I just thought I'd chime in with advice that we followed at Wunderland
last weekend.
First and foremost, Dharma Trading Co. is your friend!
http://www.dharmatrading.com/
Dharma sells their own brand of Procion reactive dyes that come in a
bazillion colors (although you really only need 3, extra colors come in
very handy to avoid the tedious and time-consuming mixing). They also
sell clothing blanks (various garments, white cotton, rayon or silk) for
super cheap -- between $4 and $2 a t-shirt, depending on how many you
buy. The Gildan shirts are very very nice, by the way.
You need a fixer for the Procion dyes, usually soda ash. Conveniently,
Dharma sells this in individual and bulk quantities. Urea helps the
solubility of the dyes or something like that, they suggest getting that
to mix with the dyes. None of our dyes had problems with clumping or
anything like that and we did use urea, so that may help. Echoing the
turquoise sentiment, Dharma actually instructs you to use twice the
amount of dye powder for the turquoise (and some other colors as well)
when mixing your dyes.
Alison says that the RIT dyes are not good for tie dye. That is all I
have heard about that.
One thing -- I think that leaving the garments for the full 24 hours to
let the dyes soak in might help make the colors more uniformly dark --
I've noticed on a few of the shirts we made that some of the colors
didn't seem to stay nearly as dark as they should have. We let the dyes
soak in for about 14-16 hours, but next time I think I'll strive to let
them soak for 24.
Make sure you have some sort of container capable of holding an entire
garment for rinsing. We had a nice big plastic thingamabob for rinsing
copious amounts of dye from fabric.
Hope you have a tie-dye project as successful as ours!
-- Josh