Iron (Ferrous Sulfate)
Historically referred to as "copperas" or "green vitriol," this food-grade powdered iron is used as a mordant for dark colors, or as an after-bath to shift and darken colors. Yellows become greens, beige goes grey or brown, and reds can take on a near purple hue. It is also used with ecoprinting to react with tannins.
Using powdered iron is a great alternative to homemade rust baths when you are working with a larger quantity of fibers (such as yarn), and need to achieve a full color shift.
Consider wearing a mask if measuring, and gloves if using a strong solution.
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