John Herschel invented the cyanotype process in 1842, rather
early in the history of photography. While both albumen and
silver gelatin prints rely on the light sensitivity of silver,
cyanotypes are produced by light sensitive iron
A piece of paper is first sensitized with a solution of ferric
ammonium citrate (an iron salt) and potassium ferricyanide (a
crystalline iron salt) and dried. The prepared paper is then
contact printed, or placed in direct
contact with the negative, and exposed to sunlight until an
image begins to appear on the paper (usually about fifteen minutes).
As contact prints, they are always the same size as their negatives.
In the final step, the print is washed in water to oxidize the
iron salts and draw out the cyanotypes brilliant blue
The cyanotype is named for its rich blue-green hue, cyan. Cyanotype
prints have no emulsion; the light-sensitive iron salts have
been infused into the paper fibers, unlike either albumen or
gelatin silver prints. Cyanotypes were far simpler and les expensive
to produce, which made them a favorite method for turn-of-the-century
amateurs who wanted to make proofs of their negatives. The architectural
blueprint is a variation of this photographic process.
1842 to present (although most cyanotypes in the A. D. White
Architectural Photographs Collection were produced between 1880