Georgies' cone 6 crackle glazes give their best and intended results when used on porcelain or tight white stoneware clay bodies. Crackle glazes are not foodsafe because of the nature of their fired surfaces. Samples on the right show color results with India ink applied after firing. India ink will highlight the crackle patterns in the fired glaze. India ink, however, SHOULD ONLY BE USED WHEN THE CLAY BENEATH THE GLAZE IS VITREOUS (fused and waterproof as a result of firing). If the clay is not vitreous, the India ink will soak into the clay body and spread, with less-than-desirable results. If the ink soaks in, there is no cure other than refiring the piece to burn out the ink.