Look up the differences between foundry type and monotype. Many times foundry gets sold as mono and the seller and the buyer are happy. Foundry are solid blocks with a half round groove. Monotype is generally hollow blocks and square grooves. Foundry is richer than mono and usually has 1-2% copper which is why it melts so funny. It sounds like you have some foundry type to me, which is good news because it goes even further as a sweetener. Probably 12-14% tin, 24-27% antimony and 1-2% copper.
I have found that foundry needs about 45 minutes after it melts to fully mix. I like to wait till the foamy stuff mixes in and flux with wax a few times while I'm waiting. After it is mixed well I will use some wood chips so I know I'm not losing the good stuff when I pull the junk out.