Mildew damages cotton's fibres, in addition to leaving a stain. The characteristic grey and black splotches come from exposure to damp. Curtains, items stored in a basement, and laundry left in the washer are all prone to mildew growth. Laundering the items as soon as possible produces the best results. The University of Illinois Stain Solutions website recommends working on the stain as soon as you can--before laundering--and warns that once you wash and dry a mildewed item, the stain may not come out. Try basic mildew stain removal techniques to restore mildewed cotton.
Take the mildewed cotton outside and brush the mildewed area with a brush. Brush briskly in each direction to help remove the mildew from the cotton fibres.
Pour liquid soap directly on the mildewed areas of the cotton, and let the item sit in the sink or bathtub for half an hour before laundering.
Set the washer at the highest temperature possible. Hot water works best for cleaning mildew stains. Add heavy-duty detergent.
Add the recommended amount of bleach for your washer and the size of the wash load, plus 1/4 cup more. Do this only if the mildewed cotton item will tolerate bleach. Add the bleach to the washer's bleach receptacle, or add it as the washer fills so the bleach distributes evenly.
Use the washer's pre-soak cycle, if available; otherwise, stop the wash cycle, put the mildewed cotton items in the washer and leave them to soak for 15 minutes. Start the washer and run it on the longest cycle, such as the setting for heavily soiled garments.
Check the mildewed areas while the items are still wet. If there are still signs of mildew stains, repeat Steps 1 through 4. Do not dry the items until you remove the stain completely.
To wash mildewed cotton without a washing machine, fill a sink or bathtub with very hot water. Use ¼ cup bleach for a small sink, ½ cup bleach for a kitchen sink and at least 1 ½ cups for a bathtub---more if you have multiple or large cotton items to wash. For delicate items, use Castile soap instead of heavy-duty washing powder. Place white mildewed items outside in the morning sun for sun bleaching. Try this with mildewed items that can't be laundered, such as cushions, upholstered cotton furniture, area rugs and fragile items. Brush the mildew off first, if possible, then leave the item the sun until the stain fades. Turn the items to treat all areas of the fabric. If you're sensitive to bleach or prefer not to use it, try an oxygen stain removal product. Wet the cotton item and pour the oxygen cleaner directly on the mildewed area. Leave it on for 20 minutes. Use a brush on the stain to work the cleaner into the cotton fibres. Wash the item according to the directions above, without the bleach--add the oxygen cleaner to the wash water instead.
Keep bleach out of reach of children.