8 Deep-etching Process
The idea of this is chiefly to increase the printing form and to make the stone more resistant in printing. In working this the principal thing is to sufficiently protect the drawing from the action of the strong acid. This is generally done by dusting with resin, which has a low melting point.
There are two deep-etching processes which are chiefly used, one in which the resin is melted with a spirit flame, the other in which the same thing is done by ether vapour.
The procedure in each case is extremely simple. In the so-called burnt-etching process, which has been perfected by Eberle, of Vienna, the already etched stone ready for printing is rolled up with a very stiff printing ink, not transfer ink, and then the drawing dusted with the finest powdered resin. The stone is then gone over with a cotton-wool pad and plenty of talc or French chalk, and then well dusted off with a broad camel’s hair brush in order to remove all traces of resin from the blank stone. To melt the resin a spirit lamp with a side flame is used. The flame is passed over the drawing, and ought only to be allowed to act sufficiently long to make the resin combine with the ink, which can be recognized by the drawing looking shiny. If it has been melted too long the sharpness of the drawing is damaged. After this the etching may be at once proceeded with, and this is effected with a solution of gum to which 8 to 10 per cent. by measure of 44 per cent. nitric acid has been added. For fine drawings 4 to 6 per cent. of acid should be used. In using this a fairly strong effervescence takes place, which should not frighten anyone.
In the ether or cold-melting process exactly the same procedure takes place, only that instead of melting with an open flame this is effected by ether vapour.
For this work a wooden stick of about 2 mm. thick is laid on two edges of the stone. On another ruler, which should be large enough to cover the whole of the stone, and which should be covered with flannel or stout cloth, ether is poured; this ruler is laid on the others, so that it forms a right angle with them, and is slowly drawn over the surface of the stone. The ether vapour falls on to the surface of the stone and dissolves the resin which thus forms the protective film for the deep etching, which is done precisely as described above.
Another melting process recommended by Scamoni consists in pouring alcohol on the stone and then setting fire to it.