Ingots with "shrink" holes in center?

[10 ga]

I've been smelting my cache into ingots and have another question. I hand sort all the lead as to type/source/whatever. example is battery cable ends, sorted and smelted out about 4lb. ie 2 ingots..... Well some of the ingots, mini, medium and regular muffin pans, have what I call "shrink" holes in the center. It is where the pour shrinks and leaves a "crystal" lined hole in the center of the ingot. This is not in all of the lead but some of the alloys have it in almost every ingot I poured. What is this telling me? The soft types seem not to have this. Any explanation? Thanks, 10

[btroj]

Alloys with more antimony will tend to do this more than pure lead. The alloy shrinks some as it cools. Not a problem at all.
Think of it as a good way to help keep them separate.

[lwknight]

Close anyway.
Pure lead just gets a depression in it.
Lead with a little antimony gets the shrink hole like you described.
Lead with a lot of antimony does not shrink at all.
I never really paid attention to the effects is tin in this case.

[sagacious]

My observations match LWKnights. Also, the higher the pour temp, the greater the shrinkage cavity on the ingots. Good luck.