Mix 1/4 cup oxygen bleach in 1 quart very hot water. Stir with a wooden spoon until the bleach powder is completely dissolved. Oxygen bleach will not damage most fibers, but don't use it on silk.
Fill a bucket or washtub with warm water. Stir in the bleach solution.
Immerse the lace curtains in the bucket or tub. Soak one to two hours. Stir with a wooden spoon every 30 minutes to ensure that all of the fabric gets wet.
Remove the curtains from the bleach solution, and rinse in cool water until the water runs clear. Rinse out your container or bucket.
Mix 1 cup white vinegar in 1 gallon cool water in your container or bucket. Immerse the curtain in this vinegar and water solution to remove all traces of the oxygen bleach.
Squeeze excess water from the curtains. Lay the curtains on the towel. Roll up the curtains in the towel, squeezing out as much water as you can.
Unroll the curtains and dry them flat on a fresh, clean surface, such as a sheet. Hanging wet curtains could cause them to stretch out of shape.