Cast aluminum?

My guess is that what most folks refer to as "pot metal" is probably cast zinc. Zinc has a higher material density than aluminum and will be heavier. It's also soft and you should be able to tell the difference with a file.

So far as your picky metal guy I would look for someone else. At an absolute minimum he should be able to accomodate what is known as "mixed scrap". Domestic casting production is based almost exclusively on recycled materials that are melted and re-formulated into specific chemistries for different alloy combination. This is especially true with die-cast materials with many high volume casters doing re-formulating in their own furnaces from base materials that they buy slightly "off chemistry".

The exceptions here would be aerospace alloys and some high stress commercial materials.

Cast AL will look real white.

Pot metal is a dull gray.

Another way is to take a 10X loop and look at a known piece of cast AL aand piece of known pot metal and look at the different grain structures between the two.

T_Bone