Photo-Lithography

by Georg Fritz

Everyone only slightly conversant with colour printing knows that the first plates are used for the general tones of the picture, and contain, therefore, few details, and with the exception of the highest lights are tolerably full; the latter plates, on the other hand, which are devoted to the darker colours, contain less toned surfaces, but, on the contrary, more details of the drawings. In order to attain this correctly the three stages of the production of the plates are carried out with this end in view.

It should still be mentioned that the more or less fine graining of the stone determines the general ratio of the grain of the picture.

The first stage is the exposure of grained stone coated with asphalt. With longer or shorter exposure it is possible to make the asphalt film more or less soluble, that is to say, to make the same more or less capable of resisting ethereal oils.

It is therefore possible at this stage, by long exposure of the asphalt film, to give to the colour stone for bright colours more tone and less details. The stones for the darker inks are printed for a shorter time, and the asphalt film is less capable of withstanding the solvents, and the resulting plate contains less tones and more details. Finally, the plates for the deep colours are normally printed; the asphalt film is still less resistant, and consequently the stone will only have tone and details on those places which in printing ought to be the darkest.

It is possible, therefore, by correct manipulation of this operation, to obtain approximately the correct colour plate.

There is, however, in the second process a corrector for the first. The development may be so performed that by the longer or shorter use of strong solvents, Hungarian or Russian turpentine, more or less of the asphalt will be dissolved. Thus if a slight error is made in printing it can be made good in developing. As a rule, the stones for bright colours, which consist of many tones, are not developed so long as those for the stronger colours. The actual stones for the deep colours are very strongly developed, so that almost all the delicate parts of the picture disappear, and only the dense strong parts remain on the stone.

Still the fine details which are to be found in the vigorous parts, and which cannot be recognized, must be brought out. The fine parts of the drawing must be quite clear if the details are to be recognized, or the picture will be a fuzzy copy of the original, wanting in structure.

This fault is remedied in the third stage of the treatment, in which a strong development of the picture is effected with strong acting oils or benzole and turpentine. As this operation, however, would also wash away such delicate places which ought to remain on the colour stone, they must be covered with solution of gum and allowed to dry. Then those parts not covered with gum, {77} in which the details of the image cannot be seen, are painted for a longer or shorter time, as may be required, with a brush dipped in the developing solution, and when correctly developed may be covered up. The lighter places are treated for a shorter time, the very dark places for a longer time.

By this treatment the particular parts of the picture become lighter and lighter, the details appear more and more; and, finally, the whole picture shows as it should do. Then the gum is washed off and the stone subjected to the already described treatment, so as to make it suitable for printing in quantities. Finally, the lithographer can make corrections with a needle or tusch.

This process may be used, therefore, to prepare colour-printing plates from a negative obtained in the camera from an oil painting or any coloured original.