F3249
Magnesia borosilicate frit — no sodium or potassium in it · Very low thermal expansion, and the usual cure for crazing · Flows around 1260°C, far hotter than a low-fire frit · High boron, and magnesia mattes a surface
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Frit 3249 is a magnesia borosilicate frit with very low thermal expansion. A small addition is a standard way to stop a glaze crazing, though it melts far hotter than a low-fire frit.
Frit 3249 is a magnesia borosilicate frit, and unusually it carries no sodium or potassium at all. Those two are the fluxes that raise thermal expansion most, and their absence does more to keep this frit's expansion low than the magnesia does. A small addition, under a tenth of the recipe, is the usual dose against crazing. It flows around 1260°C, far hotter than a low-fire frit, so a recipe taking it in loses melting power and can come out stiffer and drier than it was. It carries a great deal of boron as well, so enough of it to supply the magnesia will move the boron too. Magnesia mattes a surface and pulls colour toward pastels.
