The first time I used alum in eco dyeing, a few months ago, I'd used it as a premordant for cellulose fabrics. Meaning I soaked the fabrics in an alum/water bucket overnight, then dried the fabric, then used it to eco dye with. I wasn't at all happy with the results -- everything turned out a garish acidy yellow. I hadn't used alum since.
But alum is a commonly-used mordant in natural and eco dyeing, so I gave it another try. This time I made my bundles as usual, and steeped them for 2+ hours in a pot of water with 2 Tbsp. of alum predissolved. And I love the results! There was NO rust in this pot. Two bundles were wrapped around copper pipes and two around aluminized pipes. The rest were wrapped around wooden things.
The two top pieces are old linen. The one just above is jacquard-woven raw silk.
These are strips of old linen. The top piece was inside another bundle. The bottom piece just had can lids wrapped inside, no plant material.
These were the bundles on Monday morning after Sunday's steep and rest in the pot.
I have designs on that pot of liquid left over from this batch. I gathered some ginormous oak leaves yesterday when I was in southern Humboldt and I have a plan for them this evening. I'll use that alum brew plus add rust to the pot. Can't wait to see how it turns out!