The Dictionary Of Photography

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

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sion be soft, even though the water be ice cold, the water will be more milky, and the emulsion take up too much. Too much excess of acid bromide, too high a temperature at the time of adding the gelatine, or keeping at too high a temperature between adding and washing, will produce the same result. Having cooled the beaker of set emulsion down to 400 F., run a bone spatula or paper-knife round, and turn out the emulsion, or cut it out in lumps. If cold, it will come out almost quite clean from the glass. Place it on a piece of coarse " straining cloth " or canvas, and squeeze through the meshes into the water, the operation being performed under the surface of the water. Leave it so for an hour. Lay the straining cloth over the mouth of another pan or large jar, and pour the mixture of emulsion threads and liquid on to it so as to let the latter run through. Squeeze the emulsion a second time through the cloth into clean cold water, and immediately repeat the operation for a third time, leaving the emulsion in the last water for half an hour. When strained for the last time, place cloth and all in a large beaker, and put the latter into hot water until the emulsion is completely melted and warmed to about 1150 F. - i.e., not warmer than is pleasant to the hand. With a clean hand take out the cloth and squeeze it; very little will be lost. The emulsion should now measure about 16 or 17 ozs. Add 2 ozs. of alcohol, and mix thoroughly. The alcohol may be either pure ethylic alcohol, sp. gr. about -830, or good colourless methylated spirit not containing petroleum. (See Alcohol, Methylated.) If the emulsion now measures less than 20 ozs., make it up to that by adding clean water. The emulsion is now ready for use. It should be filtered into the coating-cup through washed muslin to free from bubbles, and plates coated in the usual way, dried and used as usual for rapid gelatine plates, using about an ounce of emulsion for a dozen quarter-plates.
Cold Emulsification. - This process is not very reliable, be cause the degree of sensitiveness depends solely, or to a great extent, upon the temperature of the air. Henderson's original process is as follows : - Allow 2 to 3 parts of gelatine to swell in 75 parts of distilled water, and then dissolve at a temperature of 500 C. (= 1120 F.), and add 3 parts of pure carbonate of am-monia, then add 22 parts of bromide of ammonium and 3 parts of 10 per cent, solution of iodide of potassium. Finally add 200
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