The audio file for this article was produced by the Ceramic Arts Network staff and not read by the author.
Preface
The following topics are by no means the only ones that I would consider special. Some may be obvious topics and others may be obscure, but chances are you might encounter some of these along your plaster journey.
Making a Perfect Plaster Slab in Any Thickness
My early attempts to make a perfectly flat plaster slab were quite unsuccessful. They were never perfectly flat, always had a meniscus at the edges of the cottles, and needed to be dressed further with a scraper. The most frustrating attempts were thin
plaster slabs, a half inch (1.3 cm) or thinner, until I had an “aha” moment.
It occurred to me to use four spacers of equal height and a glass slab. The four spacers were ⅜-inch (0.9-cm) nuts, placed on the mold table at the corners of the dimensions of my 18×18×½-inch (45.8×45.8×1.3-cm) tempered
glass plate. I mixed up 6 pounds (2.7 kg) of plaster, and waited until it started to thicken, poured it out in the center of the area defined by the four nuts, and then placed the glass over the plaster and pressed down until each corner was on top
of each nut (1). After the plaster had set up, I removed the glass and took the perfectly flat plaster off of the mold table. As both the glass and my mold table surface are non-porous, the plaster did not stick and there was no need for any parting
compound.
How to Perfectly Flatten a Plaster Surface
When a plaster surface needs to be perfectly flat, if you use a sanding block, the block surface only covers a small area and the possibility of there being minor deviations in the subsequent flatness is increased. If you use wet/dry sandpaper with your
fingers, there will be areas that are sanded more heavily than others. The best way is to have a piece of ¼-inch-thick (0.6-cm) tempered glass, 20×20 inches (or whatever square size works for you), and have both sides sandblasted. Use
four wads of modeling clay to secure the glass on your mold table, place some water on the glass, and then put your plaster piece on the slab and begin to slowly and carefully move it over the surface of the water-coated glass to abrade (2).
Change the water often. It will take a few minutes to get the feel of this process, and once the plaster is perfectly flat, it will have a tendency to want to stick to the glass. Be careful when trying to remove the plaster part so that you don’t
ding the edges of the plaster that you are flattening.
Brushing for Detail
If you are pouring plaster over a highly detailed, textured, or patterned surface, there is a good chance that, as the wet plaster flows over the dry plaster, air bubbles will be trapped and then show up on your newly-cast plaster piece. This is due to
surface tension that arises when a wet material is poured over a dry material. To eliminate these troublesome air bubbles, after you pour the wet plaster, put a soft 2-inch-wide (5.1-cm) brush into the newly poured plaster and slowly brush the entire
surface.
This will break any surface tension and eliminate any bubbles. Clean your brush of all plaster when you are done. There are inexpensive utility brushes available from many suppliers in boxes of 25 or 50 in a few different widths. These can be one-time-use
brushes or cleaned and then reused a few times. I find they are cheap brushes, and generally not well-suited for multiple uses.
Solid Casting
There are many ways to make solid casts of objects such as plates, including an open-front mold or an open-back mold. The method I like the best is a two-part mold with two pour holes in the back, 180° from each other and located on the foot (3).
To fill, I use a modified restaurant ketchup/mustard squirt bottle with the bottom end cut off (4). As slip enters one hole, the opposite hole works as a vent and will eventually fill with slip. Let the slip level in these two squirt bottles go down
as water absorbed into the mold leaves a solid plaster cast. Once the mold has reached its maximum quantity and the level of the slip stays the same, carefully remove each small bottle and slip your finger under its spout—it’s only slightly
messy. Once set, the mold is solid (5, 6).
Bottom-Drain Molds
Large molds are difficult to maneuver in the studio. Draining slip from a large mold presents a problem not only with the volume of slip, but the weight of the slip and the mold. The logistics of manually dumping such a mold are difficult. I have two
methods for creating a bottom-drain mold part. The first and easiest way to do this is to create a bottom mold part that allows the slip to drain from the bottom. Use a 1-inch-diameter spade bit and, in the center of the mold, carefully drill a hole.
Then, using a fettling knife, carve a taper into this hole with the wider part on the casting surface (7). Sand it perfectly smooth and apply parting compound to the inside surface. Find a dowel that is taller than the top of the mold, drill two small
holes about ½ inch (1.3 cm) from the bottom end of the dowel, and insert two nails, pins, or smaller dowels so that they just stick out a small amount from either side (8). Then pour a small amount of plaster into the hole in the bottom mold part, insert the dowel with the nail end down, and let the plaster cure around it. You may need to make a clay support to hold the dowel perpendicular while it sets. Once the plug has cured, push it out from the bottom (9). You should be able to insert and remove this plug easily. So when you are ready to cast, insert the plug with the dowel and fill the mold (10). When the necessary time has elapsed, remember to put a bucket under the mold, and then pull the plug (11). Every few pours, coat the inside surface of the tapered hole with parting compound to make it easier to pull upward when it is time to empty the mold. To patch the hole in the bottom of the cast-clay piece, pour some casting slip on a flat plaster slab, and place the slip-cast piece onto it (13). It will bond to the already cast clay, and when it is ready, you can remove the piece and sponge the bottom smooth. Easy peasy.
The second way is to insert an internally-threaded (female threads) PVC coupling into the bottom mold part. Make sure that you size the length of this fitting so that it is flush with both the inside and outside surface of the bottom plaster mold
part, cutting it to the proper length beforehand (14). Using a spade bit that is the same diameter as the exterior of the PVC fitting, drill a hole in the bottom mold part (15). Then rough up the outside surface of this PVC part using a Dremel tool
that will leave some grooves or marks. Use JB Weld to glue the fitting into the hole, making sure that both surfaces are flush (16). Before filling the mold with slip, use a PVC plug that is the same threading as the fitting and hand tighten it (17).
Fill the mold with slip (18), and, when the cast is ready, remove the plug, always remembering to place a bucket under the mold when unscrewing the plug (19).
Fill the hole in the cast piece in the same manner as described before (13), by pouring some casting slip on a flat plaster slab, placing the slip-cast piece onto it, and smoothing the surface after it sets (20). Another easy peasy, yes?
Tip: If you have a bottom-drain mold, make sure that you position the mold on top of a large bucket so the slip can drain into it. If you have a casting table, position the bottom drain between two dowels and make sure to place a bucket
under the outlet hole of the casting table.
Excerpted from The Mold-Making Manual: The Art of Models, Molds, and Slip-cast Ceramics by Jonathan Kaplan. Published by The American Ceramic Society. Available in the Ceramic Arts Network Shop at mycan.ceramicartsnetwork.org/s/shop.
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The audio file for this article was produced by the Ceramic Arts Network staff and not read by the author.
Preface
The following topics are by no means the only ones that I would consider special. Some may be obvious topics and others may be obscure, but chances are you might encounter some of these along your plaster journey.
Making a Perfect Plaster Slab in Any Thickness
My early attempts to make a perfectly flat plaster slab were quite unsuccessful. They were never perfectly flat, always had a meniscus at the edges of the cottles, and needed to be dressed further with a scraper. The most frustrating attempts were thin plaster slabs, a half inch (1.3 cm) or thinner, until I had an “aha” moment.
It occurred to me to use four spacers of equal height and a glass slab. The four spacers were ⅜-inch (0.9-cm) nuts, placed on the mold table at the corners of the dimensions of my 18×18×½-inch (45.8×45.8×1.3-cm) tempered glass plate. I mixed up 6 pounds (2.7 kg) of plaster, and waited until it started to thicken, poured it out in the center of the area defined by the four nuts, and then placed the glass over the plaster and pressed down until each corner was on top of each nut (1). After the plaster had set up, I removed the glass and took the perfectly flat plaster off of the mold table. As both the glass and my mold table surface are non-porous, the plaster did not stick and there was no need for any parting compound.
How to Perfectly Flatten a Plaster Surface
When a plaster surface needs to be perfectly flat, if you use a sanding block, the block surface only covers a small area and the possibility of there being minor deviations in the subsequent flatness is increased. If you use wet/dry sandpaper with your fingers, there will be areas that are sanded more heavily than others. The best way is to have a piece of ¼-inch-thick (0.6-cm) tempered glass, 20×20 inches (or whatever square size works for you), and have both sides sandblasted. Use four wads of modeling clay to secure the glass on your mold table, place some water on the glass, and then put your plaster piece on the slab and begin to slowly and carefully move it over the surface of the water-coated glass to abrade (2). Change the water often. It will take a few minutes to get the feel of this process, and once the plaster is perfectly flat, it will have a tendency to want to stick to the glass. Be careful when trying to remove the plaster part so that you don’t ding the edges of the plaster that you are flattening.
Brushing for Detail
If you are pouring plaster over a highly detailed, textured, or patterned surface, there is a good chance that, as the wet plaster flows over the dry plaster, air bubbles will be trapped and then show up on your newly-cast plaster piece. This is due to surface tension that arises when a wet material is poured over a dry material. To eliminate these troublesome air bubbles, after you pour the wet plaster, put a soft 2-inch-wide (5.1-cm) brush into the newly poured plaster and slowly brush the entire surface.
This will break any surface tension and eliminate any bubbles. Clean your brush of all plaster when you are done. There are inexpensive utility brushes available from many suppliers in boxes of 25 or 50 in a few different widths. These can be one-time-use brushes or cleaned and then reused a few times. I find they are cheap brushes, and generally not well-suited for multiple uses.
Solid Casting
There are many ways to make solid casts of objects such as plates, including an open-front mold or an open-back mold. The method I like the best is a two-part mold with two pour holes in the back, 180° from each other and located on the foot (3). To fill, I use a modified restaurant ketchup/mustard squirt bottle with the bottom end cut off (4). As slip enters one hole, the opposite hole works as a vent and will eventually fill with slip. Let the slip level in these two squirt bottles go down as water absorbed into the mold leaves a solid plaster cast. Once the mold has reached its maximum quantity and the level of the slip stays the same, carefully remove each small bottle and slip your finger under its spout—it’s only slightly messy. Once set, the mold is solid (5, 6).
Bottom-Drain Molds
Large molds are difficult to maneuver in the studio. Draining slip from a large mold presents a problem not only with the volume of slip, but the weight of the slip and the mold. The logistics of manually dumping such a mold are difficult. I have two methods for creating a bottom-drain mold part. The first and easiest way to do this is to create a bottom mold part that allows the slip to drain from the bottom. Use a 1-inch-diameter spade bit and, in the center of the mold, carefully drill a hole. Then, using a fettling knife, carve a taper into this hole with the wider part on the casting surface (7). Sand it perfectly smooth and apply parting compound to the inside surface. Find a dowel that is taller than the top of the mold, drill two small holes about ½ inch (1.3 cm) from the bottom end of the dowel, and insert two nails, pins, or smaller dowels so that they just stick out a small amount from either side (8). Then pour a small amount of plaster into the hole in the bottom mold part, insert the dowel with the nail end down, and let the plaster cure around it. You may need to make a clay support to hold the dowel perpendicular while it sets. Once the plug has cured, push it out from the bottom (9). You should be able to insert and remove this plug easily. So when you are ready to cast, insert the plug with the dowel and fill the mold (10). When the necessary time has elapsed, remember to put a bucket under the mold, and then pull the plug (11). Every few pours, coat the inside surface of the tapered hole with parting compound to make it easier to pull upward when it is time to empty the mold. To patch the hole in the bottom of the cast-clay piece, pour some casting slip on a flat plaster slab, and place the slip-cast piece onto it (13). It will bond to the already cast clay, and when it is ready, you can remove the piece and sponge the bottom smooth. Easy peasy.
The second way is to insert an internally-threaded (female threads) PVC coupling into the bottom mold part. Make sure that you size the length of this fitting so that it is flush with both the inside and outside surface of the bottom plaster mold part, cutting it to the proper length beforehand (14). Using a spade bit that is the same diameter as the exterior of the PVC fitting, drill a hole in the bottom mold part (15). Then rough up the outside surface of this PVC part using a Dremel tool that will leave some grooves or marks. Use JB Weld to glue the fitting into the hole, making sure that both surfaces are flush (16). Before filling the mold with slip, use a PVC plug that is the same threading as the fitting and hand tighten it (17). Fill the mold with slip (18), and, when the cast is ready, remove the plug, always remembering to place a bucket under the mold when unscrewing the plug (19).
Fill the hole in the cast piece in the same manner as described before (13), by pouring some casting slip on a flat plaster slab, placing the slip-cast piece onto it, and smoothing the surface after it sets (20). Another easy peasy, yes?
Tip: If you have a bottom-drain mold, make sure that you position the mold on top of a large bucket so the slip can drain into it. If you have a casting table, position the bottom drain between two dowels and make sure to place a bucket under the outlet hole of the casting table.
Excerpted from The Mold-Making Manual: The Art of Models, Molds, and Slip-cast Ceramics by Jonathan Kaplan. Published by The American Ceramic Society. Available in the Ceramic Arts Network Shop at mycan.ceramicartsnetwork.org/s/shop.
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