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There are two forms of Ferric Ammonium Citrate (FAC), brown and green.

Sir John Hershel first discovered and used the brown FAC together with Potassium Ferricyanide (PF) to create his prints. The brown hydrated form contains 16.5-18.5% iron, about 9% ammonia, and 65% citric acid.

Subsequently, Eduard Valenta, an Austrian chemist, modified the composition by reducing the iron and ammonia content while increasing the citric acid concentration in FAC. As a result, the green hydrated variation comprises approximately 14.5-16% iron, around 7.5% ammonia, and 75% citric acid.

Even so, Valenta introduced certain adjustments, it is important to note that both the brown and green chemicals can dissolve in water. Although both variations function similarly, the green FAC exhibits a threefold increase in reactivity to UV light compared to the brown FAC.