Let the poultice remain on the surface for 48 hours or until completely dry. Then remove with a plastic spatula to avoid scratching the surface. If staining remains, make a stronger poultice using whiting and hydrogen peroxide, and add a few drips of household ammonia. Spread over stains, and let dry. If these methods fail, try a water-base paint stripper.
Strippers can discolor the slate, so be sure to test a small area first.
For safety, use plenty of cross ventilation. After the floor has been cleaned and stains removed, apply a paste wax with carnauba or a penetrating tile and brick sealer. Both products can be found at home centers or hardware stores. Penetrating sealers can be reapplied from time to time to give the slate a new look. Just clean the slate, and put on another coat of sealer.