Invented by Sir John Herschel in 1841, this simple process produces a continuous tone image of Prussian Blue using a sensitizing solution of ferric ammonium citrate and potassium ferricyanide.
These iron salts, when exposed to natural or artificial ultraviolet light, are reduced to their ferrous state, producing a high contrast blue image when oxidised. Modifiers such as salt, soap bubbles etc can create a more interesting background. Often, as with the trees, the image is more interesting before they are washed with all the colours from the modifiers such as turmeric and paprika