Clay cracks primarily due to rapid heating/cooling (thermal shock) and internal stresses from moisture or mineral changes, especially around 100°C (steam), 573°C (quartz inversion), and 220°C (cristobalite inversion during cooling), but also from rushing chemical dehydration (around 450-600°C) or even freezing (below 0°C). The specific temperature for cracking depends on the clay type, moisture content, and firing schedule, but rushing through these critical points is the main cause.
Between 480–700ºC chemical water (“water smoke”) is driven off. Upon cooling, cristobalite, a crystalline form of silica found in all clay bodies, shrinks suddenly at 220ºC. Fast cooling at this temperature causes ware to crack.
Heat can dry out your clay and harden bags into bricks; cold can freeze it and cause expansion cracks.
After 800°C, the clay surface will start to seal off, trapping unburned carbonaceous materials and sulfides, which could cause bloating and black coring. Quartz inversion occurs at 573°C. When clay is refired for a glaze firing, quartz crystals change from an alpha (α) crystal structure to a beta (β) crystal structure.
Properties. High-grade fire clays can withstand temperatures of 1,775 °C (3,227 °F), but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1,515 °C (2,759 °F). Fire clays range from flint clays to plastic fire clays, but there are semi-flint and semi-plastic fire clays as well.
But I assure you that any properly formulated and fired claybody can absolutely handle going in and out of the oven 450-500 degrees to room temp. If your wares are overvitrified they become very brittle and likely to fracture from temp change like pouring in boiling water or in the oven.
As the temperature increases, organic material burns away, and chemical changes begin to occur in the clay body resulting in new crystalline structures. Temperatures upwards of 2,300°F cause sintering, which fuses particles together.
* Generally, don't open the kiln completely until the kiln temperature is below 150-250 degrees F, or thermal shock may hurt the pots and/or the kiln elements. You should be able to touch the pieces before you unload them.
Terracotta clays, on the other hand, are meant to retain their porous, earthen qualities. Terracotta is fired at much lower temperatures, typically between 1700-2100°F. This allows the clay to harden while still preserving its warm terracotta color and slightly coarse texture.
The acidity of the vinegar breaks down the clay a bit, and makes it sticky. Some artists use vinegar straight from the bottle, or add vinegar to clay instead of water to make a joining slip. All these methods work to create a join that is stronger than water or slip alone.
Avoid direct sun, heaters, or drafts that desiccate the surface too quickly. Drape plastic wrap or damp cloths loosely over work if drying too fast. Turn pieces regularly as they dry to prevent uneven stresses. Consider investing in a humidifier to add moisture back if needed in very dry conditions.
Leaving terracotta pots out in the winter can cause damage. There ARE steps that can be taken to help prevent your pots from cracking when old man winter whips through your garden. The goal is to eliminate the possibility of water being absorbed, frozen and thus causing the pot to crack.
This is air conditioning for the plant in the heat of the summer, but in the freezing winter it spells disaster for your lovely pot. Water in the microscopic pores of the clay expands as it freezes, resulting in slabs of pottery flaking away from the outer sides of the pot. This phenomenon is called spalling.
It is okay to place the kiln in an unheated building in cold weather. Avoid small, enclosed spaces such as a closet or small utility room. The kiln room must be large enough to avoid heat buildup around the kiln.
There is little control over creating an even temperature and in the case of a domestic oven, the temperatures are not sufficient to create glazed pottery. The Raku and Pit Fire methods also take a considerable amount of time to achieve the desired results and can be dangerous if not controlled properly.
For example, particular potters may remove the plugs from the peepholes when the kiln temperature reaches approximately 700°F. They may then crack the kiln lid open slightly, usually about an inch, once the temperature drops to around 400°F. This lid cracking is often facilitated by using a fire brick or a kiln prop.
Many potters are moving from cone 6 (2200F or 1200C) down to cone 5 (2160F or 1180C). The energy savings by firing 40 degrees F lower is considerable and there is much less wear-and-tear on the elements in electric kilns (as kiln elements get old the kiln can really struggle to climb the last few degrees).
Low-Fire Pottery is fired at the lowest temperatures, ranging from between 1,700°F to 2,100°F Cone 04-06 (900°C to 1,150°C). Mid-Range Pottery is fired at intermediate temperatures, typically between 2,100°F and 2,300°F Cone 5-6 (1,150°C to 1,260°C).
Leave any damp clay in a dry, warm location for a full day to let it dry completely. The clay should feel dry to the touch to ensure it dries correctly. Avoid putting damp clay in the oven, as it may crack during the firing process, risking your hard work.
That said all clays will bloat, even porcelains, if over fired. Firing Temperature: Bloating is more likely to occur at certain temperatures, typically in the middle to high range of the firing process. It is often associated with over-firing. Track your temperatures with cones or a thermocouple.
Firing Clay
Most types of clay are fired successfully within the temperature range of many kilns, which is 2000°F and 2400°F, apart from porcelain, which requires higher temperatures. If clay is fired at too high a temperature, it can change shape or melt.
During this process your clay is quite vulnerable. If the heat increases too quickly, the water turns into steam faster than it can escape the clay walls, creating pressure within. This results in cracks, or even explosions.
Did you know, the temperature in a cement kiln exceeds 2,642 °F or 1,450 °C? 🔥 That's hotter than molten lava when it erupts from a volcano!
The time it takes to let clay dry varies with the weather and environment where you are working. You will notice your work dry faster on the hot dry summer days and slower during the raining season. Most pieces can dry completely when left uncovered for three to four days.