I would always recommend fluxing every batch of lead being melted .
The ingot break does not look right but as you said could be because it was hot when dropped and broke.
If the lead is pure in the pot you値l notice an almost iridescent finish on the surface of the melt.
Could be that you need to turn the heat up a bit in the pot .
No doubt you値l get more replies from people with a lot more experience of casting than myself.
ATB
James