For developing the asphalt image rectified oil of turpentine free {62} from acid is used; the best is French or Austrian. The development may be accelerated by adding Hungarian or Russian turpentine, which, however, attack the asphalt image; it can be restrained by the addition of ligroin, benzine, or wood oil to the turpentine.
Zinc plates are best developed in a dish, stones by making a wax margin round the drawing. The oil of turpentine is poured on and rocked to and fro. Rubbing with a pad of cotton wool, etc., should be avoided. After development the plate should be well washed under a stream of water, and before gumming, the plates should be exposed for some time to light, as this makes the ground more resistant to the etching.
(B.) COATING THE STONES.
The stone or zinc plate is now coated with one of the above-described asphalt solutions, or with the following:—
| Asphalt | 20 | g. |
| Chloroform | 300 | g. |
| Benzole | 100 | g. |
| Oil of Lavender | 20 | drops. |
The solution must be thin, so that it flows over the stone well and does not form unequally-covered patches.
The subsequent manipulations must be conducted in the dark.
The stone which is to be used must be well ground, and polished with dry pumice stone, well dusted, and be absolutely flat. It ought not to have any depressions, as the negative would not lie in contact there. The places would not print sharp, and therefore a bad result would be obtained.
The necessary quantity of the solution is now poured into the middle of a well-polished stone, and distributed as evenly as possible by rocking backwards and forwards. When thoroughly distributed the stone should be placed on the whirler and kept for some time in motion. The asphalt solution must be as thin as possible, so that it appears yellowish-brown and the colour of the stone appears through it. If the stone is only covered all over the film can be very thin; on the other hand, if it is too thick it will not print through, and will develop badly, as too much of the asphalt will dissolve. If zinc plates are used for printing they should be prepared as previously described on p. [6.] Zinc plates for deep etching must be well ground and well polished, and ought not to repel water. The zinc plates are coated the same as the stone.