CYANOTYPE PROCESSES. These are certain processes, discovered by Sir John Herschel, in which cyanogen is employed.
1st Process. A piece of paper is first washed with a solution of ferrid-cyanide of potassium, (red prussiate of potass,) and dried. In this state it is of a yellowish colour and highly sensitive to light. By sufficient exposure, either in the camera, or pressure frame under a negative, the salt becomes converted into prussian blue in those parts where light has acted. To fix the picture, the paper must first be soaked in water, then in a weak solution of carbonate of soda, and dried.
Before washing, the shadows of the photograph are of a lavender tint on a yellow ground, but after washing, of a deep blue tint on a white ground. Prussian blue is not a permanent substance ; perhaps immersing the papers in tannic acid might blacken the colour and increase the permanence of the precipitate.
The action of light in this process is probably to convert the ferrid-cyanide of potassium into the ferro-cyanide of potassium (yellow prussiate of potass) and form prussian blue.
2114 Process. A sheet of paper is first washed with ammonio citrate of iron, and dried. It is then of a yellowish colour, and sensi tive to light. After sufficient exposure in the camera, or pressure frame, i. e., until an image is faintly perceptible, the paper is brushed over with a solution of ferro-cyanide of potassium (yellow prussiate of potass). This brings out the picture of a deep blue tint, which • .-omes almost black if the paper is allowed to get dry.
The photograph, in this state, is not permanent. It darkens all over in the dark by exposure to air. If washed with ammonia, or
carbonate of ammonia, it is for a few moments obliterated, but soon reappears with reverged lights and shades. In this state it is fixed, and after being thoroughly washed in water, the shadows are pure Prussian blue, which deepens by keeping. By this process negatives may be multiplied; but it is so uncertain in its results, as to be of little or no practical value, at present.
The rationale of the process appears to be that the ammonio-citrate of iron, which is a persalt of iron, is reduced by light to a salt of the protoxide, This reacts on the ferro-cyanic salt, and produces the blue precipitate.
3rd Proven, Brush over a sheet of paper a mixed solution of ammonio•citrate of iron and ferrid-cyanide of potassium, contain ing about equal proportions of these salts. Dry it in the dark. It is sensitive to light. After exposure, immerse it in water and dry it, when a blue picture will be produced. If this be immersed in a weak solution of proto-nitrate of mercury, it is quickly bleached and destroyed. The mercury salt being then thoroughly washed out of the paper, the picture may be reproduced by ironing it with a very hot iron. The colour is then brown, which fades gradually in the dark, but may be restored at any time by 'ironing the paper as before.
These cyanotype processes may be greatly varied by substituting for the ferro and ferrid-cyanide of potassium the corresponding cyanogen compounds of many other metals. A wide field is open for experiments in this direction.