Do you know anyone around you who might have one of the Lee hardness testers? IF so your "pure" should be in the 5 bhn range or lower, 7 at tops.
Pour up another few of them, let them air cool and measure, and then let them sit for about 2 weeks, measure again. Pure shouldn't grow much if any, if you have tin/antimony in it they will grow around .001-.0025", and they will be harder.
The other post on simply cast and load are correct with the proper alloy, and you don't want to water cool them at all.
Also make sure those grease grooves are slathered up full as well.
Hope this helps.
Mike / TX