SHREDDING AND WASHING.
Now this potassium nitrate, if allowed to remain, would practically destroy the sensitiveness of the emulsion, and must. therefore be removed. Fortunately, it is very soluble. and may be easily washed out. An hour's washing ill distilled water is ample to remove every trace, and even less than this may be considered safe. Of methods of washing, there is practically a choice of two. In the first the emulsion is broken up into a convenient form for dealing with, either scraping with a silver fork or squeezing through canvas or coarse netting. The shreds should measure about b in., and they are then well soaked for several hours according to the practice of some workers. In the other method, the plates are coated while still con taining the potassium nitrate, and are then washed in a tank, in the same man ner as a negative may be dealt with.
In any case, the emulsion should not be used at once, but should be set aside for a clay or two ; as a further ripening goes on, resulting in an increase of speed. it is a good plan, when adding the hulk of the gelatine of the emulsion, to include 1 grain of Thymol dissolved in one dram of alcohol. This is used as a preservative. The question might he asked, why the bulk of the gelatine has not been added in the first instance. In reply it may be stated that gelatine loses its power of setting when kept for a length of time at a high temperature, so that the less beat ing it has the better. Especially will this be the case, if the emulsion is to be used a little at a time, and consequently to be warmed up repeatedly. Sufficient only of the gelatine should be added, therefore, to form the emulsion.