Do not use RIT dye! I used that on some prefolds and the suckers are still bleeding three months later. All of dd's diapers that where white now have a faint pink tint to them. I've heard good things about procion (sp?) dyes.
student/sahm to three awesome girls who are always on the go!
If you's like to try tie-dyeing, I'd highly recommend getting one of the Jaquard brand tie-dye kits (they have a small 5-shirt kit, and a larger 15-shirt kit) and following the directions exactly. I got beautiful results with mine, and no bleeding. But I followed the instructions, and didn't cheat
Dharma Trading Co. has both of the kits I mentioned.
The Pburch site given above is a great resource. Dharma also has good info on how to using fiber reactive(aka Procion) dyes.
The key to reducing the amount of dye that gets rinsed out at the end is to let your item "cure" for the maximum amount of time(no less than 4 hours" at room temp or above. A chemical reaction between the dye and the fabric will not occur if it is too cold.
Also, make sure you use the right amount of soda ash. Too little and your reaction won't occur, hence most of the dye will wash out.
Rinsing is also alot easier in the washing machine. Do a cold rinse first, then one or more hot rinses. With PF's more is probably better since they are extra absorbant. The hot water will help to completely exhaust any excess dye. If you have or can get synthaprol, use it. It will help keep dye from redepositing on the fabric while you are rinsing it(helps prevent backstaining if you have an item with more the one color).
From reading directions on the various websites, this all may seem daunting, but it is really fairly easy.
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