The Dictionary Of Photography

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

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bear in this second melting. However careful has been one's method of preparation and washing, one is never certain that the emulsion is free from mechanical impurities, such as little bits of fibre, hair, etc., and it is therefore advisable to filter the emulsion before use. For this purpose, flannel, felt, or wash-leather may be used, and whilst the flannel is the quickest to allow the emulsion to pass, leather is the most effectual. Personally, I always use a felt filtering bag, such as may be obtained from any chemist to order, about 2s. for a pint size, which will be too large for most amateur workers, as the bag absorbs too much emulsion, but the bag may easily be cut down to half its size. If wash-leather be used, it is necessary to wash it well in weak soda solution first, about I in 20, to free it from the natural grease, and then wash thoroughly to free from the soda. When the emulsion is perfectly fluid, it should be poured into the bag, presuming the felt bag be used, or into the wash-leather or flannel stretched on a filter. When the felt bag is used, it is only necessary to suspend it by its ring from a retort stand, or any convenient makeshift, and thus less emulsion is lost, and it does not get cold quite so quickly. Gentle pressure on the top of the bag soon forces the emulsion through. With wash-leather it is almost necessary to have some pumping arrangement, and this may be either the ordinary indiarubber balls as used for spray diffusers, or the more powerful brass force pump used for filling pneumatic cycle tyres or footballs. When leather or flannel is used, the most convenient filtering apparatus I have found to be a small glass percolator, a utensil known to every chemist, and this fitted with a cork to the upper part or body allows one to affix the forcing arrangement. It is advisable before using the emulsion for coating, to add some bromide, and a little chrome alum to the emulsion, the latter especially when the process adopted has been the ammonia boiling process, as this addition prevents fog and frilling. A 1 per cent, solution of bromide of ammonium, and a 2 per cent, solution of chrome alum should be made, and 1 oz. of each should be added to every pint of emul-sion. The addition of chrome alum lowers the sensitiveness slightly ; at least, so it is said, and certainly I think it does, but there are no ill effects if the chrome alum be rendered neutral by the cautious drop by drop addition of solution of ammonia. The addition of the chrome-alum solution should be made to the