Cylinder head melted in mega

This was a bit of an experement, I was finding it very difficult to break up aluminium cylinder heads from motor cars I use these as they have the best grade of alloy to cast with So i was wondering how long it would take to melt a complete cylinder head seeing as i never had a top section to put on the furnace I was guessing may be a couple of hrs as most of the heat was escaping out the top Any way i have had lots of question as to what im going to do with the alloy, well i am going to cast headstone