Making yarn glow in the dark is a challenge, even for yarn manufacturers, because most glow-in-the-dark additives are pigments that are painted on. Painted on glow-in-the-dark pigments harden fibres and flake or wash off easily, so these paints are not useful to colour and spin yarn fibres. Luckily, there are some fluorescent dyes and brighteners available that will dye animal fibres in a bright colour that also glows under a black light.
Wind each skein of white wool yarn into a loose skein by holding the end of the yarn in one hand with your arm bent up. Then with the other hand wind the yarn down around the bent elbow and back up around between the thumb and fingers, and keep winding like rolling up a cord or rope.
Tie the coiled skein loops with a few 4- to 5-inch pieces of scrap yarn to keep the skein from unwinding or tangling.
Fill a sink with warm water or about 37.8 degrees C, check with an instant thermometer. Next, add 1 tbsp mild wool wash to the warm water. Place enough yarn skeins to complete your project into the water and soak for 10 to 15 minutes.
Fill the stockpot with water and bring to under a boil at 82.2 degrees C, checking the temperature with an instant thermometer. Put on a dust mask, and add 1 tsp of fluorescent acid dye and 3 tbsp of vinegar to the water for every 100g skein of yarn being dyed.
Drain the sink and rinse the yarn in warm water, squeeze gently to remove excess water, and place into the dye-bath solution in the stockpot. Slowly stir the yarn in the dye bath to dye an even colour.
Slowly bring the dye bath and yarn up to a simmer or 85.1 to 93.3 degrees C, but do not boil the dye bath or the yarn will felt, or become matted. Simmer until the yarn takes up all the colour and the water is clear, or for 30 minutes.
Remove the dyed yarn and place in a clean sink. Allow the yarn to cool until you can handle it, then rinse in warm--not cold--water, about 37.8 degrees C. Squeeze out the excess moisture and hang the dyed skeins to air dry at room temperature.
Dye all the yarn for your project in one batch to keep the colour consistent. Use fluorescent paints on the knitted or crocheted project after it is done for more intense glow-in-the-dark effects.
Gradually heat the wool yarn, and do not use cold water to wash or rinse the yarn, to avoid felting or matting. Use caution with acid dyes especially when measuring the powder. Do not reuse the equipment for dyeing in your kitchen because commercial acid dyes are toxic.