Chemical
Alum
Alum → Madder red
Picture to come.
A white crystalline mineral salt used chiefly as a mordant, fixing dyes such as madder into wool so the colour would not wash out; it was also used in tawing, which turned skins into soft white leather. England had no alum industry in this period: it was an imported, relatively costly commodity from the Mediterranean world, with sources in Spain, Italy and Asia Minor among those named for the medieval trade.
Caveats (confidence: medium)
Which Mediterranean sources supplied England specifically in the 12th century is uncertain; the major Phocaean (Asia Minor) and later Italian works were exploited chiefly after 1200. Tawing use is general medieval practice rather than specifically dated here.
Sources: jennydean.co.uk · en.wikipedia.org
Used to make
- Dye-bath of madder red