G1215U is a recipe having a significant amount of the super low expansion Ferro frit 3279. The result is an ultra-clear glaze that will fit pretty well any clay body without crazing. The mechanism is a high level of MgO, that means it responds differently to stains.
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
MgO is the secret weapon of craze control. If your application can tolerate it you can create a cone 6 glaze of very low thermal expansion that is very resistant to crazing.
G1215U - Low Expansion Glossy Clear Cone 6
A recipe sourcing high MgO (from Ferro Frit 3249) to produce a low expansion glass resistant to crazing on lower silica porcelains.