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.