PAPERS 625. Cyanotype Paper. Cyanotype paper (H. Pellet, 1878) yields copies consisting of blue lines on a white ground when printed under a tracing. This paper, which is impregnated with a mixture of ferric salts and gum arabic, has a sensitivity comparable with that of the fastest ferro-prussiate papers, and, consequently, can only be handled in a very weak light. As the image is only slightly visible before development (light yellow lines on a white ground), the print ing is timed by means of trial strips exposed under a tracing of the same opacity on which a few lines have been drawn in indian ink, these test strips being developed one by one as printing proceeds.
The print is developed without intermediate rinsing as soon as it has been removed from the printing frame, the paper being floated on a ro per cent solution of potassium ferrocyanide (yellow prussiate of potash). The time of development should not exceed about 30 seconds, otherwise the lines tend to spread. If placed in the developer, a sheet of unexposed paper would become uniformly blue, whilst a sheet which had been uniformly exposed to the sun for a, few moments would remain white. Under exposure is therefore shown by colouration of the ground, and over-exposure by a lack of depth of the lines. Care should be taken to avoid. wetting the back of the paper during develop ment ; the four sides of the sheet of paper are turned up so as to form a kind of dish, which is floated on the bath, the hands being gently passed over the back of the paper to ensure uniform wetting at the under surface.
After a short wash, the prints are fixed in a 4 per cent solution (by volume) of sulphuric acid 2 or a ro per cent solution of hydrochloric acid. This bath dissolves the gum which has not been rendered insoluble, and at the same time washes away the white ferrous ferrocyanide formed in the lines of the image, and which would gradually turn blue on oxidation. Wooden dishes lined with gutta-percha are generally used for this acid bath. The final washing should be done with a strong jet of water, or, if the latter is not available, the print should be brushed under water to dislodge particles of gum arabic not removed in the acid bath.
Cyanotype paper is useless for printing from full-tone negatives.
626. Papers. paper (A. Poitevin, 1861) gives copies in purplish-black lines on a light ground when printed under a tracing. Its preparation differs very little from that of cyan.otype paper, but the coating is slightly less sensitive. Printing is timed with the aid of test strips, developed as printing proceeds.
Development is carried out, without inter mediate rinsing, as soon as the paper is taken from the printing frame, by floating it on a bath made up, for example, as follows— Ordinary alum . 130 gr. (15 grm.) Gallic acid . . 90 gr. (10 grm.) Water, to make . 20 0.Z. (1,000 grin.) The gallic acid may be replaced by tannin. A very small quantity of oxalic acid may he added to obtain purer whites. Development should last about three minutes. Under-exposure causes the lines of the image to spread, the ground becoming deeply coloured ; with over exposure the lines are broken and faint. The ground is always tinted a light violet colour. An exhausted bath gives only a very weak image. The developed print is washed in several changes of water ; a very dilute bath of hydrochloric acid used between two washings will often lighten the ground of an under-exposed print.
The copies should first be dried as much as possible by pressure between blotting paper, otherwise the lines tend to spread while drying.
Papers which are developed by treatment with gallie acid, are gradually being abandoned in favour of the so-called " water-bath " papers, which only require washing in plain water. With such papers, the gallic acid necessary for development is applied, with polishing brushes, as a very fine powder on the surface of the sensi tive layer as soon as the latter has been dried. The first washing must then be carried out in a fairly small quantity of water so as to avoid excessive dilution of the very small quantity of gallic acid adhering to the paper.
Ferro-gallic papers are not suitable for print ing from full-tone negatives.