The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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purposes by three different methods : (i) Solution of the asphalt in chloroform and precipitation with from three to five times the quantity of ether. (2) Kayser's method of washing powdered raw asphalt with ether, and using the dried marc for the prepara-tion of the light-sensitive varnish. (3) Husnik's method of dissolving raw asphalt in the smallest quantity possible of German turpentine and precipitating with several times the quantity of ether. Kayser proved that the light-sensitiveness of asphalt increased with the proportion of sulphur, and Valenta based the following processes of increasing the sensitiveness on this statement. To prepare the light-sensitive preparation 7 to 10 parts of sulphur should be dissolved in a sufficient quantity of bisulphide of carbon, and added to 100 parts of powdered Syrian asphalt. The solution is then freed from the bisulphide of carbon by evaporation, and best heated for about an hour in a mortar to 1 ioo C, with constant attrition with the pestle; then in a roomy air-bath slowly heated till sulphuretted hydrogen escapes, and be kept at a temperature of about 1800 to 2000 C. for 5 to 6 hours. The formation of burnt products, which prove a decomposition of the asphalt, are caused by too high a temperature, and is to be avoided. The asphalt thus prepared, which now only smells faintly of sulphuretted hydrogen, should be kept in a well-closed bottle in the dark. For working in diffused daylight, and especially if high sensitiveness is desired, it is advisable to free this asphalt from any existent resin, and from traces of burnt products, which may be effected by powder-ing and treating the powder with ether, and with agitation in a wide-mouthed bottle fitted with a cork. After sufficient action, 2 to 3 hours, the ether is poured off and the insoluble portion dried by spreading it out in thin layers on several thicknesses of blotting paper. For use, lour parts of sulphurised asphalt, treated as above, are to be dissolved in 100 parts of benzol (not the so-called benzoline), the solution filtered, and finally diluted till the film which is formed by pouring it on the zinc plate appears a golden-yellow colour. (An exposure of from half-an-hour to one hour of the solution of asphalt in an open flask to direct sunlight is advisable.) To develop the asphalt pictures, rectified oil of turpentine free from acid, as for example French or Austrian turpentine, should be used. As accelerator of development, when there has been considerable over-exposure, an addition of so-called