| Syrian asphalt | 20 | g. |
| Chloroform | 300 | g. |
| Benzole | 100 | g. |
| Oil of lavender | 20 | drops. |
After it has been dried in the dark the impression from the black plate, which should be previously made on transfer paper, is transferred to the asphalt film as previously laid down, and before it is dry dusted with bronze powder. All lines, titles, or figures which should not appear on that particular colour-plate must be then covered with the above asphalt solution, and only those places, for example, on the one plate all titles, on another all the roads, on a third the river courses, and so on, should be left, all of which later should appear in printing. {88}
The stone is now, according to the sensitiveness of the film and the strength of the light, exposed for from fifteen minutes to two hours. Over-printing, if it is not very exorbitant, does not do any harm with the stronger letters or lines on account of the good covering. With very fine lines, however, more care must be taken. If there is any doubt as to the length of the exposure a test may be made at one corner of the plate with a pad dipped in rectified turpentine. When the asphalt will no longer dissolve the exposure may be discontinued. With some experience the photometer may also be used.
Then the whole surface should be washed with rectified turpentine and a pad of cotton wool, when the ink lying under the bronze, that is, the lines where the asphalt has been protected from the action of light dissolve, and the surface of the stone appears. The rest of the asphalt film has become insoluble from the action of light, and now forms the etching-ground for the reversed negative image on the surface of the stone.
When the stone has been well washed with water it may be etched, which is done with glacial acetic acid two parts, water 100. When there are any fine lines in the drawing the etching is interrupted at the end of two minutes, and they should then be covered with tusch, and then when the tusch is dry the other parts are further etched for a longer or shorter time according to the degree of fineness. In any case the time of etching should not exceed five minutes. The stone is allowed to dry and then coated with a solution of tusch or linseed oil, which penetrates into the etched parts and makes them capable of taking ink. After about ten minutes the whole stone should be washed with turpentine or benzole, by which the etching ground is removed, when the stone is inked up like an ordinary gravure.
In this way is obtained a stone with deep etched drawing, which according to the blocking contains the titles, the rivers or roadways, and from which subsequently the necessary gravures can be taken. This stone can either be used for direct printing or it may be used as an original stone from which the necessary transfer can be made.
The same process may also be made for intaglio printing plates on zinc or copper, only then the etching must be done with nitric acid or ferric chloride.
Instead of the transfer on the light sensitive film, a drawing on transparent paper may be used, or a photographic transparency. The exposure in this case, as the opaque ink and bronze powder are not used, must be very carefully estimated.
The advantage of this process is that an intaglio printing surface may be obtained so that engraving is entirely or partially avoided, and that this printing surface may be used as an original from which the reproductions are made.