The difference is a slow-cool firing. Both mugs are Plainsman M340 and have the L3954B black engobe inside and partway down on the outside. Both were dip-glazed with the GA6-B amber transparent and fired to cone 6. The one on the right was fired using the PLC6DS drop-and-hold schedule. That eliminated any blisters, but some pinholes remained. The one on the left was fired using the C6DHSC slow-cool schedule. That differs in one way: It cools at 150F/hr from 2100F to 1400F (as opposed to a free-fall). It is amazing how much this improves the brilliance and surface quality (not fully indicated by this photo, the mug on the left is much better).
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.
GA6-B - Alberta Slip Cone 6 transparent honey glaze
An amber-colored glaze that produces a clean, micro-bubble-free transparent glass at cone 5-6. Works well on brown and red burning stonewares.
Glaze Pinholes, Pitting
Analyze the causes of ceramic glaze pinholing and pitting so your fix is dealing with the real issues, not a symptom.
Cone 6 Drop-and-Soak Firing Schedule
350F/hr to 2100F, 108/hr to 2200, hold 10 minutes, freefall to 2100, hold 30 minutes, free fall
Plainsman Cone 6 Slow Cool
350F/hr to 2100F, 108/hr to 2200, hold 10 minutes, fastdrop to 2100, hold 30 minutes, 150/hr to 1400