone or plate, which in printing comes in the right position, and

(b.) The other, in which paper or a very thin zinc plate, provided with a light sensitive film, is exposed under an ordinary, that is not reversed negative, and thus is rendered capable of receiving fatty ink, and is then transferred to the stone or plate by transfer.

Of the many processes which, though differing in detail, may, looking to the final result, be assigned to one or other of the above-mentioned principles, there are two which have been especially tested in practice, namely, for the direct transfer, as we will call it, that process which is based on the light sensitiveness of asphalt or of an organic substance in combination with a {16} chromium salt; and for the indirect transfer, that process which is founded on the light sensitive chromium salt in combination with gelatine, or briefly on the light sensitiveness of chromated gelatine. All other more or less complicated methods have disappeared from technical practice and have only the honour of being scientifically interesting and theoretically correct, but for various reasons are not practically valuable.

It is indeed obvious that a discovery so important and useful to one of the principal departments of the graphic arts as lithography is, and which may be called even more essentially capable of variation and multiplication, and perhaps in its way also more artistic, should call forth an earnest movement on behalf of the technical experts, principally with the endeavour for simpler forms and extension of its powers of work. This, indeed, has not always been attained, and these attempts will therefore only be taken into consideration in this book as may appear necessary, and all others will be passed over in silence.

The methods used by some experimentalists for direct transfer, which consist in coating the stone with a solution of gelatine, albumen, or gum made light sensitive with a chromium salt, and after exposure under a positive or negative, obtaining a printing plate, were in execution complicated and troublesome, but in results fairly safe and satisfactory, so that they are now more and more used in practice.

Iron and silver salts have been proved as not very suitable for photo-lithography, and at the present time for direct transfer asphalt is most generally used, and latterly also organic substances rendered light sensitive by a chromium salt; for indirect transfer chromium salts in combination with gelatine, or chromated gelatine.

Asphalt exposed to the action of light undergoes a chemical change which consists in its becoming less soluble in its original solvents. This was known to Nicephore Niépce, one of the discoverers of photography, whose endeavours to produce images by the action of light were actually based upon this very property of asphalt. He used for this purpose metal plates which were converted by a species of etching into printing plates.

In the year 1852, the well-known Parisian lithographer Lemercier, in partnership with Lerebours, obtained a patent in France for a process by which they were in a position to obtain images on stone by the exposure of asphalt, and thus reproduce the same by printing in the ordinary way. They coated a lithographic stone with ethereal solution of asphalt, exposed it under a negative and developed it with ether; there remained behind then the parts which had been rendered insoluble by light and formed a positive asphalt image on the stone, the outlines of which had the property of taking up greasy ink and with suitable preparation of giving an impression on paper. Such a stone was etched in the ordinary way with acid and gum, and then it was possible to make as many {17} pulls from it as was desired in litho ink. We have here a practical photo-lithographic asphalt process, as it is at the present time practised with various modifications, and which gave satisfactory results.

Other resins besides asphalt may also be rendered sensitive to light, and it is well-known that they also may be used with as good results as asphalt for photo-lithography.

That the good qualities of asphalt were even earlier recognized is proved by the fact that Negré, before the discovery of zinc etching, exposed a copper-plate coated with asphalt solution under a negative, washed out the places not affected by light, electroplated these blank places in a gold bath, then cleaned the plate and deep etched it with acid. He obtained in this way an intaglio printing plate similar to an etching.

By using a positive with this process a relief or typographic plate may be obtained, both of which processes are known as “gold etching,” and were practised in the beginning of the ’50’s in the K. K. Hof- und Staatsdruckerei.

The asphalt which is used for photo-lithographic purposes must possess certain qualities, of which the most important is as high a light sensitiveness as possible. With ordinary asphalt success will not be attained, since it is only slightly sensitive to light; the so-called Syrian asphalt is therefore generally used, which after having been prepared is dissolved in chloroform and then benzole and oil of lavender are added.

Professor Husnik prepares an asphalt from which all constituents not sensitive to light have been extracted. Herr Valenta produces his sulphurized asphalt. Both possess a far higher light sensitiveness than ordinary Syrian asphalt, and are specially suitable for photo-lithography.

The second, and considerably more extensively used method for photo-lithography—a transfer process—is based on the light sensitive property of the chromate salts in combination with gelatine, or albumen, or gum.

That the bichromate salts possessed light sensitive properties, and could be used for photo-lithography, Mungo Ponton discovered in 1839. He printed on an ordinary paper, which had been rendered sensitive to light in a solution of potassium bichromate, drawings and silhouettes, and obtained after fixing, which was effected by merely washing in pure water, brown images on a white ground.

Later Talbot found that the chromate salts in combination with organic substances under the action of light altered the property of the latter, and particularly that exposed chromated gelatine became insoluble in hot water. Poitevin used this discovery for carbon or pigment printing, and had produced in 1855 various pictures with this process.

Poitevin discovered, further, that exposed chromated gelatine {18} would not swell up in cold water and took up greasy ink, and therefore after exposure under a photographic negative a picture could be obtained exactly inversely corresponding to the tones of the negative; and that these places affected by light could be inked-up and printed from, or could be transferred to a zinc or stone plate, and from this pulls could be prepared. By this discovery photo-lithography and collotype were actually discovered.

Led on by his success, Poitevin used later, instead of gelatine, albumen and gum arabic, and prepared with these photo-lithographic prints, which were transferred to stone.

If a solution of gelatine is mixed with a solution of bichromate of potash, and a sheet of paper coated with this mixture and exposed, or if the paper is first coated with gelatine and then bathed in a solution of bichromate of potash, various chemical changes take place as the result of the action of light on the gelatine film, which we will now consider.

As has already been mentioned, glue or gelatine is insoluble, and only swells up in cold water. In warm water, however, the gelatinous substance dissolves completely, and sets on cooling to a jelly. If a suitable sheet of paper is coated with the warm gelatine solution, either by floating or pouring it over it, and it be allowed to set, and if this film is now made light sensitive in a solution of potassium bichromate and dried in the dark, we shall have a photo-lithographic paper which, according to the greater or less quantity of the chromium salt used, has a more or less intense yellow colour. If the paper thus prepared is exposed to light under a negative, or in another way if some places before exposure are protected by black ink or strips of paper, the places affected by light become dark coloured and brown, and at the same time they have lost their power of swelling up in cold water. The whole surface of the paper, that is to say the exposed and unexposed films before being laid in water, are flat or in one plane; if, however, the print is laid in cold water the exposed parts are not affected by the water, that is to say they have lost the property of swelling and remain in their original plane. The unexposed parts swell up and appear raised up on the print, and thus make the drawing sunken in.

The exposed places have, however, now received the property of taking up and holding greasy ink, whilst the unexposed have taken up water and repel greasy printing ink.

If the paper be exposed under a negative the drawing appears sunken in after development with water; the other parts, which must in printing appear white, are raised up; by exposure under a positive the reverse is the case.

If these prints are in any way, either by rolling up or brushing over, given a coating of greasy ink, the ink only adheres to the exposed places and a print in greasy ink is obtained, which, like any other greasy impression, can be transferred to a stone or a metal {19} plate, which can be printed from direct, or the transfer may be made on to a metal plate for relief etching, that is to say for the preparation of a typographic block.

In Poitevin’s process the stone itself was coated with a light sensitive chromated film, and exposed under a reversed negative. After developing and careful preliminary preparation of the stone the ink only adheres then to the places affected by light.

This is also the case if the stone is coated with light sensitive asphalt, and the same printed on direct. After exposure the parts not affected by light can be washed off with turpentine, benzine, or linseed oil, so that the stone is laid bare; the places, the parts of the drawing affected by light, however, are not dissolved. If the stone is now prepared with the solution of gum as has already been described, and then rolled up with an ink roller, these places will take the ink, and by etching, etc., the stone may be so prepared that the same may be printed from like any drawing or engraving.

By photo-lithography only line or grained drawings can be reproduced, and half-tones, as is possible with collotype and photogravure, cannot be obtained.

Now, with the aid of autotypic transfers and the asphalt process on grained stones, we have a perfectly satisfactory method of reproducing in an excellent manner half-tone drawings by the aid of photo-lithography.

Photo-lithography in all its various branches of application is at the present time so perfected, and rests on so comparatively a simple principle, that a technical printer, with very little practice, experience, and observation of the formulæ given, can attain in very short time absolutely good results. At the same time it must be said that frequently very great difficulties have to be contended with; especially as regards the quality of the materials and negatives great care must be used.