After standing for about twenty-four hours the ether contains all soluble insensitive constituents of the asphalt; the pitch-like residue in the flask, after pouring off the solution, is again treated with ether, so that all turpentine is extracted and any stray insensitive particles are dissolved. The pitchy residue is now removed from the flask and allowed to stand in a porcelain dish in a warm place, with frequent stirring, till it is quite freed from ether, and has formed a hard, brittle, black shining substance, which can be easily broken up into powder with the hand. This product is now the light-sensitive asphalt, which is dissolved for use in anhydrous benzole. As benzole is only to be obtained anhydrous with difficulty, some chloroform is added to the solution in order to prevent the running together in drying, whence the film would become unequal and patchy.
The coating of the stone or plate with the asphalt film is very simple; it should be placed horizontal, and then whirled rapidly on a whirler.
Printing under a clear negative takes from a quarter to half an hour in direct sunlight.
The picture is developed with turpentine.
(b.) VALENTA’S PROCESS.
Valenta recommends for photo-lithography asphalt which has been sulphurized by the wet process, as it possesses a considerably higher light-sensitiveness than that which is not sulphurized.[3]
[3] Eder’s “Jahrbuch der Photographie,” 1892, p. 241.
The process is as follows:—100 g. of raw Syrian asphalt are boiled in a retort with an equal quantity of raw pseudo-cumene, which has the formula C6H3(CH3)3, and a boiling-point of about 170° C., with 12 g. of flowers of sulphur, which should have been previously dissolved in the pseudo-cumene. When after about three or four hours’ boiling the evolution of sulphuretted hydrogen has ceased, the pseudo-cumene is distilled off and the black pitchy residue dissolved in benzole in the proportion of 4 : 100, and used for the preparation of the plate or stone.
The sulphurized asphalt prepared in this way is almost insoluble in ether, but dissolves fairly readily in benzole, toluene, xylene, cumene, and turpentine, and is very sensitive to light.
With this asphalt good prints are obtained even in bad, cloudy weather. For use four parts of the sulphurized asphalt are dissolved in 100 parts of benzole, the solution filtered and diluted till a thin film on a zinc plate shows a golden yellow colour. Exposure of the asphalt solution in an open flask for from half to one hour in sunlight is advisable.