Wool and its finer compatriot cashmere require more care when laundering compared to their cotton and synthetic cousins. Neither can be bleached or whitened in the same manner as fabrics like cotton, so other steps must be taken. Getting your wool and cashmere sweaters back to their pristine white condition takes a little more effort and materials beyond laundry detergent.
Create a mixture of 1 teaspoon of white vinegar per every 2 cups of cool to tepid water. Ensure you have enough in the sink or basin to fully cover the sweater while following this ratio.
Lay the sweater flat in the water and vinegar mixture and swish gently. Do not under any circumstances twist, ball or squeeze the sweater when wet. Doing so will stretch and damage the delicate scale structure in the fibers. Soak the sweater for about 10 minutes.
Swish the sweater in cool, clean water, draining the sink and refilling with clean water until the vinegar is rinsed out.
Lay out the clean white towels. Lay the sweater flat on the towels, separating the arms an inch or two from the body to ensure proper drying.
Repeat the process as needed until the sweater is the desired level of whiteness.
You can also blot particular sections of the sweater -- gently, to avoid fiber stretching -- with a sea sponge or clean white cloth incorporating the vinegar and water combination.
Never place wool or cashmere in the dryer. High heat from the dryer will irreparably shrink the fiber.
Bleach destroys wool fibers. Avoid its use.