A positive image can be produced by exposing it to a source of ultraviolet light (such as sunlight) as a contact print through the negative (traditionally, semitransparent paper) or objects. The combination of UV light and the citrate reduces the iron(III) to iron(II). This is followed by a complex reaction of the iron(II) complex with ferricyanide. The result is an insoluble, blue dye (ferric ferrocyanide) known as Prussian blue. The extent of color change depends on the amount of UV light, but acceptable results are usually obtained after 10–20 minute exposures on a dark, gloomy day.