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.
Unfired clay is dried mud and has little value or practical use. The only purpose for Clay that has not been fired would be to put it on a shelf and look at it. But eventually, after a while, the clay would most likely crumble.
Remember that rushing the drying process can lead to problems like warping, cracking, or even exploding in the kiln during firing. So, it's crucial to exercise patience and allow the pottery to dry thoroughly before proceeding to the firing stage.
There is no time limit, however the longer greenware sits the more chance that it could sustain accidental damage from moving and handling. If you have a designated area that is out of the way the possibility is lessened.
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!
The best way to store your clay and keep it moist is by sealing it tightly in a thick plastic bag and placing it in a plastic storage container. This will help keep the moisture content of the clay and prevent it from becoming moldy.
As clay heats up the kiln, any moisture in the clay evaporates and expands rapidly. If there are small air pockets in the clay, the expanding steam goes into the gaps. This puts stress on the clay and can cause it to break or sometimes explode.
Skipping the bisque firing process can also cause the pottery to crack if the clay and glaze are not fired slowly enough and if the ware wasn't fired to the correct temperature in the bisque firing stage, some moisture will still be left in the clay.
Bone dry clay and slip repairs
There are two methods, one with slip. Slip is a clay and water mix. The other method includes white vinegar. We mix dry clay and white vinegar that we can use as a glue.
Bone dry: The final stage of clay drying. At that point, the clay has lost all water except for the water that is bonded at the molecular level. Pieces will be much lighter in color and weight than after forming and are extremely easy to break. Pieces that are bone dry are ready to be bisque-fired.
If clay goes into the kiln damp, it can break or explode when it is being fired. When moisture in the clay reaches the temperature of boiling water at 212F (100C) it turns to steam. As water turns to steam it expands quickly and massively. Steam is 1600 times the size of the water that produced it (source).
If the clay pieces begin to dry out, place them in a resealable bag and spray them with some water. Seal the bag and wait a few hours. The clay should be ready to use. If you have larger pieces that have completely dried, or old unpainted projects you want to reuse, you can still revive the clay.
Some products, such as Terra Cotta pottery, are left unglazed so they can remain porous and absorb water over time. However, this is not a desirable trait in an item intended for contact with food. Food-safe unglazed pottery falls into the category of Stoneware, Porcelain, and Ball clays.
Or you can use a technique called raw-glazing. This involves applying glaze to unfired greenware pottery. If you have looked into whether you have to bisque fire pottery, you may have read about single fire glazing. Single fire glazing eliminates the need for the bisque step because the pottery is only fired once.
The answer is technically yes, but it's not recommended. Ovens are not designed to get as hot as kilns, and the uneven heat can cause problems with the firing process. It's best to use a kiln if you want your clay project to turn out well.
Preparing the kiln:
Stack shelves with smallest work on bottom and largest on top. Kiln stilts work best in three's and when they are aligned from shelf to shelf, so the pressure is consistent. For a Bisque firing, greenware can be touching and even stacked. Glazeware should not touch at all.
It can sometimes help to sand your clay when it's bone dry to even out the surface.
Bone dry greenware is extremely brittle and will break apart very easily. Therefore, it should be handled as little as possible and great care must be taken when loading it into the kiln.
To summarize, there is a good reason why most people use two firings. You will have more loss with single firing. In addition to pieces cracking, you tend to get more glaze problems like crawling and pinholing.) For these reasons, most people believe that in the long run, single firing is not any more efficient.
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..
One of the risks of raw glazing is that the glaze can flake off the unfired pot. It can flake off bone dry and leather hard clay. However, there is a higher chance that the glaze will crack and flake off leather hard clay. The reason for this is that leather hard clay is still shrinking.
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.