My colorful, hand-painted pottery is decorated from top to bottom, inside and out. Rarely is there a surface left bare or free from pattern or texture. Whether painted, carved, cast, or stamped, I add interest to every angle of my dark brown, stoneware pottery with layers of porcelain slip, a bevy of underglaze colors, and white, breaking tin glaze. Vintage textile prints and cast glassware are my primary inspiration. I like drawing upon the familiar to evoke memory and emotion, but it’s only a touch point for a fresh and unique collaboration.
Initially, I threw round shapes on the wheel and decorated them. The patterns quickly became more interesting, leaving me to seek out new shapes and alternative construction methods to improve the quality of the overall design. After failing to make truly square pots I found a square form to cast in plaster and press mold with better success. I soon began collecting glassware, melamine, and plastic in all shapes and textures to make molds from. The shape of my hexagonal mold is somewhere just between a circle and a square. Being both round and angular, it’s always exciting to explore its many variations of form and surface.
Creating a Plaster Press Mold
For this hexagonal mold I used a thin, non-porous plastic take-out dish, which could be cast as a one-part mold. Prototypes can also be made by combining objects to achieve new shapes. For example, starting with a wide square plate placed face down on the table and stacking an upside-down shallow bowl on top of the plate creates a pasta bowl shape with an ample rim for decoration.
To prepare an object for casting, add wet clay to the object’s inner rim to secure it to the work surface and to prevent the liquid plaster from seeping under the object when it’s poured. Next, with the dish inverted and the clay rim secured to a flat surface, build up short, thick, clay walls around the thin plastic edge of the disposable dish to contain the plaster as it’s poured (1). When creating a one-part mold with a non-porous object, mold soap is generally not necessary. After the prototype is secured, mix the plaster using a ratio of 6 pounds of plaster to 2 quarts of water, adding the plaster to the water. Let the unmixed contents slake undisturbed for at least three minutes to hydrate the plaster, then slowly mix it using a Jiffy mixer or similar for 3 minutes to initiate the chemical setting process. Just as the plaster is beginning to thicken, pour it over the prototype. Pour when liquid to prevent the formation of air bubbles on the surface of the model. As the plaster thickens, but before it sets, you can manipulate it to mound evenly over the form (2). The plaster heats up as it sets.
Once the plaster has cooled fully, remove the clay walls, smooth down any rough edges with a green kitchen scouring pad, and wipe down the mold with a damp sponge (3). Allow the mold to completely dry before using it.
Prepping the Clay and Filling the Mold
I stack wedge my clay for slab rolling as I find it reduces warping by aligning the clay platelets. Slice a chunk of clay into two thin pieces, press each one flat, then slam one half on top of the other, pressing it flat again. Slice and repeat 6–8 times until the clay feels more fluid. Gradually flatten with a rolling pin, flipping the clay over and turning it 90° with each pass until it reaches its final thickness of ¼ inch. Compress the slab with a metal rib to further align the clay particles.
Use the slab while it’s soft to fill the mold, easing the ribbed surface face down into the interior (4). Gather the edges slightly to help the clay fill the mold without stretching too much. Press the clay against the mold, and add wet clay to backfilling any areas that become thin. It may take a few attempts filling the mold before you figure out the nuances of the form and process. Trim off the excess clay around the rim using a plastic tool (metal tools could chip or gouge the plaster) and use a stiff rubber rib to compress the rim, the clay on the top side of the slab, and to even out and smooth the surface (5).
Variations of Form
For a deeper bowl shape, build short slab walls around the perimeter of the bowl. When doing this, be sure to reinforce any seams with small coils on the interior of the bowl to prevent cracking (6). For a sturdier and more elegant bowl, add a coil or flattened band to the top edge to thicken and define the rim. The basket bowl has a slight swoop at the rim where a single coil has been attached to the rim with slightly more clay in the corners for extra height.
At this point the bowl is ready for its first coat of porcelain slip. A total of three coats should be applied while the clay is still supported by the mold (7). Wait for the slip to dry between coats.
When the slip-painted clay has firmed up, remove it from the mold by sandwiching a board on top and carefully flipping upside down. While upside down, use gentle but firm pressure to score and slip on a foot ring or foot nubbins.
To create a basket effect, extract cut outs from the corners of the form with a thin blade after the bowl is removed from the mold. Smooth the edges of the cuts (8). Carving or exterior painting is best done now with the piece firmly inverted (9).
Learning how to Combine Patterns
My pattern book is sorted into five sections to organize the spatial dynamics of a particular pattern. Motifs such as animals or single flowers are the first section. A motif functions as a unit that can either be used alone as a focal point or can be repeated in various ways to create a pattern.
Section two, overall patterns, is most abundant since many fabrics are printed to be used in multiple directions. These patterns are random and don’t adhere to any one orientation. They work well on any shape.
The third section, vertical patterns, is limited in use to upright forms such as vases and mugs or square and rectangular shapes that have a top and a bottom. As an example, wallpaper tends to be vertically oriented.
What I call ground patterns, collected in the fourth section of my book, are those that are more basic and subdued. They act as a filler or create an interesting background when used with a focal point motif.
The fifth and final section contains borders or scrolling patterns that can trim a form or frame an overall design.
When combining patterns I look for similar elements of shape, scale, and line. I can always unify patterns with complimentary color combinations. I also tend to mix styles and historical genres within a piece or in an arrangement of dishware, like bold Art Deco lines softened by an Ikat (a textile dying process) ground with Iznik pomegranates surrounded by calico. I draw each form once and photocopy it multiple times to play with dividing the form into zones for decorating. I can then play with adding patterns together. When I settle on a combination I may draw it once again and copy it multiple times to try color combinations (10).
I use paper stencils to transfer patterns to the form. Occasionally, I enlarge or reduce their scale to fit the form better. If your clay surface is a soft leather hard, I suggest putting a sheet of thin plastic between the clay and the paper template as the moisture from the clay will cause the paper to slough off into your precious line work. Trace the template with light pressure using a dull pencil or ball-tipped sgraffito tool. This leaves an impression of the pattern in the clay that guides your brush work (11).
When applying underglazes to create the patterns, I use wax resist thinned with water to paint more efficiently in layers. Apply wax over light areas of color to protect them when painting over with darker colors or to block the edges of motifs to make it easier to quickly fill in a solid background color. The peony and wheat pattern begins with hand painting the stamen and pink petals (12). The brown leaf dots are added with a fine-needled glaze applicator. When dry, the entire peony is brushed with wax resist. I give the wax about 20 minutes to harden before painting the background color (13). Using three coats of color provides a more saturated, solid color. Using a graphite pencil, I then sketch the stem of the wheat in alternating directions throughout the ground. I paint the wheat kernels last in a slightly darker color than the chartreuse. Any time I incorporate a transparent glaze color into the motif, it lessens the painting time but also adds a new dimension to the design.
My favorite sgraffito tool is a tight cornered ribbon tool. I find it grabs the clay more surely, allowing me to vary the pressure, which alters the line thickness (14). I also keep a variety of sgraffito tools on hand depending on the line quality I prefer.
Tip: Depending on the form, you may want to work from left to right if right handed or from the middle out to keep from smudging newly drawn lines. As you begin to carve through the layers of slip, underglaze, and wax, the remains will pile up. Simply dump them out into your glaze and clay recycle bin and continue carving. Don’t brush or blow them off as they may mar the surface and create excess dust. Brush off any straggling bits of clay after the bisque firing when the work is wet sanded.
Wet Sanding Bisqueware
Wet sanding bisqueware removes sharp burrs created by carved sgraffito lines. To wet sand bisqueware, use lots of water to keep the dust to a minimum. I work over a bowl of water with 320-grit wet/dry sandpaper alternating between sanding and sponging to remove dust from the sgraffito crevices (15). Be careful not to sand off the underglaze color. Mistakes can be easily fixed by applying more underglaze to the thoroughly cleaned bisque. Caution: Always wear a dust mask when doing any sanding.
The first stage of glazing on the peony and wheat pattern is to apply a transparent celadon green glaze to the leaf and stems of the peonies with either a brush or glaze applicator squeeze bottle (16). Finally, the entire interior is brush coated with two thin layers of transparent clear glaze (17).
Colleen McCall is a full-time studio artist living and working in Elmira, New York. To see more of her work, visit www.colleenmccallceramics.com or visit the Upstate New York Ceramics Invitational at Main Street Arts (www.mainstreetartgallery.com) in Clifton Springs, New York July 11–August 30.
10 Steps to Perfect Plaster
hether you need a drying bat, a simple hump mold, or you’re making a complex slip mold, you’ll need to mix plaster. Getting the plaster right requires a bit more than just “dumping and mixing.” Here are 10 ways to get the best results for your next plaster project.
1 Prepare your mold. A common mistake of potters is to mix plaster only to realize everything’s not set up for pouring. Before casting, make sure your model is set, the mold boards or cottle are secure, and all the surfaces you’re pouring onto are coated with a parting agent such as mold soap.
2 Prepare your work area. You will need a clean mixing container for the plaster, a scale for weighing the plaster, a measuring cup for the water and a rinse bucket. Note: Plaster cannot be permitted to go down the drain, because it will form a rocklike mass. Even small amounts will accumulate over time. Line a rinse bucket with a plastic garbage bag and fill it with water for rinsing your hands and tools. Allow the plaster to settle for a day, then pour off the water and discard the bag.
3 Use fresh water. The mixing water you use should be at room temperature or 70°F (21°C). If the water is too warm, the plaster will set too fast and vice versa. Use only clean, drinkable tap water or distilled water. Metallic salts, such as aluminum sulfate, can accelerate the setting time, and soluble salts can cause efflorescence on the mold surface.
4 Use fresh plaster. Plaster is calcined, meaning chemically bound water has been driven off through heating. If the plaster has been sitting around in a damp environment, it will have lumps in it, in which case it is no longer usable. Pitch it. Use plaster that has been stored dry and is lump free.
5 Weigh out materials. Do not guess about the amounts of plaster and water you’ll need. Once you start the mixing process, you do not want to go back and adjust quantities. To determine the amount you need, estimate the volume in cubic inches then divide by 231 to give gallons or by 58 to give quarts. Deduct 20% to allow for the volume of plaster, then refer to the table.
6 Add plaster to water. Slowly sift the plaster onto the surface of the water. Do not dump the plaster or toss it in by handfuls. Adding the plaster shouldn’t take more than 3 minutes.
7 Soak the plaster. Allow the plaster to soak for 1–2 minutes maximum. The soaking allows each plaster crystal to be completely surrounded by water and it removes air from the mix. Small batches require less soaking than large batches. If the soaking time is too short, it may contribute to pinholes; and if it is too long, it will contribute to fast set times, early stiffening and gritty mold surfaces.
8 Mix the plaster. Small batches of plaster can be mixed by hand. Use a constant motion with your hand and you will notice a change in consistency from watery to a thick cream. Break down lumps with your fingers as you mix. Mix only for a minute or two being very careful not to agitate the mixture so much that air bubbles are incorporated into the mix. Mixing time affects absorption rates—longer mixing times produce tighter and less-absorptive molds.
9 Pouring the plaster. After mixing, tap the bucket on a hard surface to release trapped air. Pour the plaster carefully. Wherever possible, pour plaster carefuly into the deepest area so the slurry flows evenly across the surface of the mold. Once the mold is poured, tap the table with a rubber mallet to vibrate the mold and release more air bubbles.
10 Drying plaster. When plaster sets, it heats up because of a chemical reaction. When it has cooled, it is safe to remove the cottles or forms—about 45 minutes to an hour after pouring. Molds must be dry before use. Drying molds properly promotes good strength development, uniform absorption and reduced efflorescence. Dry molds evenly. Don’t set them near a kiln where one side is exposed to excessive heat or the relative humidity is near zero. Place them on racks in a relatively dry location away from drafts.
Sources: United States Gypsum (USG) Company and Clay: A Studio Handbook, by Vince Pitelka, published by The American Ceramic Society, 2001.