Recently, I have seen a growing interest in slip casting. One of the great things about slip casting is that you can create a mold based on your own design, cast the piece, and implement the process into your work to create more consistent results.
Many potters are unsure where to begin with mold making. I recommend starting with handles. Even if you have no interest in creating an entire line of slip-cast ware, a handle mold can be an essential tool in any potter’s studio. Handles are so versatile and can be used for mugs, teapots, pitchers, etc.
Creating the Handle
Begin by sketching the handle you wish to make. For this mold we will cast a solid handle. After you have settled on a design, make a prototype out of clay (see 1). Remember to accommodate for clay shrinkage.
Next, make two sprues (see 1). A sprue is the opening in the plaster where you will pour the casting slip. You need two sprues so that the casting slip can be poured into one hole while the air escapes out the other hole. This is how your handle will be cast into a solid piece. Making them tall like the ones shown here creates a reservoir for the casting slip. As the casting process proceeds and water is pulled out through the plaster, the slip level lowers. The reservoir prevents the need for topping off the mold frequently during the casting process.
Creating the Mold
Place your handle on top of your sprues (1) and draw a line (lengthwise) down the center of your handle (2). Cut your handle and two sprues in half. This may seem shocking, but trust me and do it.
Draw a straight line on your corrugated plastic. I like to use this as a guideline for lining up my handle and cottle boards. Place half of your handle and two sprues down on the corrugated plastic, oriented along the guideline (3). Keep the other half of the handle intact and under plastic so it remains malleable. Then make three balls of clay, approximately one inch wide. Carefully cut these three balls in half and place them adjacent to the handle without touching it (see 3). These are your registration marks (also called keys) and will be used to line up the mold halves when you put them together.
Next, place your cottles around the handle and registration marks, leaving a one-inch gap between the clay and the edge of the cottles (4). Secure clay coils along the perimeters of the cottle boards to prevent the plaster from leaking out. This is very important. You don’t want a plaster disaster, but if one occurs, wait until the plaster is dry to clean up. It’s a lot easier.
Math Time
Now comes the best part: math. Volume is calculated by multiplying the length × width × height (5). I use Andrew Martin’s formula from The Essential Guide to Mold Making & Slip Casting, which is:
volume in cubic inches ÷ 80 = quarts of water
quarts of water × 3 = pounds of plaster
For example our handle mold measures 5 in. × 7 in. × 2 in. = 70 in.3
70 in.3 ÷ 80 = 0.88 quarts of water
0.88 quarts of water × 3 = 2.64 pounds of plaster
Plaster Mixing and Pouring
Pour 0.88 quarts of room-temperature water into a plastic bucket and sift 2.64 pounds of plaster through your hand into the water. Note: Wear a properly fitted respirator when handling and sifting powdered plaster. Let the plaster soak for three minutes. Then gently mix the plaster using a drill fitted with a paint-mixer attachment for three minutes. Starting at the corner of your mold box, gently pour the plaster into your mold.
Finishing the Mold
Let the mold set for at least an hour. The plaster will heat up and then cool down as it cures. Remove the cottles (6) and the three balls of clay used as registration marks. While the first half of the handle is still embedded in the plaster, wet the second half of the handle and the sprue then gently reattach them (7).
Take a teaspoon of Vaseline and rub it over the plaster face of the mold, to create a barrier to resist the plaster on your next pour so that the two halves of the mold will separate cleanly. Place the half mold back on the corrugated cardboard and surround it tightly with the cottle boards. Secure clay coils along the seams between the plaster and the cottle boards. Use the same water and plaster measurements and plaster mixing procedure, then pour the second half.
Allow the mold to dry overnight. The next day, use a rib to gently scrape the seam to loosen any plaster that may have dripped over the edge. Gently pry the mold open and remove the wet clay pieces. Remove the Vaseline and any residual clay by washing your mold under hot water (8). Rubber band the mold back together and let it dry completely (9). Now you are ready to start casting your new handles.
Heidi Fahrenbacher is a studio potter in Plainwell, Michigan. She firmly believes in working smarter, not harder, so she can spend more time outside. Her production line is completely slip cast. Find out more at bellajoypottery.com, or on Instagram @bellajoypottery.
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Recently, I have seen a growing interest in slip casting. One of the great things about slip casting is that you can create a mold based on your own design, cast the piece, and implement the process into your work to create more consistent results.
Many potters are unsure where to begin with mold making. I recommend starting with handles. Even if you have no interest in creating an entire line of slip-cast ware, a handle mold can be an essential tool in any potter’s studio. Handles are so versatile and can be used for mugs, teapots, pitchers, etc.
Creating the Handle
Begin by sketching the handle you wish to make. For this mold we will cast a solid handle. After you have settled on a design, make a prototype out of clay (see 1). Remember to accommodate for clay shrinkage.
Next, make two sprues (see 1). A sprue is the opening in the plaster where you will pour the casting slip. You need two sprues so that the casting slip can be poured into one hole while the air escapes out the other hole. This is how your handle will be cast into a solid piece. Making them tall like the ones shown here creates a reservoir for the casting slip. As the casting process proceeds and water is pulled out through the plaster, the slip level lowers. The reservoir prevents the need for topping off the mold frequently during the casting process.
Creating the Mold
Place your handle on top of your sprues (1) and draw a line (lengthwise) down the center of your handle (2). Cut your handle and two sprues in half. This may seem shocking, but trust me and do it.
Draw a straight line on your corrugated plastic. I like to use this as a guideline for lining up my handle and cottle boards. Place half of your handle and two sprues down on the corrugated plastic, oriented along the guideline (3). Keep the other half of the handle intact and under plastic so it remains malleable. Then make three balls of clay, approximately one inch wide. Carefully cut these three balls in half and place them adjacent to the handle without touching it (see 3). These are your registration marks (also called keys) and will be used to line up the mold halves when you put them together.
Next, place your cottles around the handle and registration marks, leaving a one-inch gap between the clay and the edge of the cottles (4). Secure clay coils along the perimeters of the cottle boards to prevent the plaster from leaking out. This is very important. You don’t want a plaster disaster, but if one occurs, wait until the plaster is dry to clean up. It’s a lot easier.
Math Time
Now comes the best part: math. Volume is calculated by multiplying the length × width × height (5). I use Andrew Martin’s formula from The Essential Guide to Mold Making & Slip Casting, which is:
5 in. × 7 in. × 2 in. = 70 in.3
Plaster Mixing and Pouring
Pour 0.88 quarts of room-temperature water into a plastic bucket and sift 2.64 pounds of plaster through your hand into the water. Note: Wear a properly fitted respirator when handling and sifting powdered plaster. Let the plaster soak for three minutes. Then gently mix the plaster using a drill fitted with a paint-mixer attachment for three minutes. Starting at the corner of your mold box, gently pour the plaster into your mold.
Finishing the Mold
Let the mold set for at least an hour. The plaster will heat up and then cool down as it cures. Remove the cottles (6) and the three balls of clay used as registration marks. While the first half of the handle is still embedded in the plaster, wet the second half of the handle and the sprue then gently reattach them (7).
Take a teaspoon of Vaseline and rub it over the plaster face of the mold, to create a barrier to resist the plaster on your next pour so that the two halves of the mold will separate cleanly. Place the half mold back on the corrugated cardboard and surround it tightly with the cottle boards. Secure clay coils along the seams between the plaster and the cottle boards. Use the same water and plaster measurements and plaster mixing procedure, then pour the second half.
Allow the mold to dry overnight. The next day, use a rib to gently scrape the seam to loosen any plaster that may have dripped over the edge. Gently pry the mold open and remove the wet clay pieces. Remove the Vaseline and any residual clay by washing your mold under hot water (8). Rubber band the mold back together and let it dry completely (9). Now you are ready to start casting your new handles.
Heidi Fahrenbacher is a studio potter in Plainwell, Michigan. She firmly believes in working smarter, not harder, so she can spend more time outside. Her production line is completely slip cast. Find out more at bellajoypottery.com, or on Instagram @bellajoypottery.
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