Plastic

aluminum foil is bad but i believe TIN foil is a litte safer but i would steer clear of any foil.

aluminum foil is not safe to smoke out of.. neither is tin.. or copper.. hehe.. gold is tho:) just get a stainless steel bowl if ur gonna smoke out of metal. otherwise use glass.. or maybe pick up a wood bowl from ur local head shop. heat treated i beleive so they dont burn =] and even if they did it wouldnt do much cuz they'd burn up slowly