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War_Hymn t1_jbsag6y wrote

Yep, they mixed the crushed snails with urine to make the dye. The urine (or the ammonia in it) acted as a mordant to fix or bind the dye to the fabric, so all that expensive purple dye won't get leached out when you washed it.

Needless to say, you probably won't want to live near a dye works back in those days.