Cordierite crystal-containing glaze

A glaze having an oxide composition consisting essentially of

• ______________________________________ SiO.sub.2 66-75 wt. % Al.sub.2 O.sub.3 13-22 MgO 3-8 B.sub.2 O.sub.3 2.5-7 Na.sub.2 O + K.sub.2 O .4-2.0 ______________________________________
• the above-listed components totaling at least 95% of the composition, and Li.sub.2 O is not more than 0.5%, the total of Fe.sub.2 O.sub.3 and TiO.sub.2 is not more than 1.5%, and at least one of ZnO and ZrO.sub.2 is included in an amount less than 2%,
• the ratio (SiO.sub.2 +Al.sub.2 O.sub.3 +MgO)/(CaO+SrO+Ba0+Cs.sub.2 O+F.sub.2) exceeding at least about 28:1,
• said glaze when fired on a ceramic body containing cordierite crystals as the major crystal phase therein and the exterior of the glaze is richer in SiO.sub.2 than the interior of the glaze.

The method of glazing a ceramic body comprising,

• preparing a glaze having an oxide composition consisting essentially of
• ______________________________________ SiO.sub.2 66-75 wt. % Al.sub.2 O.sub.3 13-22 MgO 3-8 B.sub.2 O.sub.3 2.5-7 Na.sub.2 O + K.sub.2 O .4-2.0 ______________________________________
• the above-listed components totaling at least 95% of the composition, and Li.sub.2 O is not more than 0.5%, the total of Fe.sub.2 O.sub.3 and TiO.sub.2 is not more than 1.5%, and at least one of ZnO and ZrO.sub.2 is included in an amount less than 2%
• the ratio (SiO.sub.2 +Al.sub.2 O.sub.3 +MgO)/(CaO+SrO.sub.2 +BaO+Cs.sub.2 O+F.sub.2) exceeding about 28:1,
• applying the glaze to ceramic body, and
• firing the glaze on the body,
• the glaze developing cordierite as the major crystalline phase therein during said firing and the exterior of the glaze is richer in SiO.sub.2 than the interior of the glaze.