I just had the same problem, but there were unglazed portions that had the same blistering/pimples, Pieces were bisqued to Δ04. I run a manual updraft gas kiln and did a fairly fast fire, ~5 hours to 1000°C which required ~1 hour @ 1200°C for Δ6 to mature. I figure I need to run a much slower ramp and then hold at a lower temperature for this clay.