|Monthly Tech-Tip |
'Dimpled' glaze surfaces are those that have tiny holes that do not go down to the ceramic underneath (a pinhole). These holes look as if they were produced by a pin-point being pressed into the surface of the glaze when its melt is very stiff. They are often associated with pinholes. Dimples are not always immediately evident if the glaze, closer examination is needed. Dimples are considered a glaze imperfection and are usually caused by firing too quickly or by particulates in the body and/or glaze (which generate gases during firing). Industrial producers pay great attention to the particle sizes of their body and glaze materials and process them to whatever size is necessary to assure defect free glazes (even wet ball milling, filter pressing and spray drying if needed).
Testing for pinholes and dimples is often best done using a transparent glaze over a large surface and looking at the surface in the light. In this case, an open bowl is used. Heavy pieces like this are difficult to fire evenly and encourage under fired areas where pinholes are more likely to appear.