If the bleaching powder used is Whiting or Basic White, mix it with the peroxide and make a paste. Spread the paste all over the skull and let it sit for a day or so until it dries. It will start cracking and falling off once it is dry.Rinse the skull in water and repeat the process if you want a whiter skull, if the skull is white enough for you, you can let it dry and seal it with a gloss or matte sealer whichever you prefer, then it is ready to put on a plaque.