These glazes are both 80% Alberta Slip, but the one on the right employs 20% Ferro Frit 3249 accelerate the melting (whereas the left one has 20% Frit 3134). Even though Frit 3249 is higher in boron and should melt better, its high MgO stiffens the glaze melt denying the mobility needed for the crystal growth.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Rutile Blue Glazes
A type of ceramic glaze in which the surface variegates and crystallizes on cooling in the presence of titanium and iron (usually sourced by rutile)
Ferro Frit 3134
A frit with 23% B2O3. The most common of frits used in pottery in North America. Around the world, other companies make frits of equivalent chemistry.
Ferro Frit 3249
A magnesia borosilicate frit having very low thermal expansion and melting point. Invaluable in pottery as to increase the MgO in glazes to prevent crazing.