Remove as much residual flesh and skin as you can including the eyes if there any left. Clear out the nasal cavity as best you can mashing the thin boney structures with a blunt screw driver to aid their removal. Suspend the skull in a pan of boiling water submerged just to the level of the coronets. Add a smallish scoop of biological washing powder and soda crystals.
Boil for around half an hour remove and have a careful scrape. Poke the brain about with a stick and mash it up.
Back in the boiling pan for another ten mins then another scrape and poke.
Use the jet of a hose to remove last bits and leave to dry.
A few warnings :
This process smells. Outside is best.
Don't scrape too hard as you will leave noticeable scratch marks in the bone.
Don't boil for too long otherwise you will destroy the connections holding the various skull plates together and it will fall to pieces.
Gauge your boil time by what you seeing in terms of flesh removal and most importantly movement of the plates of bone. The first to fall off tends to be the 2 spiky bits which would form the top of the nasal cavity.
Once dry you can bleach the skull (steer clear of the antler and coronets) with Domestos (or specialist bleaching gels). Paint domestos all over the skull and inside any cavities. Cake the skull in kitchen towel and soak this in more bleach. Leave it overnight then wash off.
Have fun
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