Turn Up the Volume by Mark Cole
Paying attention to the visual beginning and ending at the foot and the lip of a pot is a fundamental necessity to the success of the overall form. This may include adding transitions that separate the foot from the body, leaving a little heft, and/or creating a linear element at the lip of a pot that provide a resting place for the viewer’s eyes. These formal qualities help define the visual elements of a piece as well as provide a physical stability to the overall form. But how does this visual information translate when using flat slabs?
Deciding the right thickness of a slab in relation to the overall size of the form can be problematic. When making smaller functional forms, the slabs can be either thick or thin, because even small hefty forms can be easily lifted. With larger functional forms, it’s a different story. If a larger form is too thick, it may weigh too much for reasonable use, and if it’s too thin, it may seem too fragile.
Keeping the limitations of weight (and therefore of thickness) in mind, constructing a sizeable and trustworthy functional form that incorporates distinctive visual elements can still be done in many ways. The solution I use combines slabs of uniform thickness and the constructional integrity of clay to create a visually substantial, hollow-rimmed form.
A slump mold provides a stable form a slab can conform to while firming up. Whether the mold is made of bisqued clay or plaster with either a flat or curved bottom, it allows a form to be safely moved around the studio. For the forms shown here, I used a bisque slump mold made from an unaltered, wheel-thrown form (figure 1). Both bisque and plaster molds draw moisture out of a slab, so monitoring the consistency of the clay is very important.
Place a slab onto a cut section of a cotton sheet, trim the slab to size, then use the sheet as a support to lift it up and place it into the mold. The sheet remains between the clay and the mold as a separator, making it easier to remove the piece later.
Inventing the Interior Shape
At this point, the overall shape of the piece may take on innumerable characteristics. Is it curvilinear or geometric? Will the walls be even in height or undulating? Will the width of the hollow rim be the same throughout or will there be a wider section? Once you settle on an overall shape, draw the location of the interior wall with a scoring tool and apply slip in the scored grooves (figure 2).
Using a metal ruler (as a template) and a fettling knife, cut strips of clay to make up the interior wall. Being careful not to bend or fold the strips, score and slip the bottom and sides of the wall and firmly attach them to the form. Reinforce the connections with a thin coil (figure 3). Cover the form and the mold with plastic and let it sit overnight so the moisture evens out. The next day, the form and interior wall should be approaching the leather-hard stage. Determine the location of the outer wall, draw the outline onto the form and trim away any excess clay. Then, shape the outer edge, score and slip each edge in preparation for the final slab (figure 4).
Whenever I decide on a curvilinear form, I typically also decide for the rim area to reflect this and create a rounded edge on the piece. To do this, use a large dowel and a soft rib to shape the flat slab strips around the dowel. Score and slip each side of the rounded clay strips, carefully attach them to the outer wall and then ease the strip over to the interior wall, making a firm connection. Use a pony roller and a small paddle to force these two connections together (figure 5), reinforce each connection with coils, and score the entire surface thoroughly (figure 6). Smooth the surface, first with a metal rib, then a medium hard rubber rib, and finally with a soft rubber rib, until all of the evidence of scoring has disappeared (figure 7). At this point, I cover the form and the bisque mold in plastic and let it sit a second night.
Working on the Underside
Taking the leather-hard form out of the mold and flipping it upside down onto foam, I am now ready to finish the bottom of the piece (figure 8). I first refine the texture left behind by the cotton sheet with a soft rubber rib to ensure its smoothness. Note: Canvas leaves a very particular texture that requires more pressure to smooth out and pushing too hard during this stage can distort the form. The residual canvas texture has an unintentional effect to the glazed surface A cotton sheet leaves a less pronounced texture that’s easier to remove with a rib.
I use a needle tool to poke a hole into the hollow section. After all the compression due to ribbing, rolling, paddling and shrinkage, you’ll undoubtedly hear a hiss as this releases pressure built up inside the form.
There are many different types of feet to experiment with when working with slabs. I use four understated feet made from small clay lugs that elevate the entire piece off the table, allowing me to glaze the entire form. The only unglazed part of this piece will be the four places where it sits on the kiln shelf.
Attach the clay lugs or similar feet in the location where they will best support the piece and smooth each one into the form (figure 9). Start smoothing with your finger, then with a metal rib, then with a medium hard rubber rib, then with a soft rubber rib, and lastly with a chamois cloth (figure 10). Let the piece dry slowly over the next three days.
When bisque firing these pieces, spread grog onto the shelf, and create four piles to the outside of the foot areas before placing the pieces. The grog allows the piece to shift and shrink during the firing without added stress, and, in the case of the piles, supports the piece during the firing. If the feet were not able to move freely and one snagged on the kiln shelf, a crack could form. If not supported, the form might sag.
Glazing: More is More
Embellishing the form’s surface with glaze provides a seemingly endless array of potential designs, patterns, divisions of visual space, and glaze interactions. When glazing pieces, I apply one glaze at a time and use wax resist to mask off any desired glaze patterns. If using petroleum-based wax, which flows nicely off the brush and dries firm, let it dry overnight after each application.
Use a pitcher to pour glaze over the entire form as evenly as possible. Let the glaze air dry completely before applying wax. For more complex patterns, I sometimes draw onto the delicate glazed surface with a pencil very lightly, then follow this line with the wax. When the wax is completely dry the next day, I take a wet sponge and wipe away the excess glaze not covered by the wax. The strength of petroleum-based wax is an asset when wiping with a sponge. Paraffin wax and water-based liquid waxes do not seem to stand up. Also, waxed edges can be carved away with a fettling knife to neaten the line quality if necessary (figure 11). This process repeats with each glaze, so they’re positioned side by side. The combinations are based on anticipation of the eutectic formed where the glazes meet and melt (figure 12). When the last glaze is applied, the only steps left are to make sure to wipe any glaze beads that remain on the waxed areas and wipe any glaze from the feet.
I fire these forms directly on a very flat kiln shelf and let any deformation aid the overall form’s ability to sit flat on a table without rocking back and forth. If a piece is not level, I simply sand down one of the feet with a palm sander until it sits properly on all fours.
When making larger pieces, considering the form’s visual weight becomes very important. Making hollow sections provides a substantial looking rim and wall that weighs much less than it would if it was solid. The extra surface area offers a clear visual separation between the inside and the outside of the vessel and presents a voluminous area to explore innumerable decorative techniques and qualities. n
Mark Cole is an assistant professor in the ceramics department
at the University of Nebraska-Lincoln. To see more of his work,