It needs to stay wet for the dye to soak into the fibers. You can wait longer if you want, but the color won't improve after that.
DYE IS PERMANENT AND WILL STAIN FURNITURE, CLOTHES, WALLS, ETC.
Open the bag in a sink or outdoors, and handle it carefully!
While the lack of soda ash will make the colors lighter, some amount of dye will stay on cloth no matter what. Stains in a sink or counters may be removed with an abrasive cleanser (Eg. Comet, Ajax), but stains on fabric may never come out.
RINSE BEFORE YOU UNWRAP YOUR SHIRT
Before you remove the rubber bands, go ahead and run the shirt under a faucet or hose until the water runs mostly clear (it's okay to rinse out on a lawn, the dye is natural and won't hurt the plants). You may want to wear gloves during this process, as otherwise your hands are likely to get stained.
Next, remove the rubber bands and unwrap, continuing to rinse until the water runs clear (this may take a while).
AFTER RINSING, MACHINE WASH THE SHIRT TWICE ALONE
Before putting the shirt in with other laundry, wash it alone (or with other tie dyes being rinsed) in warm water only, no detergent. Then, wash it once more with detergent and warm water. After these it should be safe to wash them with regular clothes, but it's best to stick to similar colors the first few times just in case.
If you are washing multiple tie-dyes together, it's still best to keep dark colors away from light colors like yellow, white, or orange. If you have multiple colors on the same shirt, or you'd like to wash multiple tie-dyes together regardless of colors, it can help to get a Shout! Color Catcher or Woolite Dye Magnet. These will soak up most of the excess dye and reduce the color bleed.
The soda ash is a fairly strong alkali, so theoretically it should preserve the shirt for a long while if for some reason you can't wash it right away. Still, it's best to rinse it sooner rather than later.
Dye on the skin must wear off over several days for the most part. If you have access to a pool, going swimming with the chlorine takes it off fairly well. An industrial hand-cleaner such as "GoJo" will take most of it off, but the cuticles and under your nails won't be affected. In a pinch you can put Ajax or Comet cleanser on a nail brush or old toothbrush and apply it directly to your nails (dries out the skin but it works). Still, if you're getting married tomorrow and you absolutely must have the stains off of your nails your best bet is to use a nail brush with straight, undiluted bleach (not pleasant).
Well first off it's important to note that you shouldn't buy Rit brand dye. Rit dye is easy to find at a craft store, and is great for solid-color dying but requires high-heat and isn't great for tie-dye.
I order all my stuff from Dharma trading company . You can get a kit from them with everything you need for any size group (anywhere from 6 to 100 shirts). The dye is the right kind (fiber-reactive) which can be done with squeeze-bottles.
Short answer? The dye will only react with natural fibers. Cotton is by far the most common, though technically if you wanted to you could dye silk, rayon, hemp, or bamboo, though they each take the dye slightly differently and some fibers like silk needs special treatment since normal concentrations of soda ash can damage it.
Long answer for those interested in the chemistry (adapted from Dharma's website):
Fiber reactive dye attaches permanently to cellulose fibers using a covalent (electron-sharing) bond. These molecules carry a "chromophore" which absorbs varying wavelengths of light, allowing only certain wavelengths to reflect (the colors you see). Covalent bonding is one of the strongest, most fundamental types of chemical reactions. This reaction happens gradually over time at speeds which depend on external factors such as temperature and pH of the surrounding environment. The soda ash pre-soak raises the pH level to about 10.5. Raising the pH is what allows the dye to work at room temperature, since it allows the reaction to produce many more excess hydrogen ions without having to stop due to being over-saturated. After some time around 24 hours, the solution surrounding the garment will become fully saturated and the reaction will stop working. At this time any dye which was unable to bond to a cellulose fiber is washed away with warm water.
Anyway, that explains what happens when you dye a shirt. And since the dye attaches to the cellulose, if the fiber has no cellulose then there's simply nothing for the dye to bond with. Likewise a 50% cotton 50% polyester will only take the dye half as well, since only half the fibers are receptive to the dye.
Unfortunately this section of the website is currently unfinished. In the mean-time, you can check here to see some examples and techniques.
If you'd like to send any photos you have of finished shirts, go ahead and email them to Sharon.Damerell@gmail.com .