Many potters have had the disappointing experience at least once of their pottery exploding in the kiln. The beauty of pottery is that you can get some very unexpectedly wonderful results. However, a nerve-wracking array of things can go wrong with pottery once you hand it over to the kiln of fate. So, exactly why does pottery explode in the kiln?
The main reason that pottery explodes in the kiln is residual moisture left in the clay body even when it appears bone dry. Once the kiln reaches 212F, the moisture starts to turn into steam. It expands very rapidly and the pottery shatters to accommodate the steam.
If a piece of pottery is going to explode, it normally does so in the bisque fire. The reason for this is that prior to the bisque fire, the greenware still has some water content. After the bisque fire, all this water will have gone. So, the main reason for the pottery to explode will have basically evaporated.
However, there are other factors that can cause pottery to explode at other points during firing. This article will have a look at each contributing factor.
Moisture Can Make Pottery Explode in The Kiln
It is commonly said that air bubbles in clay cause pottery to explode. However, this is only partially true. The primary reason why pottery can explode in the kiln is water in the clay.
Greenware may look bone dry, however, even in very dry atmospheres, unfired clay will contain some residual water.
The water in the clay can be categorized in two ways:
- Mechanical water or residual moisture.
- Water that is chemically bonded to the clay particles
Moisture and chemically bonded water leave the clay at different points in the bisque fire. Mechanical water is also known as ‘free water’. It is the kind of moisture that you might be able to see with your naked eye if you were to split your greenware open.
A piece may look bone dry. However, there is always moisture nestled deeper in the layers beyond the surface area visible on the outside. Even if you weren’t able to see darker clay at deeper layers, it is still there. All unfired clay has some mechanical moisture.
The Water Smoking Stage When Firing Pottery
Free water begins to evaporate when the kiln is in its preheating stage. The expulsion of water through evaporation is called the ‘water smoking stage’. Ideally, the preheating stage is long enough so that most if not all of the free water has evaporated.
During the low ramp in the firing schedule, the temperature in the kiln passes through 212F, which is the boiling point of water. At this point, any residual moisture that has not been smoked out, turns to steam and expands very rapidly. It also expands a lot in volume. In fact, it expands by a factor of 1600 when it turns to steam1. Sometimes it is possible to hear popping noises inside the kiln during the water smoking stage. This is an indication that temperature is increasing too rapidly.
Pottery explodes in the kiln for these two reasons at this point. Namely, the rapidity of the change from water to steam and the large change in volume.
Chemically Bonded Water
This is water that is bonded to the clay at a molecular level. It starts to be driven off the clay body at around 600F. And this continues until about 1100F.
During this process, the clay is undergoing a restructure. It is being altered chemically. Once the water has left, it will have changed at a molecular level irreversibly. The changes are very significant and need to happen slowly. Pottery can explode at this point too if the water is driven off too quickly.
Air Bubbles Can Cause Pottery to Explode
I stated above that it is only partially true that air bubbles cause pottery to explode. The reason it is a partial truth is that on their own, air bubbles are not the culprit. However, they can play a contributing role.
Air itself does not expand enough when heated to cause the pottery to explode. However, when free water starts to evaporate, it looks for a way to exit the clay. It migrates towards the surface of the clay.
If there are air bubbles in the clay the moisture will evaporate into the bubble cavity. Then as it expands within the bubble cavity, it will cause the pottery to explode in the kiln. Therefore, the air bubbles themselves did not cause the clay to shatter. However, they did make it easier for the water to do damage.
Some potters argue that air bubbles in themselves are not a problem. Provided there is not too much moisture in the greenware and it is fired slowly. However, a piece of pottery made with air bubbles will be less sound than one without. Wedging clay sufficiently will give the finished product greater strength and integrity.
Pockets of Air Can Also Create Problems
Another related cause of explosions are pottery designs that involve trapped pockets of air. An example might be a handle on a pot lid that is shaped like a bulb. If the bulb is hollow, it is important that there is an outlet for any steam.
It is possible to make a small unobtrusive hole in the bulb so that steam can come out. The hole does, however, need to be big enough that it will not close up as the clay shrinks in the kiln.
Thick Walls Can Make Pottery Explode in The Kiln
Received wisdom states that the walls of your pottery ware should be no thicker than 1 inch. And this is considered to be a maximum. Relatively thin walls are better if there is any residual moisture left in the piece.
If the walls are thin, mechanical water has a better chance of moving towards their surface and escaping. If the water has less distance to travel, it is more likely to evaporate off the surface of the clay rather than cause the pottery to explode.
That being said, other potters argue that in theory, you can fire very thick greenware if it is done correctly. Even if clay is very thick, it can be left to dry for a long time. This will remove as much mechanical water as possible. Then, if it is fired on a long schedule, with a long preheat, it may survive the firing process.
Although this is put forward as a possibility, most potters work with the principle that thinner walls are better.
Pre-Heating the Kiln to Avoid Exploding Clay
As a piece is fired, the temperature in the kiln gradually rises. The rate at which it rises and the different stages are called the firing schedule. The greenware is undergoing big changes and it needs time to adjust and handle these changes. If the temperature ramps up too quickly, the clay will not cope well and this can cause the pottery to explode.
A process called ‘candling’ can be used to ensure that as much mechanical water has evaporated as possible. Candling involves setting the kiln at a very low temperature for a long period of time.
A low temperature in this context is anything below boiling point. Many potters will set a candling temperature up to 200F. Often, they will allow the pottery to candle overnight, before moving onto the bisque fire.
This is also known as the pre-heating stage. During candling, the potter will leave the kiln door open just a crack to let moisture escape. If you hold a mirror to the cracked open door you may see the residual moisture mist up the mirror surface.
How long to pre-heat the pottery varies according to different factors. These factors include how much moisture you think is present. And how thick the clay is and what the pottery design is.
Sometimes pottery needs to be candled for a few hours. However, very large sculptural pieces with very thick cross-sections may be candled for days or even a week2. Failing to pre-heat the kiln and greenware for long enough is one reason why pottery can explode in the kiln.
Firing Too Quickly Can Make Pottery Explode in The Kiln
As stated above, the water content can cause pottery to explode, especially if it is fired to quickly. However, there are other reasons why firing too quickly can cause problems.
These additional reasons are:
Clay bodies contain organic matter and organic matter contains carbon. To change from clay to ceramic, the carbon in the clay needs to be burnt off. This process needs two things.
Firstly, there needs to be enough oxygen in the kiln to allow the carbon to burn. For anything to burn successfully, oxygen is required. Also, the carbon needs to be given enough time to burn off.
Oxidation in firing happens between 500 and 2000F. As the carbon is burnt off, it is converted into gases. As the temperature in the kiln increase, the surface of the pottery starts to seal.
If the temperature during this process rises too quickly, the carbon does not have enough time to burn off completely. As a result, carbon and gases can get trapped under the surface of the pottery which has become impermeable.
This can cause the ware to bubble, bloat and crack as the fire continues. It can also, on occasion cause the pottery to explode.
The Quartz Inversion
Another factor that can be the cause of damage to ware in the kiln is the quartz inversion. Quartz inversion does not cause pottery to explode but can cause it to crack, particularly as the clay cools down.
Quartz in the clay expands sharply during two stages of the firing. Once, when the temperature is rising, and another time when the kiln is cooling down. It is called the quartz inversion because it occurs as the temperature rises and falls. The expansion occurs in a window of around 122F (50C), and this centers around the temperature of 1064F (573C).
When the temperature is rising, the ware still has a degree of plasticity and may withstand the change in the volume of the quartz. However, when the kiln is cooling the ware is more rigid and less able to cope with the strain of the quartz expanding. Cracks are therefore more likely to occur during the cooling part of the schedule.
Problems arise particularly if different parts of the ware are at different temperatures. For example, one part of the pottery may be at 1064F (and therefore expanding) and the other part may be at 1112F. In this example, one part of the piece expanding whilst the other part is not. This puts the structure under strain and can cause cracks.
The pottery may not explode during the quartz inversion. However, cracks, are more likely to occur due to quartz inversion if the firing schedule is moving too quickly through the 1064F range.
A number of factors explain why pottery explodes in the kiln. The main culprit implicated is left over water content in the greenware.
However, other factors such as air bubbles and pockets, and clay thickness play a role. Another important factor is the firing schedule. If the piece is being fired too quickly, it puts a lot of strain on the ware and explosions are more likely. All these factors need to be taken into consideration to avoid the big bang!
Also, I wanted to include a link to this video by a really nice hobbyist potter called Tammy Jo Schoppett. This illustrates that when a piece explodes, it really does shatter..
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