Q: What is the difference between slip and clay? Can I make slip just by adding water to clay?
A: Clay is composed of microscopic plates, which cling to each other’s flat surfaces. This gives it its characteristic plasticity—the quality that makes it hang together as it is being worked. The difference between slip and clay is the presence of a “deflocculant.” This is a chemical agent that is added to clay powder, along with water, which reduces the plasticity of the clay so that it becomes pourable with the addition of less water than would otherwise be the case, and reduces the tendency to form lumps. It does this by reducing the attraction that the tiny flat plates have for one another.
The deflocculants most commonly used are sodium silicate (waterglass) and sodium carbonate (soda ash), often in combination. Just a little is needed: one third of one percent of the weight of the dry ingredients. Using too much will accelerate the deterioration of your plaster molds. Mix the deflocculant into the water first, then add the clay. The water will weigh between a third and a half as much as the clay that goes into it.
Mix dry clay powder into the water/deflocculant until it won’t accept any more without getting too thick to pour or forming lumps. Try for a consistency like heavy cream. This makes a slip that will dry on the inner surface of the mold quickly, and will release without warping, cracking or sticking. Do a small-scale test before mixing a large batch, in order to get the proportions right.
Some clays won’t deflocculate properly at all, and this is good to find out. One can buy dry clay formulated especially to work as a casting slip. For small projects, it is often easiest to buy slip already mixed at a ceramic supply.