Stain is from burying, yellowing and greasy feel over time is deep grease weeping out. If you want to degrease & aren't boiling etc... Is to use 50/50 clear ammonia from the cleaning section at the store & distilled water. Warm like with an aquarium heater is even better. Takes a few weeks, refill or replace when real cloudy, and when it stays clear you're done. Disinfects, degreases, and cleans well... Then if you want to whiten, the developers from beauty supply stores works well. 20,30,40.... 40 is the higher strength. They use it to highlight hair. Heat applied with that product increases its effectiveness as well. I'm not sure you'll get the ground stain out but this degrease process works out well after boiling, maceration, or beetles. Then you whiten if desired. If you're okay with paint, that is another route. Typically if there's grease in the skull it will eventually weep through.