Dyeing polyester and rayon is much more difficult and complicated than dyeing cotton or wool. Because of the chemical make-up of polyester and rayon, the dyeing process must be performed under very high temperatures. Even under high temperatures, an assortment of chemicals are required to help the fabric to accept the dye evenly. Be very careful with the chemicals, and always make sure the room is well ventilated.
Hand wash the fabric in a pot on the stove. Fill a large stainless steel or enamel pot with enough water to cover your fabric and 1/2 teaspoon of dye activator or soda ash and 1/2 teaspoon of professional textile detergent. Heat the water until it reaches 60 degrees C. Put your thermometer carefully into the water, and read the temperature given. Once the water has reached 60 degrees C, swirl the fabric around in the pot with a wooden or stainless steel spoon. This step removes dirt, oil and sizing. Remove the fabric from the stove and thoroughly rinse it with clear water in the sink.
Dissolve the desired amount of dye powder in 1 cup of boiling water in a nonreactive bowl. You can decide how much dye powder to use by determining how dark you want the fabric to be when you're done. For a very pale colour, use 1/2 teaspoon of dye powder to 1 cup of boiling water. For a dark colour, use 3 teaspoons of dye powder to 1 cup of boiling water. For black, use 6 teaspoons per cup of water. Allow the mixture of boiling water and dye powder to cool until it reaches room temperature. Strain the mixture through two layers of old nylon stockings to remove any lumps or impurities.
Mix 1 cup of boiling water with 2 tablespoons of dye carrier in another bowl.
Heat 2 1/2 gallons of water in a nonreactive stainless steel or enamel pot until it is 48.9 degrees C. Add 1/2 teaspoon of professional textile detergent and 1 teaspoon of citric acid or 11 teaspoons of white distilled vinegar. Add the diluted dye carrier from Step 3. Add 1/2 teaspoon metaphos, which is a cholinesterase inhibitor for softening water. Only add metaphos if you have hard water. Finally, add the dissolved and strained dye powder. Stir the mixture well after the addition of each ingredient. Use a cooking or candy thermometer to measure the temperature of the water before you add your fabric. When the water is 48.9 degrees C and everything is properly mixed, add the damp fabric.
Bring the dye bath to a boil, and stir the fabric constantly. Allow the mixture and the fabric to simmer for 30 minutes. The deeper you wan the colour to be, the longer you should simmer it. For black, simmer for 45 minutes. While the dye bath is simmering, bring another pot of water to at least 82.2 degrees C. Again, use your cooking or candy thermometer to verify the water's temperature.
Take the fabric out of the dye pot and immediately plunge it into the pot of hot water (82.2 degrees C measured by your thermometer). If the water is not at least 82.2 degrees C, the dye carrier will leave residual crystals and an unpleasant odour in the fabric.
Pour the dye bath down your sink drain and refill the pot with hot water (71.1 degrees C). Use your thermometer to measure the water's temperature. Add 1/2 teaspoon of professional textile detergent to the hot water. Transfer the fabric from the first rinse pot to the new rinse pot. Stir the fabric in the second rinse pot for 5 to 10 minutes.
Rinse the fabric in hot water and wring out the excess water. If you detect a strong odour in the fabric, repeat Steps 6 and 7 to try to get rid of the odour of the dye carrier. When you're satisfied with the fabric's smell, dry the fabric by hanging it on a clothesline or drying it in a clothes dryer.
The measurements used here are for one dry pound of fabric. Increase the amounts of dye activator and professional textile detergent proportionally if you are dyeing more than a pound of fabric. Wear latex gloves when handling the dye chemicals. Wear an apron to keep your clothes clean. Look for dyeing chemicals at craft stores, or order them from online stores. Practice dyeing a garment you can easily discard before attempting to dye your prom dress or other special garment.
Do not reuse any utensils or tools used in the dyeing process for food preparation. Don't allow children in the kitchen while dyeing polyester or rayon fabric. The dyes and chemicals are caustic, and the boiling water is dangerous. Make sure that the room is very well ventilated during the entire dyeing process.