Working foundry

I built my working foundry from old barrel, broken chimney bricks, and some rock wool insulation. The crucible was parts donated from plumber i work on site with and welded together by my brother. So special thanks to them.

I dont know if this is a issue but i am getting the center of the ingot forming a hole when they cool. Is that normal? if someone can explain this, please leave me a quick comment. Thanks