- Preparation of the adhesive coat (between glass and sensitive coat): prepare the aqueous mixture by adding a junior solution of silicate of soda or potassium hydroxide to 36 ° baumés to some gelatin, as well as a very small dose of alum of chromium, according to the following formula :
|
1 / 1
|
1 / 3
|
|
|
Soft gelatin |
15 gr
|
5 gr
|
|
Water |
300 gr
|
100 gr
|
|
Sodium silicate |
20 cc
|
7 cc
|
|
Solution 10 % chromium alum + 90 % of water (*) |
9 Gtes
|
3 Gtes
|
- It is very easy to put inflating beforehand the gelatin in the quantity of cold water which suits during
a few hours. The advantage which ensues from it is a very easy dissolution when we carry the mixture in temperature in the water-bath. Without putting beforehand to soak into the cold 
water, the dissolution of the gelatin which is more laborious there, is possible but disadvised. In both cases, to put the gelatin + water in the water-bath. Shake the warm mixture until complete dissolution of the gelatin. Lower the 
temperature in 120 ° F, then put the silicate of potassium by shaking with a glass stalk. Add the drops of the preparation of alum of chromium while continuing to mix, still hanging a few seconds. By means of a sponge moistened with some warm and drained water, to cover with this preparation (in crossed coats glass put beforehand by level in the steam room, (This first coat must have passed on glass being at room temperature – 64° F minimum, but especially not warm, because the gelatin would dry too quickly) (if the coat is spread over too cold plates, the gelatin of this preparation congeals immediately, and its application is impossible!). Remove the excess, then let dry naturally, what sets little time because of the sharpness(delicacy) of this coat (10 minutes maximum). - At the end of the drying, put the drying chamber to heat for the suite of the operations – thermostat on 105 ° F, unless we prefer to postpone them. Plates so prepared can keep.
- Before putting the sensitive coat, it is advised to rinse in the cold water this first adhesive coat during about ten minutes, then to let it dry at room temperature. It is a question of eliminating any track of alum of chromium and sodium silicate which enters its composition. Or he can occur a transition of these substances towards the photosensitive coat and to cause imperfections during the edition ( abnormal grey zones).
(*) prepare a solution of alum of chromium to saturation with some water: 10 gr of alum + 90 gr of tepid water – the alum of chromium being little soluble in the cold water. (Note that the addition of the alum of chromium is not absolutely compulsory for the preparation of the sublayer). See in fine, the properties of chemicals used there phototypie (the sodium silicate).