A while ago I did some experiments with a cone 6 clear in a electric kiln that has lots of tiny bubbles in it when applied thicker, I made some small tiles (about 1 1/8" square) out of black clay so the bubbles would have good contrast and because they're fired horizontal I got a nice even spread of bubbles.
Some tests have additions of Mason stain.
The first tile has white clay inlaid in the black clay with small pieces of a glassy porcelain I have that were placed on top of the glaze prior to firing.
Tile three has 1% Mazerine stain added to the clear with a streak of white glaze on top and a ball of porcelain.
The last one is 4% Praseodymium with porcelain additions.
Tile one closer
The glaze is:
Not the most compatible of glazes but bubbly. I don't remember exactly how thick the clear was but at least 4 coats and if applied too thick the glaze becomes too cloudy.
I haven’t experimented with this enough to know if it's useful or practical to do it this way, but found it interesting.