Iron, also known as ferrous sulfate, acts as both a modifier and mordant. Iron vats use ferrous sulfate as a reducing agent to remove oxygen from the indigo vat. An iron vat is built with 1 part indigo, 2 parts iron, and 3 parts calcium hydroxide.
Ferrous sulfate can be used to darken or sadden shades for deep purple, varied grays, olive greens, and rust tones. Iron modifiers extend the range of colors produced and improve dye fastness. As a mordant, iron improves light and wash fastness.
We recommend using iron as a post-mordant with (1) Myrobalan Powder to produce grey, (2) Black Walnut Paste for an improved formula, (3) Osage orange to extract olive, and (4) Madder Paste to achieve purple and rust tones.