G2926B (center and right) is a clear cone 6 glaze created by simply adding 10% silica to Perkins Studio clear, a glaze that had a slight tendency delay-craze on common porcelains we use. Amazingly it tolerated that silica addition very well and continued to fire to an ultra gloss crystal clear. That change eliminated the crazing issues. The cup on the right is a typical porcelain that fits most glazes (because it has 24% silica and near-zero porosity). The center one only has 17% silica and zero porosity (that is why it is crazing this glaze). I added 5% more silica to the glaze, it took that in stride, continuing to produce an ultra smooth glossy. It is on cup on the left. But it is still crazing just as much! That silica addition only reduces the calculated expansion from 6.0 to 5.9, clearly not enough for this more severe thermal expansion mismatch. Substituting low expansion MgO for other fluxes will compromise the gloss, so clearly the solution is to use the porcelain on the right.
How I Improved a Popular Cone 6 Clear Glaze Using Insight-Live
How I found a ceramic glaze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed and got a fabulous more durable cone 6 clear.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Calculated Thermal Expansion
The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Band-aid solutions to crazing are often recommended by authors, but these do not get at the root cause of the problem, a thermal expansion mismatch between glaze and body.