Handbuilding Nesting Bowls with Hump and Slump Molds How to use hump and slump molds to make bowls of various sizes! Courtney Murphy

Nesting bowls are a project I have been meaning to take on for a while. I've always wanted to make a set of bowls that fit nicely together like Matryoshka nesting dolls. So, I've been trying to figure out the best approach—handbuilt or wheel thrown.

After seeing Courtney Murphy's nifty method for making nesting bowls, which I am sharing in today's post, I am leaning toward handbuilding. See what you think! - Jennifer Poellot Harnetty, editor

A few years ago, when I switched from cone 6 porcelain to low-fire earthenware, I was excited about the potential to work larger. I had come up with some ideas, and even made some sketches, but had never actually made anything very large.

I had been searching home improvement and salvage stores for years, looking mostly at lighting fixtures, but really any object with a relatively shallow continuous curve to make a mold from. Finally, a little over a year ago, I borrowed a giant mold shaped like a satellite dish from a friend. Using his mold, I created both a hump mold and a slump mold out of plaster. I was attracted to the gradual uninterrupted curve of this simple form, and the flexibility that it offered. It could be the basis for a large platter, a round-bottomed bowl or a rocking, boat-shaped vessel.

Now that I had this very large mold as a starting point, it was time to go ahead and try to create more significant pieces. I like the idea of things that nest or fit together, with each thing having its own particular place. I was also really drawn to the concept of nesting after years of constant moving—along with always packing and unpacking—for school, residencies, and other ceramic opportunities.

Forming the Bases

To create the bottom of each bowl in a nesting set, I start with circular templates cut from tarpaper. This material is durable and impervious to water, which makes it great to use over and over as a template for clay. Tarpaper can be found at home improvement stores, often in giant rolls, which are cumbersome, but will last forever. Using a protractor, I trace four concentric circles, each about two inches smaller than the next in diameter. Cut four circles out of a thin cotton bed sheet, each just slightly larger than the tarpaper circles. Since the bottoms are stacked during forming, so that each one conforms to the one below it, these fabric circles keep the stacked clay slabs from sticking together.

When rolling out slabs, rotate the slab so that it doesn’t get stretched out in just one direction repeatedly, which can weaken the clay. I roll out my clay between ¼- to 1/8- inch thick, compressing it with a rib. I then use the template to trace the largest (bottom) circle of the nesting set and to move it into the plaster slump mold, smoothing it into the curve of the mold with a rubber rib (figure 1). Working from the largest to the smallest, place one of the fabric circles down, then lay the next slab circle on top. Use the rib to conform each slab to the one below it and repeat the process until all slabs are placed and rounded (figure 2).

Building the Walls

Once the walls have reached soft leather hard, taper the top edge or rim first with a stiff rubber rib then with an elephant ear sponge (figure 3). Tapering the edges creates a visual lightness and sense of delicacy to the bowls.

After thinning out the rims, stand the slabs on edge to shape them into circles. The slabs should be stiff enough so they can stand this way without collapsing, yet soft enough that they are not in danger of cracking when curved. Keeping the slabs on the slightly softer side is also an enormous help when you’re blending the seams together.

After forming a slab into a circle, flip it over, placing the tapered top end down, and score the bottom as well as the attachment area on the corresponding base, then apply slip to both scored areas. When attaching walls to each base, work from the inside smallest piece outward to the largest so your hands have space to move around (figure 4). Use a fine-toothed flexible serrated rib to smooth the edges of the wall together. Join the two sides together at a 45° angle to enable the largest surface area point of contact (see figure 6). After the seam sets up slightly, add a thin coil to reinforce it (figure 5), then smooth the seam repeatedly with a series of stiffer to softer rubber ribs. This helps both to smooth the seam as well as to compress it and prevent cracks while drying. Continue to work from the inside out, attaching walls to each of the concentric circular bases. Since the largest bowl calls for a 52-inch long slab, which would be very challenging and awkward to work with, I piece together two or three shorter slabs to create the wall of the outermost pieces (figure 6).

After all walls have been attached to all bases, allow the piece time to set up. The entire process start to finish takes several days.

Once the pieces have stiffened to leatherhard, place the bowls into separate bisque molds (figure 7) (see box) and add a fairly thick coil on the inside to reinforce the seam, as well as to create a softer and smoother transition from the walls to the base of the piece (figure 8). Smooth the coil into the seam, first with a serrated metal rib to remove some of the excess clay, then a slightly stiff rubber rib. I’m a huge fan of the Mudtools ribs. The yellow rib number Y2 has the perfect curve to create a nice transition in the bowl where the base meets the wall (see figure 9).

At this point, the bowls should be stiff enough to pick up and place upside down to work on cleaning up the bottom. Placing the upside down bowl on foam (to protect the rim) on a turntable, very gently smooth the bottom of the piece with a rib (figure 9), filling in any indentations with very small coils of clay. Use a Surform rasp to clean up the outer edge where the wall meets the base (figure 10). After using the rasp, go back and smooth this area over with a rib. The pressure from this clean-up work sometimes creates a bit of a bulge on the inside of the pot. It’s important to keep flipping the piece over to a minimum, but if necessary, fix this bulge by adding coils or removing excess clay from the inside. Once all pieces have been cleaned up on both the inside and outside, place them all back together, leaving them this way overnight under plastic to equalize in moisture.

The next day, use a wide hake brush to apply slip to the outside of each bowl, when it is on the slightly stiffer side of leather hard (figure 11). Brush slip on only ¾ of the wall, leaving about 1–2 inches bare at the bottom. Due to all of the added moisture from the slip, allow the pots the chance to stiffen up back to leather hard before painting slip on the inside.

Using a dull pin tool or dull pencil, scratch drawings or surface decorations through the leather-hard slip (figure 12). Brush the excess burrs away with a soft brush once the drawings are closer to bone dry (this ensures that the burrs won’t get stuck in the drawn lines), then go back and color the drawings in with underglaze. Brushing a thin line with water-based wax creates a band of exposed terra cotta between the slipped surface and the terra sig layer on the bottom (figure 13). Although I cover most of the surface with either an opaque slip or terra sigillata, I like the underlying warmth of the earthenware, and I like a slight hint of it to show through in the piece.

Once the wax has dried, I apply terra sigillata to the bottom of each piece with a very soft brush (figure 14). I brush it on in two to three thin layers, or until it is just opaque enough that you can’t see the clay underneath. After it has lost its sheen, burnish the final layer with a plastic grocery bag wrapped tightly around your thumb. Bisque fire the bowls to cone 05 or 04, then brush a clear or transparent glaze over the slipped areas and fire to cone 02–03.

To see more of Courtney Murphy's work, visit her website at www.courtneymurphy.net.

**First published in 2012