Using kitchen foil as a core for polymer clay beads is an excellent idea. The beads are lighter as it uses less clay so they are great if you want to chunk up your beads, or balance the weight so your jewellery sits correctly. For example with a bracelet you want the focal piece to be lighter so that it sits on the top/front of the wrist and the clasp to be heavier so that it naturally slips underneath/behind, and for necklaces you want the opposite.
The first batch of foil core beads I made, I put holes into some and others I decided to wait and decide which orientation I wanted to use after they had been cured. The beads which had been cured with holes were fine, but the ones without had two problems:
Firstly although I had compressed the foil as much as possible, small air pockets remained under the clay which resulted in large air blisters that distorted the beads as they baked (I think due to the air expanding as it heated). Presumably for the beads which had been pierced the path of least resistance for the air was out the hole (don’t know how to say this without innuendo!).
The second problem was that after drilling through the clay veneer I could not drill the foil inside without making horrible sharp splinters (probably not healthy for the drill bit either). As the bead was unusable anyway, I decided to use force and shove through the spike I use for piercing beads. The resulting pressure from within caused the polymer clay veneer to crack.
Still, not all is lost, I intend to shave off the bubbles and cover the beads again. One of the things I love about polymer clay is that it is hardly ever wasted.
I have a tutorial on making a foil core bead.