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Cyanotype process

The cyanotype process is a non-silver, iron printing process invented by Sir John Herschel in 1841. Paper is sensitized with ferric ammonium citrate and potassium ferricyanide. Exposure time can range from one hour to several hours using an ultraviolet light source. A form of this process is still used today for making building blueprints.

The black-and-white negatives were made using a regular box-style camera
with an exposure of 1/50th of a second.

Printing  of the negatives  takes so much time. The cyanotype process is a "printing out" process. Cyanotypes use no silver to make prints; and, thus, need no developer. Just a water wash after printing. That is why it takes so long. The only chemical reaction is during the actual exposure of the negative to the sensitised paper.

by Michael Moreth