A good developing ink must have the following properties: When spread upon the print must give a clean, sharp, sufficiently dense impression, which can be easily transferred to the plate or stone; on the other hand, the drawing ought not to spread. The transfer should possess such resistance that, without rolling up, it should resist a slight etching.
5.—The behaviour of asphalt on stone or zinc.
The following are principal conclusions to be drawn from my experiments:—
(a.) The exposed asphalt, whether this exposure is made by printing or whether a drawing is first made and then exposed to the light, adheres more firmly to the stone or zinc plate than if it was not exposed. An asphalt drawing prepared in the dark and inked up does not adhere to the stone, and this may be explained in that the grease cannot act through the asphalt film. Insensitive asphalt behaves in the same way. With correct exposure under a clear negative of about 20 degrees Vogel, and subsequent development and auxiliary exposure for about half to one hour, the drawing adheres firmly to the stone. If the image is to be washed off, the turpentine must act for five to ten minutes on the film, till the exposed asphalt has dissolved. If, after washing off, it is inked up, all the places take the ink well, and the image appears clear and plain. The image can only be destroyed by strong acids or alkalies, which actually dissolve the stone itself.
From this it is clear that the prints neither require inking up nor strengthening in any other way, and my experiments have proved that in all cases the prints are, by this treatment, clogged up, and neither gain as regards a longer run nor in withstanding etching.
(b.) Solution of asphalt, to which oil has not been added, behaves as described above; if, however, oil is added to it, it acts still better as regards the keeping of the drawing, and the prints need no subsequent exposure. Too large an addition of oil has the disadvantage that, in developing with turpentine, the asphalt dissolves too quickly. The images wash off and one hardly has time to examine. The exposed asphalt, to which oil has been added, also dissolves very readily. For the preparation of colour plates this point is of some importance.
6.
Of the acids the most important is nitric acid, the purpose of which is to convert the surface of the stone into a nitrate, so that the grease cannot penetrate. This acid acts even when diluted very energetically on the stone, and produces a fine grain.
Hydrochloric acid exerts a less energetic action; it leaves the stone smoother and does not attack it so evenly as nitric acid, but has the same result finally.
Sulphuric acid ought not to be used for etching stone. It forms calcium sulphate on the surface of the stone, and this separates from the stone in printing and spoils the pulls. {10}