I am drawn to slip casting because it allows for the replication of forms and encapsulation of details impossible to make otherwise. I find solace in this process; the technical nature of mold making being extremely challenging and rewarding.
I am inspired by the things we lose and gain as we age. I collect things I view as little treasures—my kitties’ whiskers (one day they will become a glaze brush), puppy teeth, and teeth from friends and family. They are like small, morbid tokens of love because they come from the people and animals closest to me.
My background as a potter leads me to explore form through functional pots. After learning the basics of mold making in undergraduate school, I delved into more complex mold systems to make some unsettling double-walled pots—I refer to my slip-cast tableware as playfully macabre.
Some may have preconceived notions about slip casting—that slip-cast pots are simple, and that molds are a fast and efficient means of production. While this can be true, my approach requires a great deal of planning and forethought while I create a prototype and divide it up as each part of the mold is cast. This requires constant refinement throughout the mold-making process at each step. Strap in!
Design Approach and Preparation
I’ve never taken to drawing, so my sketchbook is full of handwritten notes. I tend to sketch either in clay or by paper cutouts. Paper cutouts quickly and effectively help me visualize 3D ideas. Also, by folding paper to cut, I only need to draw one line! Focusing solely on the contour eliminates overthinking the final product and designs. When I am satisfied with my cut paper contour, I can move on to throwing the form on the potter’s wheel. I use clay as my prototype for two reasons: First, clay is a natural resist to plaster; and second, I can model my prototype to be exactly what I envision.
I know what you’re thinking: no, the teeth aren’t real. I bought 200 acrylic teeth online—you can get just about anything on the internet!
Start with a Prototype
I throw my mold prototype on the wheel upside down and solid, plug acrylic teeth in (1), and spend a significant amount of time modeling with silicone tools and a soft brush. This guarantees there are no gaps that plaster would inevitably find its way into. Once the prototype is refined, I begin masking off areas I don’t want plaster getting into using clay as flashing. This ensures there are no plaster disasters. For those unfamiliar—in slip casting, two bummer things can happen:
Plaster disaster: Liquid plaster leaks or explodes out of your cottles after pouring.
Slip up: The mold bursts apart and casting slip spills everywhere.
Neither is fun!
How Many Parts?
My cup molds are usually seven parts (2). Due to the teeth on the rim and foot, there are many opportunities for undercuts and protrusions that cause the object to become trapped in plaster. I need to separate the body of the cup into five parts, one part for the sprue, and the last part is the interior wall.
I use 0.75-inch (2-cm) thick clay slabs to cottle each part before I pour the plaster. I match the contour (3), build a clay wall, and add supports with clay to hold the plaster in. I reconstitute my mold clay and use it endlessly for other molds or new prototypes. After the first mold part is poured, I use the same contoured cottle for the next one, and so on (4). Each part is cleaned up upon curing, keys are added to each so they lock together, and a 50/50 solution of Murphy’s Oil Soap and water is applied so that the parts release from each other once the mold is complete.
Once the outer edge of the form is encased, I remove the flashing from the foot and spend time cleaning up the mold. I add more keys, then roll out a thick coil for my sprue, where I will pour the slip when the mold is functional. I again cottle with a thick slab and use 0.13-inch (3-mm)-thick vinyl flashing around the entire mold to support the wall (5). Extra coils hold the sprue in place in case the plaster wants to lift it.
The final task is making the double wall. When the contour and foot/sprue are encased in plaster, I flip the mold rim-side up and carefully use Xiem metal scraping tools to clean areas that would otherwise be undercuts (6). I center and adhere the mold to the potter’s wheel with a large coil and trim the interior wall (7). Then, I refine the interior with flexible silicone ribs. Through trial and error, I have found that if the interior wall is too steep, the cast will not release from the mold. If I trim too wide, there is a risk that the final object will not be double-walled. If I make an undercut at this stage, the interior wall of the object will become stuck in the mold. The result tends to be more of a conical shape (8). Once I am satisfied, I pour the final part (9).
I wait an hour after the last part has cooled and cured. Then, I finally take the mold apart, remove my prototype, and leave the mold to dry for a week until I attempt the initial cast and diagnose any problems. I never expect a mold to work perfectly for the first couple of casts. Molds, especially with this many parts, need to season a little before you can get a proper cast. In other words, the mold works better the more you cast it. For this one in particular, I cast it four times before getting a successful cup with zero problems.
The more time you spend refining each mold part, the better the outcome. The mold will last longer, you’ll have less to refine, and your mold will lock together beautifully.
Casting
I cast my forms with a cone-9 porcelain slip recipe from Tom Spleth. Using one mold strap around the body of the mold and two extra-large rubber bands from top to bottom, I pour in the slip (10). The longer the slip remains in the mold, the thicker the clay wall becomes. Timing is crucial here; I don’t want the walls to be so thick that the double wall is lost. After twelve minutes, I pour out the excess slip and leave the mold sprueside down at an angle for no more than two hours. This allows extra slip to drip out and time for the clay inside to set up. Next, it’s time to de-mold (11).
Chasing Seams
I have a few favorite tools for chasing (or removing) seam lines: an X-Acto knife, a curved knife, a Mudtools finishing sponge, and a firm-pointed silicone modeling tool. I scrape the seams away, use the sponge to clean the scrape marks, and then use the silicone modeling tool to bring some detail back (12).
Flourishing Touches
To finish, I use a separate sprig mold of individual teeth that I cast and add to the bottom of each piece (13). This is my favorite part—it makes me feel like a manic dentist adding teeth where they shouldn’t go, and better yet, it creates a nice surprise for the user.
Water Etching
Once the pots are bone-dry, I apply shellac resist to the teeth with a soft brush, then brush on elongated roots (14). I wait 24 hours for the shellac to cure completely, then take a damp sponge and directionally wipe the work, changing my water often. The water causes the bone-dry clay to slake down while the shellac protects the teeth, thus the relief of the roots is revealed (15). I use shellac because it cures hard, repels water, and holds up better than Mod Podge or wax.
Glazing and Firing
I bisque fire my pots to cone 04, then apply a clear glaze formulated by Linda Cordell, modified by Joe Page, with rare-earth mineral oxides, and fire in oxidation to cone 9. After the glaze firing, I apply premium gold luster to a specific number of teeth on the rim and foot, fire to cone 018, and voilà! I thrive when it comes to detail-oriented tasks, and while sometimes this process may feel punishing, each task is worth it.
Mary Rhein is a studio potter in Southern Illinois. She attained her BFA in ceramics from Southern Illinois University Edwardsville in 2021. She teaches wheel throwing and mold making at Craft Alliance in St. Louis and is the exhibitions coordinator at the Edwardsville Arts Center in Edwardsville, Illinois. To see more, follow her on Instagram @maryrheinstudio.
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I am drawn to slip casting because it allows for the replication of forms and encapsulation of details impossible to make otherwise. I find solace in this process; the technical nature of mold making being extremely challenging and rewarding.
I am inspired by the things we lose and gain as we age. I collect things I view as little treasures—my kitties’ whiskers (one day they will become a glaze brush), puppy teeth, and teeth from friends and family. They are like small, morbid tokens of love because they come from the people and animals closest to me.
My background as a potter leads me to explore form through functional pots. After learning the basics of mold making in undergraduate school, I delved into more complex mold systems to make some unsettling double-walled pots—I refer to my slip-cast tableware as playfully macabre.
Some may have preconceived notions about slip casting—that slip-cast pots are simple, and that molds are a fast and efficient means of production. While this can be true, my approach requires a great deal of planning and forethought while I create a prototype and divide it up as each part of the mold is cast. This requires constant refinement throughout the mold-making process at each step. Strap in!
Design Approach and Preparation
I’ve never taken to drawing, so my sketchbook is full of handwritten notes. I tend to sketch either in clay or by paper cutouts. Paper cutouts quickly and effectively help me visualize 3D ideas. Also, by folding paper to cut, I only need to draw one line! Focusing solely on the contour eliminates overthinking the final product and designs. When I am satisfied with my cut paper contour, I can move on to throwing the form on the potter’s wheel. I use clay as my prototype for two reasons: First, clay is a natural resist to plaster; and second, I can model my prototype to be exactly what I envision.
I know what you’re thinking: no, the teeth aren’t real. I bought 200 acrylic teeth online—you can get just about anything on the internet!
Start with a Prototype
I throw my mold prototype on the wheel upside down and solid, plug acrylic teeth in (1), and spend a significant amount of time modeling with silicone tools and a soft brush. This guarantees there are no gaps that plaster would inevitably find its way into. Once the prototype is refined, I begin masking off areas I don’t want plaster getting into using clay as flashing. This ensures there are no plaster disasters. For those unfamiliar—in slip casting, two bummer things can happen:
Plaster disaster: Liquid plaster leaks or explodes out of your cottles after pouring.
Slip up: The mold bursts apart and casting slip spills everywhere.
Neither is fun!
How Many Parts?
My cup molds are usually seven parts (2). Due to the teeth on the rim and foot, there are many opportunities for undercuts and protrusions that cause the object to become trapped in plaster. I need to separate the body of the cup into five parts, one part for the sprue, and the last part is the interior wall.
I use 0.75-inch (2-cm) thick clay slabs to cottle each part before I pour the plaster. I match the contour (3), build a clay wall, and add supports with clay to hold the plaster in. I reconstitute my mold clay and use it endlessly for other molds or new prototypes. After the first mold part is poured, I use the same contoured cottle for the next one, and so on (4). Each part is cleaned up upon curing, keys are added to each so they lock together, and a 50/50 solution of Murphy’s Oil Soap and water is applied so that the parts release from each other once the mold is complete.
Once the outer edge of the form is encased, I remove the flashing from the foot and spend time cleaning up the mold. I add more keys, then roll out a thick coil for my sprue, where I will pour the slip when the mold is functional. I again cottle with a thick slab and use 0.13-inch (3-mm)-thick vinyl flashing around the entire mold to support the wall (5). Extra coils hold the sprue in place in case the plaster wants to lift it.
The final task is making the double wall. When the contour and foot/sprue are encased in plaster, I flip the mold rim-side up and carefully use Xiem metal scraping tools to clean areas that would otherwise be undercuts (6). I center and adhere the mold to the potter’s wheel with a large coil and trim the interior wall (7). Then, I refine the interior with flexible silicone ribs. Through trial and error, I have found that if the interior wall is too steep, the cast will not release from the mold. If I trim too wide, there is a risk that the final object will not be double-walled. If I make an undercut at this stage, the interior wall of the object will become stuck in the mold. The result tends to be more of a conical shape (8). Once I am satisfied, I pour the final part (9).
I wait an hour after the last part has cooled and cured. Then, I finally take the mold apart, remove my prototype, and leave the mold to dry for a week until I attempt the initial cast and diagnose any problems. I never expect a mold to work perfectly for the first couple of casts. Molds, especially with this many parts, need to season a little before you can get a proper cast. In other words, the mold works better the more you cast it. For this one in particular, I cast it four times before getting a successful cup with zero problems.
The more time you spend refining each mold part, the better the outcome. The mold will last longer, you’ll have less to refine, and your mold will lock together beautifully.
Casting
I cast my forms with a cone-9 porcelain slip recipe from Tom Spleth. Using one mold strap around the body of the mold and two extra-large rubber bands from top to bottom, I pour in the slip (10). The longer the slip remains in the mold, the thicker the clay wall becomes. Timing is crucial here; I don’t want the walls to be so thick that the double wall is lost. After twelve minutes, I pour out the excess slip and leave the mold sprueside down at an angle for no more than two hours. This allows extra slip to drip out and time for the clay inside to set up. Next, it’s time to de-mold (11).
Chasing Seams
I have a few favorite tools for chasing (or removing) seam lines: an X-Acto knife, a curved knife, a Mudtools finishing sponge, and a firm-pointed silicone modeling tool. I scrape the seams away, use the sponge to clean the scrape marks, and then use the silicone modeling tool to bring some detail back (12).
Flourishing Touches
To finish, I use a separate sprig mold of individual teeth that I cast and add to the bottom of each piece (13). This is my favorite part—it makes me feel like a manic dentist adding teeth where they shouldn’t go, and better yet, it creates a nice surprise for the user.
Water Etching
Once the pots are bone-dry, I apply shellac resist to the teeth with a soft brush, then brush on elongated roots (14). I wait 24 hours for the shellac to cure completely, then take a damp sponge and directionally wipe the work, changing my water often. The water causes the bone-dry clay to slake down while the shellac protects the teeth, thus the relief of the roots is revealed (15). I use shellac because it cures hard, repels water, and holds up better than Mod Podge or wax.
Glazing and Firing
I bisque fire my pots to cone 04, then apply a clear glaze formulated by Linda Cordell, modified by Joe Page, with rare-earth mineral oxides, and fire in oxidation to cone 9. After the glaze firing, I apply premium gold luster to a specific number of teeth on the rim and foot, fire to cone 018, and voilà! I thrive when it comes to detail-oriented tasks, and while sometimes this process may feel punishing, each task is worth it.
Mary Rhein is a studio potter in Southern Illinois. She attained her BFA in ceramics from Southern Illinois University Edwardsville in 2021. She teaches wheel throwing and mold making at Craft Alliance in St. Louis and is the exhibitions coordinator at the Edwardsville Arts Center in Edwardsville, Illinois. To see more, follow her on Instagram @maryrheinstudio.
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