Cyanotypes
One of the oldest photographic printing processes, having been invented in 1842 by Sir John Herschel. The essence of Herschel’s discovery was that paper impregnated with certain iron salts was light sensitive : a Prussian blue image being formed when exposed to light.
The general process of cyanotype is the photochemical reaction of Ferric ammonium citrate and potassium ferricyanide. When mixed together in dim light, applied to paper (or fabric) then exposed to UV light, for example sunshine, a negative process occurs and areas where light is not reached the paper will remain white. So.. any object be either natural material or a stencil will remain white against the blue background.
