I stopped by a hardware store today where I picked up an aluminum plate. I used one of my credit cards to match the thickness. So, assuming these cards are not dramatically thicker than an average credit card (which they do not look like that's the case on photos), it would be a good approximation.

I cut a blank using a Visa credit card as a stencil. I placed the test "card" in the fireplace at home that was burning regular pine wood.

Once and for all: aluminum "credit cards" melt in about 2 minutes, proof: http://www.youtube.com/watch?v=RF35HGdxZNA&feature=youtu.be

This is obviously MUCH better than a paper or a plastic card could do.