The Frustrating Reality of Pottery Making
As a pottery instructor, one of the most disheartening experiences for both myself and my students is opening the kiln after a firing, only to find a shattered, exploded piece of pottery. It’s a moment that can feel like all your hard work has literally blown up in your face. But why does this happen? Is it bad luck, a faulty kiln, or something else entirely? The truth is, pottery explosions in the kiln are almost always preventable and stem from a few key factors related to the unique properties of clay and the firing process. Let’s delve into the science behind these frustrating blasts and how you can avoid them, ensuring your pottering projects emerge perfectly from the kiln. Expand each section for more information.
The Main Culprit: Water
The most common reason for pottery to explode in the kiln is trapped moisture. Clay, even when it feels bone dry, still contains water. This water exists in a few forms:
- Mechanical Water: This is the water that is freely held between the clay particles. When you are shaping clay on the wheel or by hand, this is the water that makes it pliable. As the piece dries, this water evaporates.
- Pore Water: This water is trapped in the tiny pores and capillaries within the clay body.
- Chemical Water (Hydroxyls): This water is chemically bonded to the clay molecules themselves.
When you load your greenware (unfired clay) into the kiln, the temperature begins to rise. As the temperature reaches and surpasses the boiling point of water (212^\circ F or 100^\circ C), any remaining mechanical or pore water attempts to turn into steam. Here’s the critical point: If this steam cannot escape the clay body fast enough, the pressure builds up internally. Imagine a tiny pressure cooker inside your pot! If the pressure becomes too great for the clay to withstand, the piece will literally explode. This is especially true for pieces that are too thick, have uneven thickness, or have internal air pockets.
Other Contributing Factors:
While water is the primary cause, a few other factors can contribute to kiln explosions:
- Air Pockets: During the clay preparation or hand-building process, it’s possible to trap pockets of air within the clay. Just like trapped water, this air will expand when heated, and if it has no escape route, it can cause the piece to burst. This is why thorough wedging is so important!
- Rapid Heating (Thermal Shock): Firing too quickly, especially in the early stages, can also lead to explosions. The rapid temperature increase can cause the water to turn to steam too quickly, not allowing enough time for it to escape. It can also cause uneven expansion and contraction, stressing the clay.
- Uneven Drying: If a piece is not dried evenly, some parts may still contain significant moisture while others are bone dry. When heated, the wetter areas will experience more rapid steam conversion and pressure buildup, leading to cracks or explosions.
- Inclusions: Sometimes, foreign materials or even large particles of grog (a sand-like material added to clay) can create weaknesses or areas where pressure can build up, contributing to a blow-out.
How to Prevent Pottery Explosions:
The good news is that most kiln explosions are avoidable with careful preparation and attention to detail. Here are my top tips for happy, intact pottery creations:
- Dry, Dry, Dry! This is paramount. Ensure your greenware is absolutely bone dry before loading it into the kiln. This means no cool spots, no dampness to the touch. Place pieces in a well-ventilated area, and for thicker pieces, consider a longer drying time. You can even use a candler or a low-temperature pre-heat setting on your kiln to slowly evaporate any residual moisture before the main firing.
- Wedge Your Clay Thoroughly: Wedging not only homogenizes your clay but also removes any trapped air pockets. This is a crucial step in the pottering process!
- Ensure Even Thickness: Aim for consistent thickness throughout your piece. Avoid drastic changes in thickness, as these areas will dry and heat at different rates, leading to stress. If you’re creating a piece with varying thicknesses, consider hollowing out thicker sections.
- Slow and Steady Wins the Race: When firing, especially the bisque firing (the first firing), a slow ramp-up during the initial stages is essential. This allows ample time for all the mechanical and pore water to escape safely. Most kilns have programmed firing schedules that account for this.
- Ventilation: Ensure your kiln is properly vented, especially during the initial stages of firing, to allow the water vapor to escape.
By understanding the science behind pottery explosions and implementing these preventive measures, you’ll significantly increase your success rate in the studio. Learn more about pottery on the Potterings blog, full of tips and tricks for beginning handbuilders. Happy pottering!