Tips and Tools: Gravity Filter Press Henry James Haver Crissman

Appears in the February 2016 issue of Ceramics Monthly.

One of the best ways to thoroughly mix a clay body is to mix it into a slip or slurry and then dry it out. A home-built gravity filter press is an effective tool to turn that slip into a workable clay body.

While in North Carolina, this past summer as a resident artist at STARworks Center for Creative Enterprises, I was fascinated with the prevalent use of local clays by area potters and by their systems for refining it. After the raw clays were dried out, they were mixed as slips, passed through a screen or a series of screens, and then blended with other clays to make the potters’ custom clay bodies.

Mixing these materials as a slip allows them to be readily passed through screens and it also addresses the biggest issue when making any clay regardless of whether or not you’re using local materials—the difficulty of thoroughly mixing it. The simplest way to mix materials thoroughly is by blunging them together as a slip, but then you have all of this slip, and how are you supposed to dry it to a workable clay state? You could dry your slip on a plaster block, but plaster dries your clay unevenly, and if it’s already saturated with water from a previous round of slip drying, you have to wait until the plaster dries out.

The Solution?

Clay drying beds—or the gravity filter press, as I like to call it (1). Much like a traditional filter press would pump pressurized clay slip into a cloth, pushing water out through the cloth, not allowing the clay to pass through, and quickly leaving you with workable clay, the gravity filter press accomplishes the same result with just a little more time and far less technical equipment. Instead of pumping the slip in, you just pour the slip on top of a suspended cloth and watch as crystal clear water drips out the bottom (2). With a rudimentary wooden frame, some chicken wire, an old bed sheet, some plastic, a bucket, and a few thumbtacks, you’ll be thoroughly mixing and drying your slip or reclaim into a usable clay body like the pros in no time!

Many of the North Carolina potters had drying beds larger than 4×8 feet, but with limited space and resources, I built mine to stack on top of each other using the wood from a couple of old pallets. Their drying beds were often outside, with the draining water from the clay just falling to the ground. Being indoors, I stretched a sheet of plastic under mine that directs the water to drip into a bucket on the floor. The bed sheet I use has a standard thread count and it would work just fine as a single layer, but I’ve doubled it over for strength. The slip seems to dry most evenly when it is no deeper than 2 inches thick in the drying bed (3). With more heat and/or air circulation the process is sped up and you can more readily dry a deeper layer of slip.

This can be a great tool to use when making a fresh batch of clay or to reconstitute your reclaim. Now get out there, mix your clay bodies as a slip, maybe get some local clays involved, and enjoy your perfectly blended clay!

the author Henry James Haver Crissman currently resides in Alfred, New York, and is the studio manager at the Wellsville Creative Arts Center in Wellsville, NewYork. Send your tip and tool ideas, along with plenty of images, to editorial@ceramicsmonthly.org. If we use your idea, you’ll receive a complimentary one-year subscription to CM!