THE FERRO-PRUSSIATE PROCESS 622. Industrial Papers for Copying Tracings. Ferro-prussiate papers and fabrics are used in considerable quantities for making "blue-prints " from engineers' and architects' tracings. They In all cases where the differentiating reagent gives a coloured precipitate with the ferrous salt only, the reagent can be mixed with the ferric salt before sensitizing the paper, the reaction then taking place partly during the exposure to light and partly in the first wash water (or in a common solvent of the salts used). A positive image is obtained in this way if the print is made under a negative. If the differentiating reagent forms a coloured precipitate with the ferric salt, it should be used separately as a kind of developer in which the print is placed without intermediate washing. In this way a positive picture is obtained when the print is made under a positive.
A large number of other processes have been suggested which make use of the ability of ferrous salts to reduce various metallic salts (copper, mercury, gold, etc.). There are also those processes which are based on the differen tial actions of ferricyanide and ferrocyanide on various metallic salts (uranium, etc.) . This last are manufactured by continuous machinery by means of which the paper is superficially im pregnated with a mixture of various ferric salts and potassium ferricyanide. Such paper is sup plied in rolls of 10 or 20 yd. in various kinds and of very different sensitivities, the most sensitive papers being usually those which can only be kept in good condition for a short time. A good quality, freshly-manufactured paper is of lemon yellow colour on the sensitive side. It gradually turns greenish-yellow, light green, and finally blue. It is then practically useless. It can be kept for an almost unlimited period in a per fectly dry atmosphere (metal cases with double bottoms containing desiccating materials). In a tightly-rolled spool the outer coils and edges deteriorate faster than the central parts of the inner coils.
On exposure to light (chosen as strong as possible), the unprotected parts of the paper turn progressively a deeper and deeper tone of bluish or purplish grey, finally becoming metalized with a silver-grey or bronzed appearance. If
the exposure to light is considerably prolonged the exposed parts become lighter and lighter, while the incompletely protected regions darken. The image then appears reversed.' If this reversal is only partial, washing in water is often sufficient to restore the image to its ordinary condition. On the other hand, various reagents can be applied which will restore the print to a normal condition. The more vigorous prints are usually those which have undergone slight reversal.
Printing (the production of photo-copies) is carried out by exposing to the light the coloured surface of the sensitive paper, which has been placed in contact with the back of the tracing, the exposure being continued until the lines appear of a greenish tint, and the ground is of a pale blue colour. If in doubt as to the appear ance the print should present, a strip of the same paper should be simultaneously exposed to the same light under a tracing of the same colour and transparency on which a few lines have been drawn in indian ink ; from time to time a small piece of this test paper should be torn off and washed. When very large tracings have to be printed, or in an establishment which turns out large numbers of prints, con tinuous printing machines are used. In such machines, an endless belt carries the tracings to be printed, together with a band of ferro prussiate paper, round a glass half-cylinder, which is illuminated from the inside. The paper is washed by jets of water, arranged on the same or on a separate machine, and finally dried between endless canvas belts passing round one or more heated cylinders.
When the washing is carried out sheet by sheet in a dish, it is advisable to do so in three or four washings, taking about io minutes in all. The first wash water, which very quickly becomes turbid with excess of salts, should be thrown away almost immediately. As large sized dishes are usually required, they are generally made of wood or wood lined with zinc, although the zinc is attacked in time by the salts removed from the paper.