Mike Avery gave serious info on how to improve the development of the
crumb in the bottom of the loaf.
My solution is a bit different.
I make regular bread (with commercial yeast) and sourdough by normal
preparation methods. However, I have a 14 inch cast iron pizza pan
which I put in the oven when I start it up in preparation of baking a
loaf by either method. When the temp in the oven reaches 450F the
cast iron pan is pretty hot (I don't know precisely the temp). I put
the risen loaf (usually crowned ~ 1 inch above the pan) into the oven
on top of the cast iron pan and allow it to bake for about 35-45
min. The heat of the cast pan seems to enhance the rise in the
bottom of the loaf and the holes in the crumb appears about the same
from the bottom to the top of the loaf. The extra heat must push the
expansion of the crumb quite a bit.
all best,
bill bowers