Glaze pustules (often referred to as glaze blisters, bumps, or boils) are small, raised bumps, domes, or sharp-edged craters that form on the surface of pottery when trapped gases burst or freeze near the end of a kiln firing.
What causes Glaze Pustules?
Trapped Gases: Gaseous carbon, organics, carbonates, or sulphates from the clay body or raw glaze materials decompose and try to escape as the glaze melts.
Early Surface Sealing: If the glaze melts and seals the surface too quickly, expanding gases get trapped underneath, forming bubbles.
Thick Application: Applying the glaze too thickly makes it harder for subsurface gases to break through and heal before the kiln starts cooling.
High Viscosity: A stiff, high-surface-tension glaze cannot flow or "heal" over the broken bubble crater once the gas pops.
Overfiring or Rapid Firing: Pushing the kiln to too high a temperature or ramping up/down too fast can cause severe gas evolution or leave unhealed pits.
How to fix pustules
Add a Peak Soak: Program your kiln with a 15- to 30-minute drop-and-hold (soak) at or near peak temperature to give the molten glaze time to fluidize and heal over escaping gas
Thin Your Glaze: Apply a thinner, more even coat of glaze to prevent sealing gas pockets inside.
Adjust Bisque Firing: Fire your bisque ware to a slightly higher temperature to burn off organic materials and carbon before the glaze is ever applied
Slow the Cooling: Slow down the initial cooling phase of the kiln so bubbles have time to settle out smoothly
