Is aluminium foil pure aluminium?
From the Institute of Minerals, Materials and Mining,


  Most aluminium foil is made from pure aluminium, but increasingly alloys are used to improve properties and reduce the thickness required.  


If you want to make sure that the Aluminium foil you're using is pure, the only way would probably to ask the manufacturer. Other than that, it could be the case that the foil is either pure or alloyed, depending on how it's made.