The result of holding a kiln for too long depends on the type of clay and glaze you are using. And on how long the hold ran over. Low fire clay will tend to melt. High firing clays will tend to become brittle, and glazes will become discolored or run.
It is generally said that clay can take up to 7 days to become bone dry. When clay is bone dry, it is pale and feels warm and dry to the touch. To prevent your ware from exploding in the kiln, it needs to be bone dry before it is fired. Some potters will put clay in the kiln when it is a little damp.
After you've made your pot from clay it will be ready for its bisque firing once it gets to the bone dry stage (about 1 week after the making of it). Your pot needs to be bone dry to go into the kiln for the first firing to prevent it from exploding!
However, all manufacturers and most potters will recommend not doing this. Things can go wrong with kilns. At best this leads to a kiln of ruined pottery. At worst it can lead to a damaged kiln or a studio fire.
Firing clay too high can cause it to deform or even melt, too low and it will not be durable. Firing glazes too high can cause run-off on the pot, too low and they will be dry and rough.
THE MOST IMPORTANT THING TO REMEMBER IS, DO NOT OVER-FIRE!
(This is unlike glaze which must be fired to the exact specified temperature range.) If clay over-fires, it will first slump and bloat, and then will melt and potentially cause a lot of damage to your kiln.
Drying the Clay
Water turns to steam at 202 °F. if there is still moisture in the clay in can actually explode the piece as the steam tries to escape. This is not dangerous but it can cause damage to the kiln and other pieces in the firing. Hold a piece up to your cheek to feel the temperature.
Kiln elements will last between 100-200 firings. This assumes you are bisque firing half the time and glaze firing at cone 6 the rest of the time. Certain types of kiln elements will last longer than others. There is a big difference between 100 and 200 hundred firings.
Kiln Ventilation
Because kilns pass off vapor elements on heating, a basement is not safe if it is not well ventilated. Use the space as is, and you will have airborne fumes spreading through other sectors of your home. A kiln in the basement should be vented for safety.
A Pottery Kiln can be opened once it has cooled to a least 125° F (51° C). Experts recommend keeping it closed until then to avoid injury and ensure the piece doesn't crack from thermal shock. You can open all the peepholes to let the heat out, but only the top one is typically recommended after 400° F (204° C).
Pottery can be reglazed and refried multiple times. Most pottery glazes need to be applied in 1-3 layers. Pottery that has already been fired with a glaze can be re-glazed and fired 2 times. After the 3rd or 4th time, pottery starts to become brittle and weak, but that's because of the firing and not the glaze itself.
The first firing of the pottery is used to bring the clay to a state where it can be manipulated and shaped. The second firing is what gives the pottery its final form and appearance. The first firing is referred to as a bisque firing. During this firing, the clay is heated to a temperature of around 1640°F.
Even after the atmospheric water is gone, the clay still contains some 14 percent of chemically bonded water by weight. The pot will be substantially lighter, but with no physical shrinkage. This chemically combined water's bond loosens when heated.
You can put slightly wet pottery in a kiln, provided you set it at a low heat for several hours. This is called candling and is a way of pre-heating the kiln before firing. Candling dries the clay out completely before the firing schedule starts, and prevents pottery exploding.
Clay is normally fired twice. The first firing, or bisque fire, takes around 8-10 hours. And the second, or glaze firing takes around 12 hours. So, in total, it takes about 22 hours to fire clay in a kiln.
Clay goes through several physical changes when fired. The first step is the evaporation of water from between the clay particles. Pots must be completely dry before firing, otherwise the steam escaping could cause them to explode.
Kilns utilized asbestos in furnaces, cast houses, and ovens for decades in the United States—that is, until 1979, when asbestos was finally banned due to its confirmed link with mesothelioma cancer and other dangerous diseases.
Your kiln should be in a covered, enclosed space. Even if you live in a dry location, dew will form on the kiln if it is located outside. A basement or garage is usually a good location. Preferably the floor should be concrete.
As long as there is nothing overtly flammable - i.e. spilled oil, gasoline, etc and as long as you keep any flammable items like wood and paper at least 36" away from the kiln you should have nothing special to worry about.
Kiln manufacturers give different guidelines about what temperature you can open a kiln. Their recommendations usually range between 125 and 250F. A good rule of thumb is to open your kiln when it is at room temperature.
Heat removes the molecular water in the clay. The heat converts clay molecules to molecules that do not dissolve or slake in water. In modern societies pottery and brick is fired in kilns to temperatures ranging from 1,800 F to 2,400 F.
One technique you may not have used is multiple firings. Some people fire a single piece 3, 4 or even more times until they get exactly what they like. The only rule in multiple firings is that you can't re-fire at a hotter temperature than a previous firing, or you will burn off the lower temperature glaze..
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. The steam expands very rapidly into any small air pockets in the clay and shatters the pottery.
Here are the typical ways explosions occur in bisque firings: 1 - pot is too wet, placed in a bisque firing, temperature is brought to the boiling point too fast, water in the clay expands exponentially as it turns to steam, explosion occurs.
PLASTER IS THE ENEMY OF FIRING. It is most important that small pieces of plaster do not make their way into recycled clay because they will explode/spit out in the kiln once heated causing disastrous effects on pottery.