Blue Print or Cyanotype Papers.—This name is usually given to the process by means of which blue prints of engineers' and architects' plans can be reproduced. It was discovered in 1842 by Sir John Herschel. It is a useful method of reproducing drawings, and incidentally is of great value to the amateur photographer because of the facility with which it can be applied for getting proofs from negatives quickly and easily without special baths and chemicals. The process is based upon the reduction of a ferric salt to the ferrous condition by light, and the formation of Prussian blue by the action of potassium ferricyanide. The negative cyanotype gives white lines on a blue ground. Various formulæ are in common use.

— Herschel. Clark. Watt. Rockwood.
Solution 1.
Potassium ferricyanide16274810
Water100100100100
Ammonia—2·3——
Saturated solution of oxalic acid—20——
Solution 2.
Ammonia-citrate of iron20305030
Water100100100100
Boric acid——0·5—
Dextrin———5

Equal parts of the two prepared solutions are mixed when required and spread evenly over well-sized paper. The paper is hung up, dried, and preserved in a dark dry place.

The positive cyanotype gives blue lines on a white ground, being the reverse of the ordinary blue print. That is, no image is formed where the light acts, and the reaction is the formation of blue due to the union of a ferrous salt with ferrocyanide of potassium.

Pizzighelli in 1881 gave the following formula:—

— Solution 1. Solution 2. Solution 3. Solution 4.
Water100100100100
Gum arabic20———
Ammonia-citrate of iron—50——
Ferric chloride——50—
Potassium ferrocyanide———20

Mix the first three solutions in the following order in the proportions stated:—

Solution 1. 20parts.
Solution 2.8„
Solution 3.5„

As soon as the solution, which at first gets thick and cloudy, is clear and thin, it is spread over the surface of well-sized paper, which is then dried in a warm room.

The print, which appears yellow on a dark yellow ground, is treated with the developer (solution 4) by means of a brush dipped in the solution. When the image is deep blue in colour, the print is washed in water and then placed in dilute hydrochloric acid (1 part of acid to 10 parts of water) till the ground is quite white. A final washing with water is then necessary.