How was common madder used to make a red dye?

Common madder (Rubia tinctorum), a member of the Gentianales order, was historically cultivated for the red dye known as alizarin, which was extracted from the plant's ground-up roots.123

To apply this dye to textiles like cotton, wool, or silk, the fibers required prior treatment with a metal oxide, known as a mordant. The final color produced depended on the specific metal used:
* Aluminum: Yielded a red shade.1
* Iron: Produced a violet shade.1
* Chromium: Resulted in a brownish red.1

The dye was also famously utilized in the complex "Turkey red" process. While synthetic alizarin largely replaced the natural product after its commercial introduction in 1871, the natural root extract is still occasionally used today as a lake for artists' colors.12