When the film is absolutely dry, which will be in about fifteen minutes, printing may be proceeded with. For this purpose the negative is either laid on to the asphalted stone, film side down, and pressed down with screw bands, or it is laid, the glass downwards, in a specially-constructed and very strongly-built printing frame, which is provided with a sheet of plate glass, and the {63} asphalted side of the stone laid on this, the printing frame closed, and then exposed to light. The exposure continues according to the strength of the light and the nature of the negative from 15 minutes to 1½ hours. If the film of asphalt be thin and the negative clear it may be printed to 18 or 20 degrees in Vogel’s photometer. This kind of printing always requires some experience, as various factors, such as the thickness of the asphalt film, the density of the negative, also the fineness of the drawing, have to be taken into account. It will be understood that the asphalt film must always be coated as evenly as possible.
If it is printed too long the stone will not develop at all, or only in parts; if it is printed too little the drawing wholly or partially dissolves. In both cases an unsatisfactory result will be obtained. Negatives very clear in the lines of the drawing, and well covered in the ground, are here more essential for success than with the chrome gelatine process.
(D.) DEVELOPING AND INKING UP.
When properly printed the frame and the stone should be brought into the dark room and development proceeded with. For ordinary Syrian asphalt rectified turpentine can be used, and one of two methods may be adopted. The stone is laid in a pan as straight as possible, and then flowed over with turpentine, which should be allowed to remain on it for some time, and then poured off into the pan. Fresh turpentine is now applied till the drawing appears clear and distinct. The turpentine which is allowed to run off can be used for washing purposes. An edging of warm gutta-percha may also be made round the drawing, and then turpentine poured on to it and the stone rocked up and down till the drawing is developed.
The very sensitive kinds of asphalt of Husnik and Valenta are much more readily soluble than the ordinary Syrian asphalt. In developing one must go carefully to work, and not allow the turpentine to stay on the stone a long time, but to pour it over the stone, which should be inclined so that it can run off. This can be repeated if required. It can be well and safely developed if half and half of turpentine and linseed oil is used, which, however, has the disadvantage that the stone becomes greasy, but this can be taken off by the expert printer.
After developing the stone should be well washed under a strong stream of water, then allowed to dry in the light, as by this the asphalt film becomes more capable of standing the etching, and then be coated with neutral or very slightly acidulated gum. After gumming, the stone is again dried and allowed to stand for some hours to rest, after which it can be inked up and etched, and finally sharp etched.
The zinc plate is, after development, also well washed, then allowed to dry, treated with thin gum solution, then with an {64} etching solution of tincture of galls or gallic acid and phosphoric acid, and inked up.
Prints on zinc plates for deep etching are gummed after developing, and if the asphalt film is perfect without rubbing up with a pad, which thickens the drawing, etched for some minutes in very dilute nitric acid.
Many lithographers are accustomed, and indeed it is necessary with some transfers, to rub the prints up with greasy ink. The process is as follows:—Greasy transfer ink is diluted with some turpentine, and after the stone has been gummed and allowed to dry, then washed, the drawing is wiped over with a soft pad with this dilute ink, when the ink adheres to the drawing, strengthens it, and if great care is not taken, thickens it also. With asphalt prints this is absolutely useless, and for other direct prints, as well also as for good chromated gelatine prints, in most cases unnecessary. The exposed asphalt combines so intimately with the stone that strengthening with a greasy ink is quite unnecessary, is indeed absolutely purposeless, as the ink can neither penetrate the hard asphalt film nor combine with it. The film of asphalt alone is so resistant to every etching solution that it is for this reason absolutely useless to strengthen it.