Photo-Lithography

by Georg Fritz

may be used and allowed to act for half-a-minute. The stone is then well washed under a strong stream of water, allowed to dry, and covered with linseed oil. After about five minutes the whole of the stone may be washed with turpentine. Then it should be inked up with a pad, care being taken that all lines take the ink well and evenly. This pattern stone serves as the original plate for the subsequent ruled transfer.

From the original ruled stone can be made, according to the nature of the original, a single, double, triple, or four-fold transfer.

For preparing a stone with a ruled transfer, a smoothly ground stone well polished with oxalic acid is evenly coated with sensitive asphalt solution. On this, when dry, the desired single or crossed transfer from the original stone is made, and dusted with bronze powder. This adheres to the lines of the pattern, and in combination with the printing ink protects the asphalt film underneath from the action of light. During the subsequent exposure, which, according to the sensitiveness of the asphalt and intensity of the light, can last from a half to two hours or more, the particles of asphalt not covered by the bronze powder become insoluble, whilst the protected parts preserve their solubility.

After correct exposure the surface of the stone is carefully washed with a tuft of cotton wool soaked in oil of turpentine, when the particles of asphalt underlying the bronzed lines dissolve. The stone at these parts is laid quite bare, and therefore made capable of being subsequently etched. The deep etching of the pattern is effected in the same way as was suggested in the preparation of the mother pattern stone.

By this method a positive pattern is obtained, that is to say, the lines print, and the impression shows therefore a black network.

A second method of making a pattern transfer consists in making a pull from the original stone on to chalk transfer paper. {81} This pull is then damped on the back, and when just properly damp is laid on the polished stone (without any asphalt) and the transfer made. In order to remove the chalk paper without damaging the pattern the stone is covered with warm water, when the lines will dissolve. The simple pattern will now be found on the stone, which should now be well washed, then dusted with resin powder, which should be melted on with ether vapour. Then if the direction of the first lines was vertical, the second transfer is arranged with the direction of the lines horizontal to the first, and there is thus obtained a simple crossed pattern. With four-fold crossed patterns the subsequent pulls are made in opposite diagonal directions. After each transfer the stone is well washed, allowed to dry, then dusted with resin powder, and with ether melted on to the pattern.

The stone is now deep etched with dilute nitric or acetic acid and covered with linseed oil.

By this method a grain instead of lines is obtained. By polishing the stone and transferring on to it, as well as by etching the places not covered by the ink, we have the pattern reversed, that is to say, it is converted into grain.

For further operations of litho-heliogravure a very clear and soft negative is required, from which a glass transparency is taken. Under this transparency pigment paper sensitized on a 20 per cent. solution of potassium bichromate is printed to 8 or 10 degrees Vogel. Printing must be very carefully performed, as with under-printing too many details are lost. The exposed pigmented paper is washed in cold water, laid film side down on the previously prepared stone, and pressed into contact with an india-rubber squeegee. It should then be allowed to dry in a not too dry or too warm place.

The stone should now be placed in a trough with warm water of about 40 to 45° C., and care should be taken to obtain water of as even a temperature as possible. After about five minutes the paper will be free from the stone, which should remain in the bath and be developed so long by shaking in order that the water may flow evenly over it till the image appears quite distinct; a negative image is thus obtained on the stone. The film is then allowed to dry spontaneously for four or five hours till the pigment image has become quite hard.