Why are the leaves of blue-dried works dark?

The leaves of blue sun-dried works are dark because of two chemical reagents: ammonium ferric citrate and potassium ferricyanide.

Blue sun drying is an ancient iron salt printing method, also known as iron cyanate printing. It is the first practical non-silver salt photography process after photography appeared. Sir John Herschel (1792- 187 1) found that many iron compounds can be sensitized in 1842. Later, it was found that two chemical reagents, ammonium ferric citrate and potassium ferricyanide, were prepared in proportion, coated on paper or other media, irradiated by ultraviolet rays in the sun, and the parts that were not exposed to the sun would show different colors, and then washed with clear water, and the fixing was completed.

Blue printing process is simple, non-toxic or low-toxic, efficient and artistic, and images can be developed with water, which is usually an introduction to classical photography. The whole process does not need to be operated in a dark room, and contact printing is adopted. The most interesting thing is that the image gradually appears in the sun, just like the magic of the sun.