PELLET PROCESS An iron (blue-print) process—the true cyano type, which gives blue lines on a white ground when a copy is made from a line tracing. It is a " positive from positive " process, and unsuit able for use with ordinary negatives because negative prints would then be obtained. The process is also known by various other names, such as " Cyanofer," " Positive Ferrotype " and " Cyanographic," and is largely used for the reproduction of technical drawings. Pellet's own formula has always been kept a trade secret, but Dr. Liesegang gives the following, which answers quite well: Common salt . . 144 grs• 33 g• Tartaric acid. . . 156 „ 36 „ Ferric chloride . . 384 „ 88 „ Gum arabic . . . 2.5 oz. 275 „ Water . . . . fo „ 1,000 ccs.
Dissolve the gum in one half of the water, the other ingredients in the remaining half ; mix, apply to paper in the manner described under the heading " Blue-print Process," and dry quickly. An exposure, under the tracing, of one or two minutes is sufficient in bright sun light. The print is developed by floating it face downwards on a saturated solution of potassium ferricyanide (none must reach the back of the print) ; then wash for a minute or two in water and immerse for about ten minutes in a clearing solution of water I00 oz., hydrochloric acid 8 oz. sulphuric acid 3 oz., finally thoroughly washing and drying.
A modern formula, due to Pizzighelli, is given on the next page A. Pure gum arabic . 264 grs. 6o•5 g.
Water . . . 3 oz. 30o ccs.
B. Ferric ammonio citrate . . 220 grs. 50 g.
Water . I oz. roo ccs.
C. Ferric chloride (crystals) . . 220 grs. 5o g.
Water . . . r oz. loo ccs.
The gum solution does not keep well, but the others do, if stored in the dark. For sensitising
paper take of— Solution A (gum) . 2} oz. 25o ccs.
„ B (citrate) . 1 , loo „ C (chloride) . 5 drms. r ,, Add B to the gum, shake well, add C, and shake again. If mixed in any other way the gum may coagulate. The paper is coated and dried like blue-print paper and exposed under a tracing. Exposure is very brief (a minute to a minute and a half in strong sunlight), the image showing faintly. The print is developed in a solution of r oz. of potassium ferrocyanide (yellow prussiate of potash) in io oz. of water. The lines should develop to a brilliant blue, without any blueness in the ground, which would indicate under-exposure ; broken and feeble lines are due to over-exposure. The print is washed for a few seconds in order to remove most of the developer, and then fixed in an acid bath, sometimes called a bleaching bath, made by mixing i oz. of strong sulphuric or 2 oz. of hydro chloric acid with 20 oz. of water. The prints, face upwards, remain in this bath for five or six minutes, and are then thoroughly washed. A light blue deposit is often seen upon the white parts of the paper, but this washes off, or it may be removed with a very soft brush or cotton wool. Any blue stains (or, in fact, the whole of the image) may be removed with a solution of about 7o grs. of potassium oxalate in oz. of water (4 g. in 25o ccs.), washing well afterwards. (For selection of suitable papers, method of sensitising, etc., see " Blue-print Process.")