Remove the lace curtains from the curtain rods carefully. Examine the tag for cleaning instructions. If you have vintage curtains with no tag, do not wash them yourself. Take them to a dry cleaner. Likewise, don't try washing curtains that say dry clean only. Most lace curtains, however, can be washed in nondetergent soap.
Fill your bathtub up with cold water and 1 cup of nondetergent soap. Nondetergent soap, which does not have water-attracting properties, is often sold in camping stores to clean sleeping bags.
Submerge the lace curtains in the bathtub. Let them sit in the bathtub overnight.
Drain the water in the bathtub, and rinse the lace curtains with cold water. Much of the soot should be gone.
Launder your curtains in the washing machine on the delicate cycle, using laundry soap. If the curtains are still stained, add 6 tablespoons of trisodium phosphate to the washing machine.
Hang the lace curtains out to dry in the sun.