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If you get the flu or nausea or vomiting from food poisoning, mushroom poisoning, charcoal is your best natural remedy. Comment is correct about meds though. It will absorb the bad and the good so be careful with people taking meds.
I have never had charcoal stain anything. It whitens your teeth when you brush with it. I use it often and have never had it stain anything. Maybe there is an additive to medical charcoal? That would make it stain?
We did this as kids and we just made a berm with snow and flooded with water. I dont understand why you would buy pvc or wood for a rink. we started with a small amount and added layers until it was thick enough. So much fun!