Several recent posts inquired about pin holing in glazes: and asking if clay was involved. With that, I thought I would share some USB pics from a study I did a couple of years back.

Nep Sy is the flux of choice in most commercial bodies because of the price point; nearly a third less than the cost of potassium spars. However, Nep Sy.contains 14-20% soluble salts that migrate as the clay is drying to a bone dry state.

Those white patches you see in this sample are sodium crystals from migrating soluble salts. The larger deposits and deposits near the surface of the clay can cause serious pin hole problems even when you follow a slow firing cycle. This pic of the same clay as shown above illustrates how vigorous the off gassing spars become. It was taken at 20X magnification, so you only see the pin holes that break the glaze surface. The numerous other bubbles are not detected upon visual inspection.

Hope this helps those dealing with this issue. Often times pin holing has nothing to do with your glaze or your firing cycle: it is the body flux used in the clay body recipe.

Tom