Here's how you can increase your chances at success with Rit:
1. Start with distilled water:
Since local tap water varies widely from place to place, and you need a neutral to mildly alkaline solution, distilled is the easiest way to start from zero--actually, 7.0 or 'neutral' pH. It also eliminates the possibility of mineral content which can hinder the dying process.
2. Slightly increase the alkalinity of your solution:
This can be as simple as adding a small amount of sodium bicarbonate (orange box, in the kitchen, white powder, not blow
unless you're buying really cheap blow
). About a teaspoon should do the trick for the average dye 'vat'. Adding more is not necessary and if your solution is too alkaline, the water can't 'carry' the dye. There are other household items that would make your water more alkaline (bleach, sodium hydroxide), but these can, and will, release toxic fumes into the air when heated--which brings us to the last part...
3. Stay near boiling point:
While 'dying' a disc, use a thermometer to stay above 200F(90C), or just keep your pot at a slight simmer. Since you will have evaporation loss at constant temp, make a little scratch at the starting fluid level, of whatever concoction you've decided to go with, so that you can add distilled water as the level goes down. Constant temp will find any weak spots on your masked edge, so be sure to thoroughly burnish down the edges of your design.
Following these simple guidelines will ensure that, whatever dye job you're up to, your dye will be at it's best. Good luck, and, keep dippin'!