STATES OF CLAY
Clay is a liquid solid: It acts as a liquid when hydrated and in motion but as a solid when it is still. This is why it is easily formed and can hold its shape.
In its raw state (before it has been fired), clay can be powder, liquid (slip), plastic, leather hard, and bone dry. In order to work with clay, it is important to understand the consistency of the material as it progresses through its various states.
Each of these five states allows for certain manipulation but also has limitations. The following information is important to read through carefully so that when you begin to work with clay you will have a point of reference and understand each state’s inherent qualities. A clay’s consistency is determined by moisture content, plasticity (ability to be manipulated and hold the shape), and ability to endure the stress from the shrinkage during drying. These are the three most important features to pay attention to when working with unfired clay.
In this blog we will be talking about the powder, liquid (slip), and plastic states of clay. Next week we will cover the leather hard and bone dry states of clay.
POWDER
Clay powders can be mixed to formulate clay bodies as well as added to glazes. They are mixed with water to create clay.
SLIP
Slip (which is also called slurry – liquid) refers to clay that is super saturated with water and is runny and slippery. Slip cannot be used to hand build or throw pots on the wheel; however, it can be poured into molds (slip casting). Toilets, sinks, commercially made dinnerware, housewares, and other industrial ceramic products are made by using specifically formulated slip. These slips have a specific shrinkage rate, density, and firing temperature. They are poured into plaster molds, dried, cured in a controlled environment, and then fired.
Ceramic artists pour slip into molds to create multiples of a specific form. Potters and hand builders use slip to lubricate clay on the wheel and to attach leather-hard clay pieces together. For example, if you want to attach a handle to a mug, put a small amount of slip on the mug where the handle will be attached. Then place the handle on top of the slip. When two pieces of clay of different moisture content come together to be joined, the slip hydrates both pieces thus allowing the clay to shrink and dry evenly, reduc ing the chance of developing cracks.
PLASTIC
Clay in the plastic state is used to throw pots on the wheel, roll coils for coil building, pinch pots, extrude shapes, and roll out slabs for slab con struction. Clay will hold its shape in this state, but it is susceptible to oversaturation and the effects of gravity. Clay particles are flat and vary in size. Some clays have many different-sized particles that are bonded by water. When the clay is thoroughly mixed with just enough moisture, it is considered plastic and can withstand forming without deforming and can be stretched and thrown with great ease.
Plasticity is the most appealing and versatile state of clay. It refers to the responsiveness of the material to touch. Ceramic artists often will use the terms short, tight, greasy, very plastic, or as having tooth (strength and grittiness from grog) to refer to the properties of clay. Stonewares tend to have a lot of tooth because of the particle size variation and the addition of sand or grog. Porcelain is an example of a clay body that has much larger and uniform particle size and there fore is less tolerant of too much manipulation. It has less plasticity than stoneware and is susceptible to over saturation and collapsing.
If you have purchased clay that doesn’t seem workable, try adding some water to the bag of clay and leave it overnight. If the clay is stiff, wrap the clay with a moist towel and place it in a plastic bag overnight. Check to see if the clay is then workable or if it needs more hydration.
One way to make your clay more plastic is to wedge and knead it. By rotating the clay into itself, particles become aligned and the water molecules are spread evenly, creating a homogenous mass of clay that can be easily manipulated and will hold its form. Wedging is an essential part of working with clay.
Another way to make your clay more plastic is to age it. Potters will allow clay to sit in a plastic state for a period of time. Whether you buy or mix a fresh batch of clay, it may take a couple of weeks for all the particles to become evenly hydrated. Organic matter in clay and the water create a colloid, a suspension of small particles of clay displaced in water, that impart plasticity to the clay.
The late Doc Crespi, a professor at Southern Connecticut State University, often told a story about people adding a bottle of beer to a fresh batch of clay because yeast encourages plasticity (and a strong stench). He also shared that in some cultures clay bodies are mixed years in advance for the next generation of potters.
