iron blue

noun

1

: steel gray

2

: any of various strongly colored pigments that range in masstone from reddish blue to jet black and possess good hiding power, that are now usually made by bringing together solutions of ferrous sulfate, ammonium sulfate, and sodium ferrocyanide and oxidizing the white precipitate formed, that consist of complex compounds regarded as both ferrocyanides and ferricyanides containing positive ions (as ammonium and sodium ions), and that are used chiefly in blueprints, inks, laundry blues, paints and enamels, crayons, and linoleum: such as

a

: prussian blue

b

: turnbull's blue

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