Printing with Ferric Salts
Poitevin's Process with Ferric Chloride and Tartaric Acid
In Poitevin's process another property of a ferric salt is brought to bear, viz., the fact taht it makes gelatine insoluble. A 6 per cent. solution of gelatine in water is prepared, with which is mixed any suitable pigment. Paper is floated on it whilst still warm. The paper now presents a uniformly coloured surface. To sensitise the paper it is immersed in a solution of
Ferric Chloride ... 10 parts
Tartaric acid .... 3 parts
and after drying in the dark it is ready for exposure. When exposed to light, the gelatine, which is now insoluble, becomes soluble in hot water. If, therefore, the paper be exposed beneath a positive (reversed as regards right and left), an image may be developed by simple immersion in hot water. The parts which are insoluble remain next the paper, hence a perfect image may be developed with care. The student should compare this process with the autotype process (p.162), and not the comparative advantages and disadvantages of the two. it seems to the writer that there is a possibility of a great future development of this process.