FERRIC SALTS, PRINTING WITH The light-sensitiveness of the iron (ferric) salts is the basis of a large number of printing processes, including chrysotype, cyanotype, kallitype, the sepia printing process, amphitype, the ink process, and platinotype. In all these the ferric salt is reduced by light to the ferrous state. The following table (due to Eder) shows the comparative light sensitiveness of the various iron salts :— Ferric chloride and oxalic acid . too Ferric oxalate . . . . 89 Ammonium ferric oxalate . . 8o Potassium ferric oxalate . . 78 Ferric tartrate . . . . 8o Ammonium ferric tartrate. . 8o Ammonium ferric citrate . .
Ferric chloride and citric acid . 19 Ferric chloride and tartaric acid 25 Many of the inorganic ferric salts are com paratively stable to light, but in contact with organic matter are readily reduced, as in the case of ferric chloride with oxalic or citric acid. According to Abney the spectral sensitiveness of the iron salts is chiefly in the indigo blue, about G IP, and extends to in the green and well into the ultra-violet.
Printing with salts of iron is known as the iron-printing or heliographic process. The four
principal processes, each of which is described under its own heading, are : the blue print pro cess (ferro-prussiate), white lines on a blue ground ; Pellet, blue lines on a white ground ; ferro-gallic, black lines on a white ground ; and brown line (better known as, and described elsewhere in this work under the heading of, " Kallitype "), white lines on a brown ground.
An interesting process of printing with a ferric salt is Shawcross's Amphitype (which see), in which advantage is taken of the fact that these salts have the property of attracting or repelling greasy inks. This is again shown in the black line " True-to-scale," or Ordoverax, process, where an undeveloped blue-print laid on a gelatinous surface will so affect the latter as to enable the lines to take ink while the other parts repel it.
The table on p. 239 (due to Eder) gives a very clear precis of the principal iron printing pro cesses and the developers necessary to produce full vigour of the images, which as a rule are only faint. (See also separate headings.)