INK-PRINTING PROCESS. By this process positive prints may be obtained in common writing ink.
The paper is first immersed in a nearly saturated solution of bichromate of potass, and dried in the dark. It is of a bright yellow colour.
It is then exposed to light under the negative in the pressure frame, until all the details of the picture are brought out. The time required is less than that in ordinary sun-printing. The picture is of a pale brown tint upon a yellow ground.
The picture is then washed in water, in order to remove the whole of the undecomposed chromium salt. The water should be changed several times, and the print left to soak in it for two or three hours. This should be done in the dark room.
When the print has been sufficiently washed it may be taken into the light, and the picture, which is now a pale brown upon a white ground, is permanently fixed, the dark material being an oxide of chromium in combination with the lignin of the paper.
It now remains to colour the print black or purple. This is done by causing writing ink to adhere to the dark parts of the picture, which act as a mordant.
Immerse the print for a few minutes in a weak solution of proto sulphate of iron, say 5 grains to the ounce of water. Then remove it, and wash it as before in several changes of water, and let it soak for two or three hours in water. The iron salt adheres to the
image or mordant, but in much less quantity to the bare lignin of the paper.
Next, immerse it in a solution of tannic acid, of moderate strength. The dark parts of the picture are rapidly blackened by the formation of tannate of iron, or writing ink; the lights remain ing unchanged. When fully developed wash the print well in water, and dry it. It is now finished.
The difficulties of the process consist in thoroughly removing the chromium and iron salts from the paper, so as to preserve the purity of the whites ; and also in obtaining blacks of sufficient depth and vigour. The prints are permanent, and uniform in colour, and the detail very good ; the process is also very economical. The paper may either be albumenized or gelatinized by mixing albumen or gelatine with the bichromate of potass. These organic substances are fixed and rendered insoluble by the reduction of the chromium, and are washed out from the parts where light has not acted. The details of the picture are therefore glazed upon a dead ground of white paper. The most vigorous prints are obtained upon unsized paper, but this is very liable to get torn in the operations of washing.