To prepare the dye solution 0·5 g. of the dry eoside of silver and 1 g. of ammonium acetate are gently heated together in 30 ccm. of alcohol till dissolved, then 120 ccm. of alcohol and 10 ccm. of glacial acetic acid are added, and the whole filtered. For plates to be exposed wet, 5–10 ccm. of glycerine are added. The emulsion is sensitized by adding one-tenth of its volume of dye, and should be well shaken before coating the plates. The glass plates should receive a preliminary substratum of gelatine or india-rubber, so that the frilling of the film and impurities of the glass plates may be totally avoided. {44}
For small sized plates the emulsion is used somewhat thicker than for larger, and for the latter it should be diluted with alcohol and ether. After coating they should be dried at about 15° C., and then for some minutes exposed to a temperature of 30° C.
The plates should be developed with the following mixture:—25 g. of sodium sulphite should be dissolved in 40 ccm. of warm water, then 10 g. of glycine added and 50 g. of potassium carbonate, at first in small quantities in consequence of the evolution of carbonic acid. When cool about 75 ccm. of a thin pasty liquid will be obtained, which may be kept as a stock solution, and before use must be well shaken and diluted with from 12–15 times the quantity of water. Wet plates should be washed before development, and the developer flowed on; dry plates should not be washed, and should be developed in a dish.
They should be fixed in sodium hyposulphite. The intensification may be effected with metol (see p. [39]), and the blackening as described on p. [38.]
Obviously the collodion emulsion may be used for ordinary work undyed.
The qualities of the cross-lined screen and the distance of the same from the sensitive plate will be described in Chapter V.
5. PREPARATION OF REVERSED NEGATIVES.
For the different photo-lithographic work direct and reversed negatives are used; for the transfer process direct negatives are used; for printing direct on to the stone or plate, reversed negatives.
For making the latter various methods are used, the principal of which are as follows:—