Iron Modifier Calculator
Ferrous sulfate dose for saddening/darkening dyed color, by weight of fiber (WOF) — with a fiber-damage risk warning as the dose climbs.
2 g ferrous sulfate (2% WOF)
At or below the cited source's own 2% WOF sample dose.
Safety: ferrous sulfate is harmful if swallowed — wear a dust mask when weighing the dry powder, keep it away from children and pets, and seek medical attention if ingested. It stains permanently. Dispose of the used bath down a municipal or septic drain — never into a stream, lake, or other waterway.
The cited source uses 2% WOF in its own color samples and states iron should never be used above 4% WOF — excess iron makes fiber brittle over time, especially wool and silk. See the reference chart for sources.
Mordanting before dyeing? Calculate your alum and tannin dose.
Frequently asked questions
- What does iron do in natural dyeing?
- Iron (ferrous sulfate) is used as a modifier after dyeing to "sadden" a color — shifting a bright, clear shade toward a deeper, more muted or greyed tone. It's applied at a much lower dose than a primary mordant like alum.
- Why is there a risk warning?
- Excess iron makes fiber brittle over time — wool and silk are especially prone to feeling rough or weakening after a heavy iron treatment. This calculator defaults to a conservative 2% WOF and flags doses above 4% WOF as caution and above 10% WOF as high-risk, based on craft-practice sources that converge on that range for routine use.
- Is a higher dose ever appropriate?
- Some specialized techniques — particularly certain cellulose eco-printing methods — use iron at higher doses. That trade-off (stronger color shift for more fiber-damage risk) is a deliberate choice for an experienced dyer to make, not this tool's default.