The cyanotype relies on the reaction of ferric or iron salts to light, where they are reduced to the ferrous state. The ferrous salt reacts with the potassium ferricyanide to from insoluble ferric ferrocyanide, also known as Prussian Blue. Separate solutions of ferric ammonium citrate and potassium ferricyanide are made. Just prior to exposure equal parts of the solutions are combined and the paper is coated. Once dry the paper is exposed to UV light. The image is developed and fixed by rinsing the paper in warm running water for about 10 minutes. The image will gradually darken as the cyanotype completely oxidizes. The print can be oxidized by placing in a solution of 3% hydrogen peroxide. Other colors are possible by toning with a number of solutions.