Modal, a type of rayon, is a man-made fiber made from beech wood chips. This fabric has a silky feel and is often blended with cotton, wool or polyester. This type of fabric absorbs pigment in the same way cotton does, making it an easy fiber to dye. Remember, however, that polyester stitching is colorfast, meaning that any attempts at dyeing will result in the modal being dyed but leaving the thread its original color.
Wash the garment with a detergent made to prepare your garment to take the dye. These are often called PFD (prepare for dyeing) detergents. Any waxes, oils or even fingerprints present on the garment will prevent it from absorbing the dye fully, resulting in a patchy finish. Therefore, the washing step is crucial.
Select your reactive dye. Reactive dyes are best for all cellulose-based (plant-based) fibers, including modal and cotton.
Mix your dye in a small bowl according to the manufacturer's instructions. Pour it into the bucket along with the required amount of lukewarm water.
Add the non-iodized salt, if required by the dye manufacturer. Some brands may also suggest that you add a teaspoon of Calsolene oil to aid in evening out the dye distribution.
Place the modal garment into the water and allow it to soak for the recommended amount of time, stirring regularly.
Remove the garment and rinse it until the water runs clear. This will take a while, but failing to do so will result in the garment dyeing your skin if you get wet while wearing it.
Wash your garment once more to rid it of any excess dye. Ensure that the washing machine provides a cool rinse, as this will work to close the fibers and lock in the dye.