The center mug is clear-glazed, at cone 6, with G2926B (notice it is saturated with bubble clouds). This dark body, M390, is exposed inside and out (the other two mugs have the L3954B white engobe inside and midway down the outside). G2926B is an early-melter (starting at cone 03) so it is bubble-susceptible to dark-burning bodies that generate more gases of decomposition. It appears to perform well on the inside engobed surface of the mug on the left but actually, the bubbles are just less visible against the white background. The honey colored glaze on the outside mugs is GA6-B, it fires with very few bubbles and is a good choice for use on dark clay bodies.
These two glazes are both brilliant glass-like super-transparents. But on this high-iron stoneware only one is working. Why? G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated by the body during firing. G2926B (right) works great on porcelain but it cannot clear the clouds of micro-bubbles coming out of this body. Even the glassy smooth surface has been affected. The moral: Two base transparents are needed, each being able to host colors, opacifiers and variegators. But there is a caveat: Although reactive glazes leverage melt fluidity to develop interesting surfaces they are more tricky to use and do not fire as durable.
Clouding in Ceramic Glazes
There a many factors to deal with in your ceramic process to achieve transparent glazes that actually fire to a crystal-clear glass