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Natural wool ranged from white through gray to brown and black, but Celtic weavers wanted color—using plant dyes to create the vibrant hues that made tartan visually striking.
The Dye Sources:
Different plants produced different colors:
- Woad: Blue (from fermented leaves)
- Madder: Red (from roots)
- Weld: Yellow (from the entire plant)
- Lichen: Various colors depending on species and mordant
- Onion skins: Oranges and browns
- Oak bark: Browns and blacks
The gathering and processing of dye plants was specialized knowledge—which plants, which parts, which season, which preparation methods. This knowledge passed through apprenticeship, preserved in oral tradition, guarded as valuable craft secret.
The Mordants:
Most plant dyes required mordants—chemical substances that helped the dye bond to fiber. Common mordants included:
- Alum: Brightened colors, readily available from natural sources
- Iron: Darkened colors, created blacks and deep browns
- Tin: Intensified reds and oranges
- Copper: Created greens when used with yellow dyes
The mordant choice affected the final color significantly—the same dye plant with different mordants produced different hues.
The Process:
Dyeing required multiple steps:
- Mordanting: Soaking yarn in mordant solution
- Dyeing: Simmering yarn in dye bath for hours
- Rinsing: Removing excess dye
- Drying: Hanging dyed yarn to dry
- Setting: Sometimes a final treatment to fix the color permanently
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