INK PRINTING Poitevin's pigment process depends upon. three facts : ferric chloride renders gelatine insoluble, ferrous chloride has not that effect, and light converts the ferric into the ferrous chloride. If paper is coated with gelatine and ferric chloride, printed under a negative, and placed in warm water, the shadow portions (those acted upon by light) are made soluble. Thus a negative is made from a negative and a positive from a positive. i oz. of gelatine is soaked in 15 oz. of water, dissolved by heat, and zoo grs. of Indian ink mixed in very thoroughly. Paper is floated on the still warm mixture for three minutes, dried quickly, and immersed in the following sensitive mixture until limp : Iron perchloride . . z oz. z zo g.
Tartaric acid. . . 14.o grs. 32 „ Water . . . . To oz. 1,000 ccs.
The paper is dried in the dark, printed under a positive, and developed and fixed simply by washing gently in warm water.
Another method invented by Poitevin is to float plain paper for three minutes on a sensitive mixture prepared as follows : Iron perchloride . . 144 grs. 33 g.
Citric acid . . . 96 „ 22 „ Water . . . . io oz. i,000 ccs.
The paper is dried quickly in the dark, printed under a positive and developed by immersing in a weak solution of Indian ink in glycerine. The ink will affect only those parts that have not been acted upon by light. The developed print is finally briefly washed and dried.
Most of the photo-lithographic and collotype processes may be described as ink-printing. The sensitised bichromate image is rendered susceptible to printing ink by exposure to light. The Amphitype process, and the Ordoverax processes (which see), also show that the iron salts are capable of being made to take up fatty inks.