A Zeiss anastigmat is used for making autotypes. Duration of exposure from three to five minutes. Coloured objects, oil paintings, etc., are taken in the daylight studio. They are printed in the daytime in the open air, and in bad weather or under pressing circumstances in the night by the electric light.
2. THE GENERAL QUALITIES OF NEGATIVES FOR PHOTO-LITHOGRAPHY.
The negative for photo-lithographic work, for whatever method it may be used, must before all things possess two principal qualities; when looked through it should be as clear and as clean as possible, on the other hand the ground as well covered as possible. The deposit must not be of a black colour; from a well-drawn original absolutely satisfactory negatives can be prepared without the black colour. When a drawing is reduced which contains grey lines, dots, and points, as well as full black ones, toned lines will be visible as well as the transparent; with careful treatment, the negative may be so far corrected that it may be used. If this is not effected, or is not possible, the retouching on the stone will be somewhat troublesome. The toned lines print later than the transparent; these will therefore be overprinted {35} when the former have scarcely reached the correct degree of printing. The choice is then only left either to weaken the lines which are too strong or to draw afterwards the faint or not printed lines.
If the ground is not sufficiently opaque it will print through. This can be remedied by intensifying the negative, or when this is not possible to paint over it as well as possible.
Transparent spots are formed on the negative by an impure silver bath or by dust flying about the room. These must be spotted out if increased work later on on the stone or zinc plate is not desired.
In reproducing fine copper engravings, it generally happens that the fine grey hair-like lines and the light interstices, if the plate was not properly polished, appear less transparent in the negative than the other parts of the picture. In developing the print the result is a partly broken image which can only be retouched with considerable trouble. The negative can be corrected by taking care to paint over the transparent parts. Professor Husnik recommends, when the picture is not too large, to fasten a transparent paper on the back of the negative, and to cover over the glassy places with a soft lead pencil. These parts then print simultaneously with the hair-like line and develop also equally. If, however, the negative is large, and the picture very complicated, a glass positive picture is prepared in the following manner: A polished sheet of glass is levelled and coated with the following solution:—
| Gelatine | 6 | parts by weight. |
| Sugar | 1 | part by weight. |
| Ammonium bichromate | 1 | part by weight. |
| Water | 80 | parts by weight. |
When the solution has dried the glass plate is laid on the negative and exposed till the deepest shadows have printed, and it is then developed in water. When dry this glass positive is laid on the back of the negative so that every part of each picture fits. The plates are then bound round the edges with paper strips so that they cannot shift. By this—although somewhat troublesome—operation can an unsatisfactory negative be rendered more even.
This correction can also be effected in the following manner: The back of the negative is coated with raw collodion which is faintly coloured with aniline red, eosine or fuchsine, allowed to dry, and then with an engraving needle or a narrow knife the less transparent lines may be scraped out, so that the red collodion only remains on the transparent places. Obviously also this can be done in the reverse, by painting a faint red on the transparent places with a brush.
By this last method a tolerably extensive retouching may be {36} effected on unequal negatives, which are the result of not quite equally black or also very close drawn originals, from subjects with very fine lines, such as copper-plate engravings, or from finely-executed lithographs with very great reduction. Finally it should be mentioned that with intelligent retouching any bad negative can be used for photo-lithography, and the only question now is whether it is not better to take the shortest way and prepare a suitable negative when the original really permits of its being done. In printing on chromated gelatine paper a good result can be easier attained from somewhat foggy or thin negatives than by printing with asphalt or other light sensitive substance.