The slip casting of clay is different than other clay processes.  Slip is liquid clay that is used to pour into molds for hollow sculpture. When you pour the slip into a mold, the walls of the mold absorb the moisture and create a casting. Fine grade clays are used to make slip so that they do not have any gritty particles, and they are either high fire porcelain bodies, or low fire talc bodies. There are several properties to slip that do not apply to molding clays.  One is viscosity  which is the measure of the thickness of a liquid.   You can measure the thickness of a liquid with  any container you want to with a hole in the bottom,  and  measure the amount of time it takes for this liquid to drain out of this cup. That's how you measure viscosity. The second factory is the slips gravity, which means it's weight relative to water.  So, the viscosity is really important because we need it to be fluid enough to flow in and out of the mold easily. And we adjust the viscosity by not just adding a bunch of water because we don't want to super saturate these molds, we want to add just enough water to make it liquid and then we add a material called a deflocculent, which expands the water molecules, and makes it more liquid without adding water.  So, you add just a tiny bit of these deflocculents to moistened clay powder and mix until it becomes really liquid.  These are the most critical elements of slip casting.