The Dictionary Of Photography

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

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The selection of a gelatine suitable for emulsion making is a most important point, but the following leading characteristics of a suitable kind may be of service to the amateur plate manu-facturer : - The weight of the ash left after incineration should never exceed 2 per cent., proving the absence of earthy or mineral matter. The amount of water it will absorb should in no case exceed ten times nor be less than five times its weight. For emulsion making by the boiling process it should give an acid reaction, and its solubility should not be affected at a lower temperature than 70o F., nor higher than noo F. Another most important test is its expansive power, for upon this depends to a great extent its frilling or non-frilling properties. The following table of the chief characteristics ol the best com-mercial brands will be of practical use ; but it may be stated that the best film can, as a rule, be obtained by a mixture of one part of hard and two parts of soft gelatine : -
Purification of Gelatine. When crystals of sodium sulphate are stirred into a warm solution of gelatine, a point is reached at which the gelatine begins to separate as a fibrous clot, and by