Iron or Ferrous Sulphate tends to darken the dye on textiles or yarns. Ardent dyers make their own Iron however to have some control over the content of iron you expose your fabric or yarn, it is best to use the fine powdered mordant. Please use gloves when using this and make sure you keep the pot where you use iron separate from other mordants.
Iron needs to be controlled when you use protein fibres as wool and silk as it tends to make them brittle. Use sparingly or as directed by your mordant formula.