There are commonly considered to be 7 stages of clay that it can exist in between being raw clay and a finished completed ceramic object.
The stages are related to the amount of water the clay contains (its dryness), and its status with respect to firing. The use and preparation of each of the stages of clay are described below.
Note: This article relates to pottery clay rather than polymer clay.
1. Dry clay stage
Dry clay in its raw state. Dry clay is easy to store and to ship since it is light, and doesn’t go bad (unlike wet clay).
You can buy your clay in its dry state and mix it with water to prepare it for use. Many potters like to do this as it allows them to create their own mix with grogs or other additives to create their ideal clay body.
2. Slip stage
Slip is basically watered-down clay. It has a consistency between that of paint, and of toothpaste.
There are many uses for clay in the slip stage. Some common ones are:
- It is used as a glue to attach pieces of clay together, such as attaching a handle to a mug, or sticking slabs together when handbuilding.
- It can be used to decorate, like a glaze.
- It can also be used for casting, using a plaster mold.
Slip is easy to prepare, simply mix clay and water together until the desired consistency is achieved.
3. Plastic stage (aka wet clay)
Plastic clay is drier than slip, and easy to mold and model. It can be worked and will hold its shape. The clay feels cool to the touch as it is still a mixture of clay and water.
This is the best stage for most types of ceramics and pottery making: throwing, pinch work, coiling, etc.
You can prepare plastic clay from dry clay (as described above). Just mix dry clay with water, and then wedge it thoroughly.
Slip can be turned into plastic clay by removing some of the water. This is commonly done by pouring it onto a plaster surface and leaving it until enough of the water has been absorbed that it becomes workable. Wedge the moist clay on plaster to remove more water if required.
4. Leather hard stage
The leather hard clay stage occurs once the clay starts to dry out. The clay has stiffened up and can now be handled without leaving marks.
It feels cool to the touch and is not able to move much without cracking. It has the consistency of hard cheese or stiff leather (hence the name).
This stage is the best for carving or turning the clay object, though this form of clay can also be used for slab work if it is still in the early stages.
As the clay becomes drier the type of finishing work that can be done will become more delicate as the clay becomes more fragile.
This clay stage does not need any preparation other than careful control of the drying process following the construction of an object during the plastic stage.
You can slow down the drying process by wrapping the clay in plastic, or some pottery studios often have damp cupboards that will slow the drying process allowing the potters more time to work on their creations.
5. Bone dry stage of clay
When clay is at the bone dry stage all water has evaporated and is ready to fire.
Clay at this stage appears much lighter in color than at the previous stage, and will no longer feel cool to the touch. Bone dry clay is very fragile. Clay at this stage is sometimes referred to as greenware.
This is the last stage during which you can recycle your clay. Once it is fired you cannot revert it to any of the earlier stages of clay described above.
You can apply underglaze to your pottery at this stage, but it must also be allowed to fully dry out before firing.
To get clay to the bone dry stage it can just be left to dry out naturally.
The time it takes to air dry clay varies with climate. It can take a week, or sometimes even longer, for leather hard clay to reach the bone dry stage.
A word of caution: clay that is fired before it is bone dry will explode in the kiln, so it is important to ensure that clay actually is bone dry before firing.
6. Bisqueware stage
The bisqueware stage is clay that has been fired once in the kiln.
Bisque firing removes any chemically bonded water in the clay, and also any impurities. Now it can never turn back into wet clay as it has undergone the ceramic change.
Bisqueware is the best stage of clay for applying glaze. Clay at this stage is much harder than clay at the bone dry stage, but it is still porous enough to absorb the glaze.
The glazeware stage of clay is clay that has been fired again with glaze.
Glazing firings can be one of three temperatures: low- mid- or high-firing. It is important to know what type of clay you have and match the firing to the temperature of the clay.
Low-fired clay fired too hot will melt in the kiln, whereas high-fired clay fired too low will not vitrify correctly, and may not be waterproof.
So there you have it, the complete guide to the stages of clay. Each stage has its own properties and uses.
The different states of clay are important to understand because they dictate how the clay will behave and how it can best be used to create beautiful and lasting ceramic objects.
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