What medieval craft produced ink and dyes, often using plants and minerals?

The story behind the answer

This craft specialized in extracting and fixing color from natural sources. Practitioners steeped and fermented plants and minerals, then used mordants like alum or iron salts to bind color to materials—and similar know-how produced writing inks. A staple formula was iron gall ink: tannin-rich oak galls mixed with iron sulfate and gum arabic, yielding a deep, lasting black.

In medieval towns, color-makers formed guilds and closely guarded recipes. Workhouses often sat at the edge of cities because vats could smell powerfully during fermentation. Common sources included woad for blue, madder for red, and weld for yellow; minerals like ochres and verdigris added earthy tones. Colors carried status, too—sumptuary laws sometimes restricted vivid hues to elite wardrobes.