How was alizarin obtained from the madder plant?

Alizarin is a red dye that was historically obtained from the roots of the common madder plant (Rubia tinctorum).12

Within the plant, the dye occurs in combination with the sugars glucose and xylose. To extract the pigment, the roots of the plant were ground up. This ground root material was then used in a complex process—notably the Turkey red process—to dye textiles. Because the dye required a mordant (a metal oxide) to bind to fibers, the final color could be adjusted based on the metal used: aluminum produced red, iron produced violet, and chromium produced a brownish red.12

While natural madder was used for centuries, it was largely replaced in the commercial market after 1871, when synthetic methods for producing alizarin from anthraquinone were introduced. Today, the natural product is primarily used as a lake for artists' colors.12