Have you ever? Carefully crafted, decorated, and glazed your work, only to be disheartened when you open the kiln and see tiny pinprick holes or, even worse, blisters and a rough surface like frozen air bubbles all over your piece.
Today, Pottery Clay will delve into the causes and share secret tips for dealing with and preventing these issues from ruining your work again.
Understanding the "Culprits" Behind the Problems
Both Pinholes and Blistering stem from the same cause: gases or air bubbles trying to escape from the clay body or glaze precisely when the glaze is melting.
- If the gas pushes out and the glaze doesn't flow back in time to close the hole, it results in pinholes.
- If the gas pushes out but the glaze is too thick or viscous, blocking the gas and trapping it inside, it becomes blistering.
What to do if you find them after opening the kiln?
- Re-firing: Re-fire the problematic piece in the kiln at the same temperature or slightly lower, but add a Soaking technique (holding the temperature) at the peak temperature for about 15–20 minutes. This gives the glaze time to flow back and heal the open surface, making it smooth again.
- Applying glaze to fill deep grooves: For very deep pinholes, use the same glaze formula, mixed thickly with water. Dab or fill the holes completely, let dry thoroughly, then re-fire.
If you don't want to encounter this again, try these steps:
1. Adjust the "Firing Cycle"
Increase soaking time: Once the kiln reaches the peak temperature for that glaze, don't turn off the power immediately. Hold the temperature steady for about 15–30 minutes. The boiling and bubbling glaze will gradually settle and flow to coat the surface smoothly.
Slow down firing during bisque firing: The temperature range between 700 °C – 950 °C is when organic matter and carbon gases embedded in the clay are released the most. If you slow down the bisque firing during this period, the gases will evaporate completely, preventing any residual gases from erupting during glaze firing.
2. Adjust "Work Preparation and Glazing Techniques"
Always wipe off dust before glazing: After bisque firing, small dust particles often adhere to the piece. Use a damp sponge to wipe them clean, as these dust particles can trap air during glazing, eventually leading to pinholes.
Control glaze thickness: Glazing too thickly or applying multiple layers (especially with different glaze types) will make it difficult for gases to escape, leading to blistering.
Check glaze viscosity: If the glaze is too thick, air bubbles beneath the glaze won't be able to push through. You should mix water and sieve the glaze to achieve the right viscosity.
3. Adjust "Raw Materials and Chemistry"
Reduce high-gas-releasing raw materials: If your self-mixed glaze formula contains a large amount of Calcium Carbonate (Whiting) or Barium Carbonate, these materials release a lot of carbon dioxide gas during melting. Try adjusting the formula by using Wollastonite or Frits instead to reduce gas formation in the glaze.
For those currently facing this issue, we recommend starting with the easiest solution: try increasing the Soaking time during glaze firing by 20 minutes in your next kiln load. This method doesn't require changing your clay or glaze formulas, but rather allows time for the glaze to self-heal. If your work's surface becomes smoother, you're on the right track!