Staying in Shape Jim Wylder

Appears in the May/Jun 2015 issue of Pottery Making Illustrated.

All of my high-school students were struggling to make globe forms on the pottery wheel. As I studied their efforts, there was a pattern to their failure. They would throw a cylinder and then attempt to belly out the middle. As they pushed the middle out, the rim always turned into an oval, the pot wobbled, then collapsed. We needed to figure out how to keep the rim from distorting.

The first attempt was a modified one-gallon mustard jar. A 1-inch section was cut off of the top of the jar, then inverted and put inside the rim of the thrown cylinder. The cylinder was thrown against the plastic top to keep it in place and hold the rim’s shape, but it was too flimsy, distorted easily, and stuck to the clay. The second attempt put the plastic top on the outside of the thrown cylinder, which proved to be equally problematic. After numerous attempts with other materials we had a breakthrough. The answer was a wheel-thrown bisqued piece (figure A) similar to a bracelet. The best ones were shaped like a short funnel about 1 inch high, 4 inches in diameter, and ¼ inch thick. A rib needs to pass through the opening and that determines the inside diameter size. We dubbed it the rim keeper. The bisqued piece released from the clay without any trouble because it pulled just enough water out of the rim to be separated from it.

Second Generation Rim Keeper

Today the rim keeper has evolved into a plastic model (figures B and C). The bisqued rim keeper was not quite perfectly round because clay often distorts slightly while drying and firing. The plastic rim keeper is slightly lighter in weight and is perfectly round. To keep it from sticking to the wet clay, I line the rim with a band of thin dry-cleaning plastic.

How It Works

The rim keeper fits inside the rim of a thrown cylinder and keeps it from distorting while being expanded into the globe form. When the clay is being ribbed out on the right side, your forearm is against the left interior of the rim keeper. The rim keeper remains centered above the base and allows a much more aggressive push out with a rib than normal. If this was done without the rim keeper, the rim would distort and the pot would collapse. A side benefit is that the base isn’t being stressed by being moved and so it retains its strength and supports the form. The angle of the cylinder wall at the base set in the beginning starts the globe’s direction. As the globe expands outward the rim moves down. If you’re able to keep the rim keeper from moving sideways, the only clay affected is the part touching the ribs on the inside and outside of the form. The remaining part of the globe will maintain form because it hasn’t moved (remember those physics lessons?) Once the bottom is ribbed, everything else will try to move with it. For this reason, it’s best to expand the bottom third of the pot last in each cycle and not at all when finishing the rim.

Pueblo potters have used a similar technique for centuries but on the bottom of their pots. A shallow bisque bowl supports the base of their coil-built forms and facilitates handling. They use the bowl almost every time. Putting a rim keeper at the top of the pot is an extension of their idea.

Prepping the Form and Setting the Rim

Clay preparation is important. I’ve used four different clays with this method, including porcelain, and each one had shorter or longer firming cycles and slower or faster expanding cycles. Be careful not to start with clay that’s too soft.

Using a plastic bat on the wheel head, center and open 6–8 pounds of clay, then stand up for the rest of the throwing. A standing position provides a mechanical advantage as your forearm becomes a lever when expanding the globe.

Throw a cylinder as tall and thin as possible. Clean off the slurry from the inside and outside wall with a rib, then sponge the water out of the bottom.

Use a stiff rib on the inside of the bottom of the cylinder and a stiff rib on the outside of the bottom to create an angle of about 20° (figure 1). Force the clay against the outside rib from the base inside. If your base is ½ inch thick, the angle on the outside will start ½ inch above the bat. This step provides a starting point of soon-to-be-firm clay that will support a globe form. Adjust the rim’s inside diameter to 4½ inches then smooth and recenter the rim (figure 2). A small amount of cylinder wobble is acceptable. The cylinder will be put back on center within the first few turns after the rim keeper is in place. Let the cylinder firm up but not dry out. Heat guns and propane torches can be very troublesome, but a fan set on low while the potter’s wheel is slowly spinning can be very helpful in the firming stages.

Cut a strip of thin plastic about 2 inches wide and 20 inches long. Wrap plastic around the rim about one inch down the outside and then fold it over the rim to the inside (figure 3). Ignore the wrinkles. The plastic keeps the rim clay soft and allows removal of the rim keeper later. Gently set the rim keeper in the rim (figure 4). This may require throwing the clay into the rim keeper while the cylinder spins at a low speed. The threads of the plastic rim keeper will grab the inside of the rim and hold it there.

Expanding the Form

In order to be successful throwing this form you need to follow a rhythm of throwing and fan-drying cycles. The rhythm should go something like this: throw the cylinder/ 20 minutes fan drying/ expand to a globe/ 20 minutes fan drying/ expand to the final form/ 20 minutes fan drying/ throw the rim. Most pots will take about two hours to complete. Note: When in the drying cycle, use a fan pointed directly at the pot from a few feet away and set to a slow speed while the pottery wheel is slowly turning.

Start by holding one rib against the outside of the cylinder and one against the inside so they are parallel and in line with one another. The outside rib catches the clay pushed from the inside and lightly compresses the surface. The inside rib controls the form. Expand the pot at a medium wheel speed. Don’t add water. Start about 1⁄3 of the way down the pot and push the clay out from the inside. A ½-inch push out to the side with the inside rib is a good place to start. Once the electrical bond between clay particles is broken with the pressure of the rib, the clay will be much more plastic. Move toward the rim, sandwiching the wall of clay between the ribs. Do this several times until the clay angles out about 30° from the bottom of the rim keeper (figure 5). This angle is what keeps the pot from collapsing and the rim keeper from shifting. You should now have a barrel-shaped bottom transitioning into a globe form on the top.

Now begin to work your way from the center to the bottom, pressing out with the rib on the inside of the pot (figure 6). Stop when it approaches a balloon form or begins to show signs of strain. Leave the rim keeper and plastic in the rim and let the form firm up. Repeat the throwing cycle. Change ribs as needed to control the form, using smaller ribs on the inside to show more throwing lines on the outside. The clay will conform to the profile of the inside rib so if you want a 6-inch curve use a 6-inch rib. Using a small rib on the inside will create some unevenness on the outside but a larger 4-inch or 6-inch rib used on the inside will clean up the curve (figures 7 and 8).

When the body of the form is completed, let everything firm up once more, then remove the rim keeper and plastic (figure 9). If the rim keeper sticks, simply unscrew it or try holding it while spinning the pottery wheel in reverse.

Throw the rim to its final opening size or cut it with a needle tool in preparation for a lid (figure 10). You can use water when throwing the rim in the last cycle but be sparing. Water flowing on a thin wall can cause havoc very quickly. Note: I use a homemade wooden brace to keep my stands steady when throwing taller forms (see “Control at the Top,” Ceramics Monthly, June/July/August, 2014).

Trimming and Finishing

Trim like you usually do at the leather-hard stage. A bisque-fired chuck or Giffin Grip works well. I like to brush on underglaze then burnish my forms. I bisque fire then finish them with a naked raku technique.

Final Thoughts

These pots take less clay to make, dry faster, use less energy to fire, are more forgiving to handle in the greenware stage, and are very pleasant to handle after firing. The down side is that they’re thin. As the thickness of the clay approaches the thickness of the glaze the tension between the two can cause cracking. Also, these pots dry fast and you may need to adjust your work cycle by covering them with plastic to slow the drying down. My students who have tried this method report a rewarding experience in all respects. It’s fun to throw and test the limits. The clay can be expanded much farther and faster than normal because the compressed surface in combination with the rim keeper allows it to take more abuse while continuing to hold its shape.

The hardest part for most potters to adjust to with this method of throwing is giving up water. The second is the time commitment. There’s no way to make the clay dry faster without pausing at certain intervals for the clay to firm up before moving on. It all works, you just need to be patient and experiment.

Jim Wylder received his MFA from the University of Puget Sound, and his MST from Portland State University. He is a retired art teacher devoted to failure as the surest path to enlightenment.