In ceramics, deflocculants are materials that you add to your slips and glazes to make them smoother, flow a little better, and even apply a little better. How, exactly, do they work though?
Sometimes when a dry, powdery material is placed in water, it dissolves evenly, creating a homogeneous solution. Other chemicals, however, will not dissolve completely, but may form clumps or settle to the bottom. In ceramics, and especially in slip casting, clumps and non-homogenous solutions can be problematic. If you are pouring liquid slip into a mold, you want the mixure to be uniform and to flow well. Deflocculants allow this to happen by decreasing the positive electrical charge that can occur between particles that become attracted, or 'flock', to one another. The addition of a deflocculant to a medium, such as casting slip, evenly suspends the particles and causes a more fluid flow.
If increased flow is the goal, why not just add more water? Why use a deflocculant? The advantage of using a deflocculant over water is that the addition of water will alter the shrinkage rate of your slip, or the application properties of your glaze. The more water, the higher the shrinkage rate of your casting slip, and high shrinkage is typically not so desirable. The more water, the thinner your glaze application will be, and that may not be so desirable, either.
In ceramics, there are a number of different deflocculants available.
Darvan is a liquid deflocculant that is often preferred for use with plaster molds because, unlike some other deflocculants, Darvan does not degrade the plaster. There are different varieties of Darvan. Darvan 811 is a deflocculant that is similar to Darvan 7, but is more powerful. It takes only a small percentage to deflocculate slip. Darvan 7 is typically preferred with porcelains and high iron content clay, while Darvan 811 can be used for stoneware, high fire slips and red low fire slips. Sodium silicate, also referred to as liquid glass, is another commonly used deflocculant in slip preparation. In ceramics, sodium silicate can also be used in surface decoration to create a crackled surface texture on pots.