If a wet clay piece is joined to a drier clay piece, the wet clay piece has more shrinking left to do than the drier piece, and so (as it does shrink more) it will create stress and want to crack away from the drier piece.
While clay is wet
Water can be added to Air-Dry Clay to soften or join pieces. If too much water is added and clay is very soft, allow excess to evaporate before molding pieces; otherwise, cracking may result.
For bone dry or greenware, don't just wet the surface. Rehydrate it by letting the water sink in for 2 minutes or so, actually reactivating the clay and the binder, making a lovely “self-slip” for joining.
If a wet clay piece is joined to a drier clay piece, the wet clay piece has more shrinking left to do than the drier piece, and so (as it does shrink more) it will create stress and want to crack away from the drier piece.
A final technique for encouraging good attachment of wet and dry clay is to wrap wet newspaper around the completed object, then wrap in plastic. This causes the moisture levels throughout the piece to even out, resulting in less cracking. Great for those darned feet that keep falling off that too dry pot.
As it is heated in the kiln, the residual water turns to steam and evaporates from the clay. However, if it is heated too fast, it may turn to steam while still trapped in the clay. This is what can cause the pottery to explode.
Particles of clay, when mixed with the proper amount of water, cling together in a soft, sticky mass. A lump of wet clay is said to be plastic because it can be squeezed or pressed into any desired shape.
Slip is clay with added water to make it into a paste or liquid. Use: Slip is most commonly used to join pieces of wet or leather hard clay together. It can also be used decoratively.
You can add paper clay to a piece of air dry clay that is completely dry.
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.
Repairing Bone-Dry Ware:
To do this, make a small amount of very thick slip with the same clay your piece is made of. In another container cover a wad of toilet paper with water and let it sit until the paper has broken down into pulp. You can speed this process by using an immersion blender.
As they get wet, the clay minerals absorb water molecules and expand; conversely, as they dry they shrink, leaving large voids in the soil.
Once your pottery has been designed and formed, it is vital that it is completely dry before firing. Failure to do so can compromise the end result, causing the pottery to become frail, cracked or even breaking once in the kiln.
No, you cannot bisque fire a pottery piece twice. Bisque firing is a low-temperature firing in which the clay is fired to a temperature lower than its final firing temperature. This process causes the clay to become more durable and less porous, allowing it to be glazed and painted.
Some potters will put clay in the kiln when it is a little damp. But if you do this, it needs to go a long pre-heat of around 12 hours. During a preheat the kiln needs to stay below 212F, so the water evaporates off before the firing schedule starts.
Step1: Preheat your oven to 200 degrees Fahrenheit and bake the clay for about 15 minutes. This will help to draw out the moisture from the clay so that it dries faster. Step 2: Another option is to put the clay in a dehydrator or desiccant chamber. This will help absorb the clay's moisture and speed up drying.
Air dry clay needs something to cling to and if the surface is too smooth then the clay will have a hard time properly sticking to it. But if you plan on using metal wires as a base for your sculpture then you won´t have any problems. Air dry clay will stick to it, no problem.
Bakeable Glues or Bonding Agents
But bonding agents and bakeable glues are generally recommended when bonding raw clay to baked clay or when baking baked clay to another piece of baked clay. While some products are specifically marketed for this purpose, there isn't much difference, chemically, between them.