Photo-Lithography

by Georg Fritz

We have, however, in autotypy still the physical law, which is very useful to us, and that is the greater or less distance of the point from the light surface. The nearer a point or a figure is brought to the wall on which it should throw the shadow, the greater the size of the original, and so much sharper will be the outlines of the shadows. The more the point or the figure is {73} removed from the wall the less the shadow will correspond to the shape of the original; it will be smaller and less defined. The shape will also frequently change, and a square will become a round. We have then in the near or distant position of the lineature to the sensitive plate, on which it throws its shadow, an important assistance to act on the general tone gradations of the image. Let us remember that in focussing the shadow-forming object on to the surface on which the light falls, we obtain the same formed in all its actuality on the receiving surface; therefore, when this refers to a lineature with equal proportions of black and white, we obtain these in equal ratio. When, however, we remove the lineature from the light-receiving surface, the shadows which it throws contract, the lights predominate, we obtain no longer the ratio of 1 : 1 between light and shadow, but about 1·5 : 1 and so on.

By placing the lineature close to the sensitive plate, we obtain more closed tones, and as this gradation of tone is carried naturally throughout the whole of the image, also less high lights, less deep, middle shadows, whilst the deep shadows appear in their natural strength. With the removal of the lineature from the sensitive plate, on the other hand, the ratio of tones for the whole image, as well as for the details, is displaced. In the first place we shall obtain a picture which contains more contrasts of black and white. The light tones become considerably lighter by the reduction of the shadows. The dark parts, on the other hand, attain full vigour. In the first place, therefore, the original has to be considered as regards the distance of the lineature from the sensitive plate. Obviously, therefore, no accurate directions can be given in a case where feeling and experience are required. Next to the original, the number of the lines on the screen has to be considered. As a rule, one uses lineatures with five or six transparent and opaque lines to the millimetre (= 125–150 to the inch), and for these a distance of ½ to 1 mm. (= 1⁠/⁠50–1⁠/⁠25th inch) from the sensitive plate is generally sufficient for most cases. With a less number of lines the distance can be increased.

Autotypy is a process in which unprintable half-tones can be broken up by the use of a well-defined physical law into a mathematically determined grain, and the size of the grain can also be regulated to a certain extent with certain precautions.

It is, therefore, easily understood that the autotypic process is now very general and most used, and not only in the limited department of photo-lithography, but in a more extended way, does excellent service for the preparation of illustrations of all kinds, for the simplest picture for the daily paper as for the finely-printed magazines and journals, for simple monochromatic printing, and for the highest colour printing.

With accurate knowledge and command of the technique of {74} photography and lithography, there is required before everything, for successful work, a faultless screen plate. The indispensable requirements which must be found in the screen are absolutely opaque black lines and bare glass in the white lines. The home preparation of such a screen plate is a somewhat difficult matter, and will not be absolutely successful if one has not a faultless ruling machine, a composition which is thoroughly opaque, which adheres well to the glass and gives clean, sharp lines in ruling, and which does not chip, and it will require a tremendous expenditure of patience and perseverance. Glass screens of excellent quality may now be obtained commercially.

For reproduction of the very highest quality, a drawn and etched screen plate is more suitable than one merely ruled on pigment, which never gives such sharp results.

From various firms etched screen plates may now be obtained either as single or crossed-line screens, which fulfil all requirements.

The best screen to use is one with five or six opaque and transparent lines to the millimetre, which corresponds with 2,500 to 3,600 points to the square centimetre. The above number of lines will suffice for nearly all work, and such screens give reproductions in which the grain is no longer visible to the naked eye, but appears as a closed tone. It is not advisable to go beyond this number of lines, except in special cases, when the details of the drawing in the original are specially small. With a smaller number of about three or four lines to the millimetre, too coarse a grain results, of which the individual points become too plainly visible, and these are distracting and act roughly, and do not accurately reproduce the gradation of tone.

The best proportion between opacity and transparency is 1 : 1. From this is obtained, according to my opinion, the most beautiful and restful tones. It must also be remembered that the opaque spots actually lose considerably in strength in the brighter portions of the picture by the spreading action of the light.

The printing may be done on bichromated gelatine paper, with which, however, extremely accurate inking up of the print is essential, and the prints must not have too much ink and must be very carefully printed. If, however, very good, clear negatives are used, either of the direct printing methods, as already described in Chapter IV., is to be preferred. (See No. 4 Supplement.)