There is a lot of water in clay (around 20%). Therefore the evaporation of the water from the clay can be inconsistent. When one part of the piece dries more quickly than another it puts the entire structure under strain and thus causes cracks.
Air Dry Clay is primarily a water based product. The longer it is out of the packaging and in the open air, the more moisture will evaporate from it. Most often times too much moisture evaporating from the clay is the reason for cracks.
Clay has a memory. So in an effort to erase the memory of a crack, score the area in question deeper and larger than the crack itself (2), then place a bit of scored soft clay into the space you've made and compress it with a rib (3, 4). This will heal a crack in most cases, depending on how dry the cracked clay is.
In order to dry properly, air dry clay projects should be exposed to air on all sides at the same time. This will help prevent warping and cracking. We recommend drying pieces on a screen or cookie rack, for example, where air flow is the same on all sides.
It is sometimes possible to see very small cracks in the edges of a piece you have just finished. This is because, as you work with clay it dries out quite quickly. These tiny cracks create points of vulnerability. The reason for this is that the tiny cracks begin to dry out more quickly than the surrounding clay.
Sago Brothers modeling clay is very soft and smooth, no odor, doesn't crack, fade, or lose shape.
Crazing is the effect on pottery which causes it to have a web of tiny cracks over its surface.
DAS Air-Hardening Modeling Clay is good for beginners and skilled crafters alike. It isn't initially very flexible, but after some water and manipulation, it becomes more pliable. Because of the fibers in the clay, pieces created with it are sturdy and less likely to crack once dry.
Since clay is slightly alkaline, the acidity of vinegar makes it act like a glue for clay. Apply it to both surfaces and press the pieces directly together without slipping or scoring.
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.
Water will make your air dry clay soft again unless it is sealed properly, and the sun might cause cracking in your air dry clay especially if you are living in a very humid environment.
Because they're porous, they absorb moisture like a sponge. Changes in the temperature, or freezing rain and snow can cause them to crack as the clay expands and contracts.
Porcelain Clay
Revered as the best quality type of clay, and sometimes referred to as kaolin clay, it is the most expensive and hardest type of clay to work with.
Don't sculpt too thinly
Air-dry clay is actually quite sturdy if you make it at least 1cm (~0.4'') thick. So if sculpting larger pieces, you can build thicker walls - do that! Sculpt in layers to ensure that it dries evenly and creates stronger bonds between the tiny fibers in the clay.
Air-dry clay usually takes 24-48 hours to dry completely.
Baking air dry clay can cause it to warp or crack. It is best to avoid this method altogether if you can. If you need to speed up the drying process, try putting the clay in a dehydrator or desiccant chamber.
How Long Does It Take for Clay to Dry? Most air-dry clays will dry in about 24 hours to 72 hours when placed in a cool and dry space. However, some may take several days to dry, depending on what you are working on and the environment. Thicker clay takes longer to dry, and humidity can slow down the drying process.
The best option to provide a water-resistant, durable seal on air dry clay objects is to use a polyurethane or acrylic sealer.
Once your clay has dried, it may still have some bumps and cracks along its surface. Before you get to painting, you should smooth these imperfections out with low-grit sandpaper.
If it dries too quickly, the water in the clay can start to evaporate unevenly. Then one part of the clay is moving, and the other part is not. This puts pressure on the clay particles and causes the clay to crack. Slow drying is the best way to dry your clay evenly.
If you add too much water, fold the edges of the bag down and let it sit open in the sun so some of the moisture can evaporate. You can also do this if the contents are just too wet to begin with. Once you've got your clay well mixed, tie off the bag and let it sit for about 24 hours.
If the clay is heated too quickly, the water will turn to steam right inside the clay body, expanding with an explosive effect on the pot. By the time the boiling point of water (212 F and 100 C at sea level) is reached, all of the atmospheric water should have evaporated out of the clay body.