Thanks for the suggestions. It was also suggested that i try firing a few samples to C/1 (oxidation). My initial samples mentioned above were fired to C/06. The C/1 results are a deep red/brown I really like. The commercial glazes mostly bubbled with the exception of a Duncan Coal Black, although it may have micro-bubbling. There is a texture that suggests that Is the case. It's hard to tell because it's on the inside of the vases. I might just have to break a vase to have a close look.
For a better liner I'm also looking at glaze recipes that mature at C/1. If anyone has recipe suggestions, I'm all eyes.
Lastly, I am researching low-fire casting slip recipes with a Redart base. I think i already know the answer to my next question but here goes: do folks think I should jettison the commercial products and come up with a C/1 terracotta casting slip and base-glaze combo that play well together?