Don't build thicker than 1 inch. It's possible to fire a whole 25 pound bag of clay without explosions. But it takes some patience and a very long kiln firing time. But for most projects, less than 1 inch of clay thickness is a good rule of thumb.
It's often said that as a rule of thumb your pottery should be no thicker than your pinky finger. The reason for this guideline is that clay contains moisture. Even bone dry clay which looks and feels completely dry will still contain a little water.
The ideal maximum thickness for hand built pottery is 1-1½ cm. Radical variations in the thickness of pottery items will promote uneven heating and therefore, cracking. Keep checking the thickness of your piece as you go and rectify unevenness before the piece dries hard.
Thick Walls Can Make Pottery Explode in The Kiln
And this is considered to be a maximum. Relatively thin walls are better if there is any residual moisture left in the piece.
Glazes having sufficient clay to produce excessive shrinkage on drying will crack (and crawl during firing) if applied too thick. Fluid melt glazes will run off ware if applied too thick.
Cracking is normal in air dry clays: it's caused by shrinkage because of the loss of the water inside the clay body. Cracking in air dry clay is typically caused by sculpting over an armature or using a lot of water, either to mix the clay or to help it adhere onto a previous layer.
Since air bubbles in the clay can cause pieces to break when fired, encourage students not to overwork the clay, and keep this in mind when recycling extra scraps. Try to keep finished pieces not more than about an inch thick. If too thick, the clay may not be able to dry completely and can break when fired.
Slow and low is the only way to go with a solid Sculpture. Most Potters I have talked to said the same thing. If you don't fire slow and low, she will crack or even explode.
Dunting is a special type of crack which occurs from stresses caused during firing and cooling. These stresses primarily occur during two critical points of firing called silica inversions which occur at 1063 degrees F (573 degrees C), and 439 degrees F (226 degrees C).
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.
Intuitively, we are comfortable with clay bricks as a fire resistant material because of their use as a liner in fireplaces and kilns where temperatures exceed those expected in most construction fires.
Rolling the clay out too thin can lead to cracking and breaking once dry. I have found that about a quarter inch thick works well. It is thick enough that it is less likely to break, but still thin enough that it won't increase drying time too much.
Keep your clay pieces no more than the thickness of your thumb. This will allow the clay to dry and fire properly.
For example, many soils contain a relatively low amount of clay in the surface layer, a higher amount between the depths of 25 to 75 centimeters (10 to 30 inches), and a decreased amount below a depth of 100 centimeters (40 inches).
Starting with the finest, clay particles are smaller than 0.002 mm in diameter. Some clay particles are so small that ordinary microscopes do not show them. Silt particles are from 0.002 to 0.05 mm in diameter. Sand ranges from 0.05 to 2.0 mm.
Be careful not to roll out your clay too thin. This can lead to cracking and breaking as it dries. I've found that rolling the clay to about a quarter of an inch works well. It's thick enough not to break but not too thick that it won't dry.
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.
Sago Brothers modeling clay is very soft and smooth, no odor, doesn't crack, fade, or lose shape.
Meanwhile, air dry clay has the tendency to dissolve in heat or water. Applying a glaze to air dry clay is one way to combat its tendency to dissolve in heat or water.
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.
While it's not possible to glaze air dry clay in the traditional way using a kiln you can use varnishes and sealants to create a glazed pottery effect. Unlike traditional clay, air dry doesn't need to be fired to set hard.
Raw clay should not be thrown away, it can be used, reused and reused again... The important part to mention, is that only greenware clay (clay that hasn't be bisqued or glaze fired) can be recycled or reclaimed. Once clay has been fired, the chemical composition has changed and the clay can no longer be recycled.
Conversely, too much moisture can cause the lump clay to become too sticky or too wet to work with. The metal clay industry calls this over hydrated phenomenon slip or paste, and there is a use and a need for these products, but when lump clay is needed, too much moisture is problematic as well.
When the clay content exceeds 40 percent, poor drainage and compaction become an issue. Amending the soil helps you improve the structure to better support plant growth.