Thanks for sharing your very interesting findings.
The clayart reference you posted includes the following
The fact is, the bubbles in a glaze don't move due
to buoyancy. They are formed right where they are
Don't take my word for it, though. Break any piece
of pottery with bubbles in the glaze and examine the
cross section of the glaze with a good microscope.
The magnification doesn't have to be very high--25 to
100X will do. The average distance from the center
line of the bubbles to the glaze-body interface will
be about the same whether the sample is from a part of
the pot with the glaze above the body, below the body,
or on a vertical surface. If bubbles rose in a glaze
these three samples would not look the same. In my
observation they just don't vary. The bubbles don't
move... Dave Finkelnburg
Which makes me wonder what would happen if you fired a vertical test tile.
BTW Dave Finkelnburg seems to have written an Alfred MSc thesis called
Bubble Evolution and Sintering in Whiteware Glazes, anybody know how to