Definition
Slip casting is a traditional making and manufacturing technique dating back to the 18th century, in which liquid clay, known as slip, is used to create and reproduce ceramic vessels. Slip is poured into a permeable mold, where liquid is drawn out of the slip, via a capillary action. A layer of solid slip (‘clay’) remains adhered to the walls of the mold. Once the required wall thickness of the casting has been achieved, the residue liquid is drained out of the mold. Once dry, the casting is removed from the mold.
Process
To create a ceramic piece using a slip casting process, liquid clay (slip) is poured into plaster molds fabricated from an original form. The slip forms a cast on the inner walls of the plaster as water from the slip is absorbed into the walls of the plaster mold. The thickness of the cast is proportional to the amount of time the liquid spends in the mold.
The type of clay used for casting is similar to that of prepared clay used by potters on the wheel. The key difference is that slip is deflocculated: a process by which the electrical charge of the clay particles is reversed. When deflocculated, the clay particles resist each other and slide more easily. The result is a liquid clay that weighs almost as much as prepared, solid clay but functions very differently.
Quality molds, in my case a series of recovered, vintage American made plaster molds from the 1960’s, and 70’s, and fine slip are critical to achieving a high-quality cast and for this reason, I make my own slip in small, manageable batches. How the slip behaves, which is largely a function of its consistency, depends on a range of factors such as temperature, humidity, and the ratio and distribution of the ingredients used in its formulation. The correct proportions coupled with the correct environmental factors create a liquid suspension that is a joy to cast. Liquefied and properly deflocculated clay is mesmerizing to make and witness. It’s almost magical, enchanting with its creamy fluidity.
An area of my small art ‘lab’ where I make the slip, glazes, and hand build my work.
Here’s a photo of what I found in that old building in Wyoming. Many were too degraded to be useful. Fortunately, there were two other rooms with molds in better shape. Some of which were so incredibly large it would require an industrial workspace, industrial sized equipment and insanely vast quantities of slip to pour. Most were much too large and heavy for my limited space.
This is my L&L e18T-3 set-up in 2025. I use a wifi extender that connects to my main router then to my mobile device so I can monitor firings via the KilnAid app. The security camera mounted in the lighting fixture allows me to keep a close eye on things remotely throughout the firing process.