Reasons for Pinholing and Pitting
All glazes contain volatile materials and will undergo a certain amount of agitation as these burn off during firing. Most pinholes and pits are due to this off-gassing. In a real sense, the glaze became frozen while it was still boiling.
Underfiring a glaze can leave it with pinholing and pitting. It is not surprising, therefore, to find that matte glazes are more subject to pinholing and pitting, since they are glazes made to purposefully be underfired.
Other contributers to pinholing and pitting include high levels of zinc or rutile in the glaze. In addition, if the kiln enters into reduction during the early stages of firing, carbon may be deposited on the ware and can contribute to pinholing and pitting as it later burns off.
Solving Pinholing and Pitting
- increase the firing time overall
- increase the soak time at the end of the firing cycle
- down-fire (keep the burners on while allowing the kiln to slowly cool) until the glaze has re-solidified
- apply the glaze more thinly
- add a small amount of flux to the glaze
- increase the final temperature you fire to
- reduce the rutile or zinc in the glaze, if any is present
- ensure that, if you are using a fuel-burning kiln, the kiln does not enter reduction during the early stages of the firing cycle.