What’s causing my glaze to pinhole?

Pinholing in glazes can be caused by a variety of reasons. Firstly it can be caused by volatile materials burning away from within the clay passing through the glaze layer. This is normally due to the biscuit firing being either too fast or too low to allow the complete burn-out from the clay. Pinholing may also be caused by the glaze being underfired. All glazes bubble during the firing, an essential part of the glaze firing process, and if the firing is too short or too low in temperature then the bubbles will not have time to completely ‘heal’. Highly refractory glazes, such as those containing a high quantity of clay or white opaque glazes utilising zircon as the opacifier are more prone to pinholing problems than others. These often need slower firing cycles to allow the glaze to develop its maximum fluidity.