With Indigo, it's a slightly different process.
You make the dyebath through a bunch of science like mixtures, and 'oxidize' the water (which sounded really high-tech to me). You don't need to pre-treat the wool at all, except wet it. The tricky part comes when you actually get down to the dyeing part. You can't leave the wool in too long or it will 'contaminate' the rest of the dyebath. And, get this- although you're dying it blue the dyebath is yellow. YES YELLOW. When you lift the wool out of the dyebath it changes magically before your eyes. MAGIC.
I nearly squealed in delight at my potion brilliance. (Slughorn would be proud.) You can only use the dyebath once because as soon as it gets oxygen in it- it turns blue and you can't reuse it. You can, however, re-dunk wool to create darker shades.
This is a sample of my first Indigo experiments (done in Nov 2012).
Also, because I'm an overachiever (Not really, I just get excited) I tried overdying with some wool that had previously been naturally dyed Yellow.Yellow + Blue = Green!Because it was my first time, I only tried little bits and played around with times. The darker blue I 'dunked' several times. I even tried leaving some bits in the remainer of the dyebath once I was done (for like an hour) but it didn't look much darker.You like? Kimxo