There are so many amazing things that can be done with slabs, and while slab building can seem pretty straightforward, it does take some practice to learn how to handle slabs with finesse. In today’s post, Marcia Selsor demonstrates how tarpaper can be used as a molding material for slab building. These tarpaper molds are surprisingly durable and can be used to make a variety of different shapes. – Jennifer Poellot Harnetty, editor.
Discovering new forming methods for handbuilding is a motivational experience. A few years ago, Bill Daley came to Montana’s Archie Bray Foundation to do a workshop on tarpaper molds. Well known for his large handbuilt vessels, he has used a variety of forming methods including using tarpaper as a molding material. The advantages to using tarpaper are that it’s fairly stiff, waterproof, and it’s relatively inexpensive. Most of all, using tarpaper as a form adds support to the slabs and relieves the natural stresses when constructing a large vessel.
When working with tarpaper, you should utilize and appreciate its properties. Curves work well but must be engineered to become a strong supportive structure. Angular shapes can also work but must be designed to avoid a weak wall that could slump. Molding clay with tarpaper can be approached in two ways. First, you can suspend tarpaper and create a cradle-like support, or you can drape slabs over an exterior form.
Getting Started with the Tarpaper Molds
Tarpaper is also called “roofing felt” and can be found at any home center. The most common grades are 15 lb. and 30 lb. weight, which indicate the thickness. Neither is very expensive and a roll (about 200 sq. ft.) should last a long time. Use the sturdier 30 lb. grade for making large forms. Note: Tarpaper is also an excellent material for making cottles for pouring plaster molds. (See how to mix and pour plaster!)
To illustrate how tarpaper forms work, I’ve made a large four-sided vase-like vessel with a bottom. To construct a tarpaper form, cut manageable sheets of tarpaper from the roll and lay them flat so they can uncurl. Lay a sheet of tarpaper on a hard surface and use a utility knife to cut out the shapes (figure 1). The tarpaper is too thick to be cut cleanly with scissors and the tar gums up the blades.
The shapes for the vase I’m making have sweeping curves. If you want to create a more geometric or architectural type form, straight cuts will do. The sides of the form are fastened together with a series of tabs. Cut a generous amount of 1-inch wide by 4-inch long tabs and set aside.
Preparing the Form
To construct the form, you’ll need to first glue the tabs to the tarpaper sides using a hot glue gun. Be careful and wear leather gloves to avoid burning your skin. Use a heavy duty glue gun to get stronger bonds since good connections are needed to keep the form together under the weight of the clay slabs. Fold the tabs in half before attaching them to the sides, space them out evenly, then hot glue each tab in place (figure 2).
Glue the tabs to each of the larger sides, then glue adjoining sides together to form two halves (figure 3). Once the two halves are assembled, glue them together (figure 4).
Densely stuff the form with crumpled newspaper. Keep the newspaper from falling out by gluing several straps over the top (figure 5). The resulting form is rigid and strong enough to support slabs of clay.
Forming the Vessel with Tarpaper Molds
The completed form needs to be positioned so you can lay slabs onto it. Since the shape I created has curves, a flat surface won’t work so I created a cradle to rest the form in. The cradle is easily constructed from sections of six-inch tall corrugated cardboard banded together with duct tape to form an oval ring, then filled with crumpled newspaper. Place the form in the nest (figure 6).
Roll out four, 5/8-inch thick, rectangle clay slabs and a square base. Use the finished, constructed form as a template to trace over the slabs or make paper equivalents to cut exact shapes. Consistent thickness in the clay slabs promotes even drying and prevents cracking and warping in the drying and firing stages. Roll out the slabs ahead of time to keep them at a similar wet/dry consistency while working.
Place two slabs on the form and attach them by slipping and scoring at the seams where there is no potential stress (figure 7). When constructing a large vessel with 90° angles, pay close attention to good joining at all the seams and corners. These are the places the vessel will want to pull apart when drying and firing.
After the clay firms up, but is not quite leather hard, rotate the form until all the slabs have been added. The advantage still exists in that the stress points from the shapes are overcome by the seams being located elsewhere. As the piece firms up, rotate it in the nest to encourage even drying. A hair dryer can speed up the process.
You can leave the top open or seal it closed with another slab. If you completely cover the top, slice it open after the entire form has stiffened and can hold itself up, and remove the crumpled newspaper and tarpaper from the inside. After removing the form, reseal the seams. Note that the clay on the tarpaper side of the form is wetter since the tarpaper is waterproof. These forms are only good for one-time use but they also don’t occupy space in a crowded studio.
After the clay is leather hard and the newspaper stuffing and tarpaper are removed, you can refine the form. Fill divots with small additions, and add definition to edges using a curved Surform tool (figure 8). Refine the surface to whatever extent you desire, even leaving some pieces with shaving marks. The curved blade cleans ups edges with a clean consistent line that’s visually strong. Add a lip to the rim to give the form a more visually substantial presence and also to reinforce the rim area. After completing the form, place it on 2×4’s to ensure even drying (figure 9).
The finished vases shown in this article were decorated with broad brush strokes and fired in a soda kiln. The resulting surface and shape reflect the same drawing movement that prompted the original tarpaper form.
- Tarpaper (30 lb
- Utility knife
- Leather glove
- Hot glue gun
- Sculpture clay
- Slab roller or rolling pin
- 6-in. wide cardboard for “nest”
- Hair dryer (optional)
- Surform tool
Marcia Selsor is past president of the Potters Council (now the International Ceramic Artists Network) and professor emerita at Montana State University. For comments and more information, go to www.marciaselsor.com.