Hi,
if a greyish metal feels "a bit heavy" it's probably of the ZA family of Zinc alloys. Pure Al has a density of 2.7, for every litre of metal, it weighs 2.7Kgs. If you measure the volume of water displaced by the metal when it is submerged, and weigh the metal, if it is a Zinc based alloy it will weigh more than 2.7Kgs per litre. I do this if I'm not sure, A bowl of water, full to the brim in another bowl, emerge the metal in the water, the water that spills into the second bowl is the same volume as the metal. Measure the volume of the water either with a measuring jug or by weighing the water. Water has a density of 1,0 so 1 Kg of water equals 1litre. Weigh the metal, if the weight of the metal is 2.7 times the weight of water, it's Al! Zinc has a density of 7.134, so it's nearly three times heavier per litre than Al. The Zinc alloys aren't very popular as they are often badly used they are difficult to weld and are brittle. The good side, they have good vibration damping characteristics (close to cast iron), Cast very well, machine beautifully and make very good bearing material for steel shafts. The ZA Zinc alloys can be cast at much lower temperatures than Al, with a melting point of around 450°C. The thing to avoid with Zinc is overheating it. Over 700°C, it boils and gives of white smoke (Zinc vapour) that can give you (fume fever) flu like symptoms! Don't breath it!
I hope that helps! Regards, Matthew