aka Ferrous sulfate
Iron can be used to darken (sadden) colors after dyeing.
Recommended procedure for post-mordant:
Thoroughly dissolve 0.5-2% WOF ferrous sulfate in warm tap water. Add to cool water bath, then add material with stirring. Allow to soak ten minutes, then evaluate color change and repeat if desired. Works with myrobalan(to grey or olive), black walnut paste(improved formula), osage orange extract (to olive), madder paste extract(to purple or rust tones).