Pinholing is a glaze fault where tiny holes or pits appear in the fired glaze, usually reaching down towards the clay. They form when gas bubbles burst through the melting glaze late in the firing and don’t heal over.
What Is Pinholing?
Digitalfire lists many sources of the gas: carbon and sulfur in the clay that weren’t burned out in the bisque, carbonates in the glaze, air trapped in a porous or grogged surface, and moisture in the ware. As the glaze melts, bubbles rise and burst, and if the glaze is too stiff or the firing ends too soon, the craters stay open.
Common fixes are to bisque fully in a well-ventilated kiln, smooth the clay surface at leather hard, apply glaze evenly and not too thin, and add a hold at the top of the glaze firing so the melt can smooth over. Slower cooling and a drop-and-hold schedule help some glazes heal.
Pinholes are more than cosmetic on functional ware, because they leave openings in the glaze. Refiring with a thin extra coat sometimes cures them, but it is better to fix the cause.
If only one glaze or one clay pinholes, test them separately to find which is at fault.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.