This is Plainsman Buffstone, fired at cone 04. The piece emerged from the kiln without crazing. The mug was heated to 300F and plunged into ice water (the 300F-to-ice-water IWCT test). This is what happened. Water is being absorbed into the porous body through the craze lines. This is G1916J, a variation on the G1916Q recipe. J is just two materials, 85% Ferro Frit 3195 and 15% EPK. What recipe adjustment is needed? Substitute some of the Frit 3195 for low expansion Frit 3249. We have found that a 55:30:15 of 3195:3249:EPK recipe will work. Since both 3195 and 3249 melt transparent at cone 04, blending them together does not change the appearance (actually 3249 is glossier and actually improves the surface).
G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04-02 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.