This paper describes a classroom experiment aiming to prepare Prussian blue by the reaction of iron(III) chloride and potassium hexacyanoferrate(II) or, alternatively, ammonium iron(II) sulphate and potassium hexacyanoferrate(III) in Petri dishes. The unusual crystal structure of Prussian blue, i.e. iron(III) hexacyanoferrate(II), is discussed along with the short explanation of the origin of its colour, by the charge transfer via cyanide ions. There is also a short history of its discovery (H.

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... Diesbach, 1704), as well as its role in art history (Van der Werff, Watteau, Cézanne, Van Gogh, Hokusai).