The following is a practical formula for pre paring a sensitive solution of asphaltum for coating zinc plates for etching : Dissolve f5o grs. of finest powdered Syrian asphaltum in 2 oz. of chloroform and 3 oz. of anhydrous benzole. Add 3o grs. of Venice turpentine and ro drops of I'll of lemon or oil of lavender. The film should be of a transparent golden yellow tint. The coating should be done by means of a whirler (which see). The exposure is best made under stripped films, which may be treated with a bath of glycerine to make them adhere when squeegeed down to the plates. It is not necessary to use a printing frame. The exposure may vary from half an hour in the sun to two or three hours in the shade. In bad weather the exposure may extend to days. Development is usually done with turpentine, which dissolves the soluble parts of the image. The scum is rinsed away, and the greasiness removed from the surface with soap and water or a weak solution of soda. Prof. Valenta has published a process for augmenting the sensitiveness of bitumen by incorporating sulphur with it. By this process, zoo g. (3f oz. r r grs.) of raw Syrian asphalt are boiled in a retort with an equal quantity of raw pseudo-cumene—which has the formula and a boiling point of about 170° C. and 12 g. (r86 grs.) 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 dis tilled off and the black pitchy residue dissolved in benzole in the proportion of 4 to zoo, which solution is used for the sensitive coating of the plate or stone. The sulphurised asphalt pre
pared 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. Recently Prof. Valenta has simplified his method by dissolving the asphaltum in carbon disulphide and adding sulphur chloride.
A number of processes mostly used for photo lithography take advantage of the fact that when bitumen is dissolved in both alcohol and ether in suitable proportions it will split up into a grain on drying. The film can then be printed under an ordinary continuous tone negative and yield a picture in half-tones. The Frey process is a successful method worked on this basis.
Bitumen processes are not much employed nowadays, owing to the slowness and uncertainty of the exposures. Bitumen powder is, however, largely used for dusting on plates to strengthen the resist for etching, and the solution is used as a resist varnish for the backs and margins of plates, and for stopping out.
Alberini's reversed bitumen process consists in removing the exposed bitumen from the metal plate instead of the unexposed bitumen as in the ordinary process. This makes it possible to expose the bitumenised zinc plate under an engraving, print, or inked drawing on thin paper, dispensing with the use of a negative. The preparation of the plate and exposure follow on the lines of the ordinary process, but develop ment is effected by immersing the plate in a dish of 4o per cent. alcohol. The principle of the process is that the alcohol dissolves the part which has been acted on by light, and not the unexposed part, which is usually developed with turpentine.