Ah, the great plaster controversy, gritty and sordid, stealthily sifting into the studios of ceramic hobbyists and artists alike. Ask any potter or dabbler in clay and she will most likely hold an avid opinion about the powder: material of miracles, or dust of demons. Often, I have heard a ceramicist say with animosity, “Oh, I do not allow plaster into my studio!”
And perhaps the anti-plasters have a point. Plaster is messy. Clouds of fine white particles waft from open plaster bags, flying free to settle on every surface, creating dusty skims on tables and walls, shelves and chairs. It is impossible to mix a batch of plaster without finding the milky droplets have splattered on work benches and puddled on floors. And if care is not taken, plaster in the studio is plaster everywhere in the studio.
But the real reason for the hostility towards plaster lies in its insidious ability to spoil good clay and ruin ceramic pieces when they are fired.
PSA: Keep your plaster far away from your clay!
Be that as it may, I have thrown caution to the kiln (as some might see it), and opted to graciously invite plaster into my studio. And as I have avoided most plaster-related misfortunes, I have not yet come to regret my supposed folly. In fact, plaster has served me faithfully for years, becoming an indispensable component of my tile making process. It has allowed me to accurately and quickly replicate my tiles, basically enabling me to build my business. So, forget all the naysayers - hurrah for plaster!
One of my pressed tiles, made possible with the help of a plaster mold.
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And now, after you, dear reader, have waded through a few paragraphs of my babbling, let me put away my megaphone, and get down to the nitty-gritty, the minutia of mold making. In a number of previous posts (Carving Clay and Carving Plaster) I mention in passing creating molds from a master tile. Now, after long last, I will finally describe the casting process in a bit more detail (I can hear the excited cheers of ceramicists hungry for knowledge).
Project:
Making a Plaster Press Mold of a Tile
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Materials:
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I call on my mold making skills mainly to cast detailed accent tiles so that I can replicate them later. Even though this tutorial will be focusing on molding one of my tiles, the techniques are applicable for creating a mold from any number of items. I will, however, only cover the process of making a simple, one-piece mold, as this is a good beginning point. For a more in-depth look at the mold making process, there are a bunch of other resources out there. As a gift one holiday, for instance, I received The Essential Guide to Mold Making & Slip Casting, by Andrew Martin, which introduced me to techniques for creating more complex molds.
A Note on Plaster: There are a number of different plasters available for the ceramic artist, including Plaster of Paris and Hydrocal (Ceramics Arts Daily published a brief article about the types of plaster and their uses.) But the most widely used plaster, and the plaster I use exclusively, is called Pottery Plaster #1, and is available from basically any ceramic supply store. It is durable, versatile and inexpensive. It is also quite porous and is therefore wonderful for press and slip casting molds since it draws the water out of the piece and allows it to release from the mold easily. I buy it in bags of 50 lbs, from which I can make 8 to 12 molds, depending on the size of the mold.
Tip: Plaster comes in paper bags that are not moisture proof. Since plaster can absorb moisture from the air, I like to store my plaster in 5-gallon lidded buckets, which are more water resistant than the original plaster bag. This allows me to keep the plaster for a longer period of time if I am not going to use the whole 50 lbs right away. The buckets are also easier to move around, and not as messy since they are sealable. I can fit about 35 lbs of plaster in one 5-gallon bucket.
Step 1: Prep the Mold Form
♦ Determine the size of the mold needed, and find or create a mold form of an appropriate size.
The mold form is basically just a container that holds the liquid plaster while the tile is cast. Mold forms can be made from pieces of wood, strips of thin sheet metal, foam insulation sheets, plastic containers and other found objects. Keep in mind that the bottom of the mold form should be flat and smooth.
The mold form is basically just a container that holds the liquid plaster while the tile is cast. Mold forms can be made from pieces of wood, strips of thin sheet metal, foam insulation sheets, plastic containers and other found objects. Keep in mind that the bottom of the mold form should be flat and smooth.
My go to mold form is a set of four adjustable cottle boards (sides for the mold form) I constructed from some scrap wood I had lying around and a few pieces of hardware (see my mold and deckle post for more information). For the bottom of the mold form, I use two pieces of picture glass taped together and set on top of a piece of plywood for added support. The glass creates a smooth top side to the finished mold, and the plaster releases easily from the surface. Plexiglas can be used as a substitute for the glass, and is probably a safer bet due to the fragility of the alternative. I just happened to have glass around, so I used it.
The cottle boards and clamps of my homemade mold form.
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The size of your finished tile will determine the size of the mold form. Most of my tiles are around 3 to 6 inches square, so I like to have the mold at least a 1 ½ inches larger than the tile on each side, and a few inches in thickness. For larger tiles, I increase the border size, and decrease the border size for small tiles.
Step 2: Prep the Tile
♦ Place the model tile face up on the mold form bottom.
Since I am using adjustable cottle boards, I begin with my mold form unassembled. Depending on your preference and the type of mold form you are using (perhaps a piece of sheet metal bent into a circle), it may be easier to assemble it before hand.
Since I am using adjustable cottle boards, I begin with my mold form unassembled. Depending on your preference and the type of mold form you are using (perhaps a piece of sheet metal bent into a circle), it may be easier to assemble it before hand.
The model (my Knot Number 1 Tile) is centered on the bottom of the mod form (two pieces of picture glass taped together and placed on top of a wooden board for support).
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I center the tile I am molding (my Knot Number 1 Tile) on the glass bottom of my disassembled mold form. The clay tends to stick slightly to the glass (or Plexiglas), which should be encouraged since you do not want the plaster to flow under the tile. I actually spray a fine mist of water over the surface of the glass before placing the clay. This allows the clay to better adhere to the glass, though too much water can cause the clay to slide around. I also like to seal the edges of the clay where it meets the glass with a rubber-tipped clay shaping tool.
Sealing around the edge of the tile model with an angled clay shaping tool. This will prevent plaster from seeping under the model during the pour.
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Step 3: Assemble the Mold Form
♦ Apply a release agent to the sides of the mold form.
Plaster has a tendency to stick to wood, and to some extent, the sheet metal with which I have experimented. Therefore, it is a good idea to coat the inside surfaces of your mold form (those that will be exposed to the plaster) with a releasing agent. Mold soap is commercially available and works quite well. I have also experimented with diluted dish soap, which has worked for me in a pinch. But, I have to admit, I have fallen into the bad habit of forgoing the releasing agent. Since the four sides of my form detach from each other, I can pull the wood from the plaster with little problem. However, the sides of my molds come out looking very rough (I really don’t care about the aesthetics of my molds), and every once in a while I have to clean off the plaster stuck to the wooden form.
Plaster has a tendency to stick to wood, and to some extent, the sheet metal with which I have experimented. Therefore, it is a good idea to coat the inside surfaces of your mold form (those that will be exposed to the plaster) with a releasing agent. Mold soap is commercially available and works quite well. I have also experimented with diluted dish soap, which has worked for me in a pinch. But, I have to admit, I have fallen into the bad habit of forgoing the releasing agent. Since the four sides of my form detach from each other, I can pull the wood from the plaster with little problem. However, the sides of my molds come out looking very rough (I really don’t care about the aesthetics of my molds), and every once in a while I have to clean off the plaster stuck to the wooden form.
The edge and corner of a finished mold. I have smoothed down the corners, but the side of the mold is quite rough from the residual plaster on my cottle boards. This is what the lack of a releasing agent will get you.
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♦ Place the cottle boards around the tile and secure with clamps.
If your mold form is already assembled per the previous step, feel free to ignore the next paragraph.
Since the cottle boards are adjustable, I can easily alter the mold form to the size of my tile. I place the cottle boards around the tile, abutting the end of one cottle board to the side of its neighbor. I then use 1.5-inch clamps to secure the boards to each other, ensuring the sides of the form are strong.
If your mold form is already assembled per the previous step, feel free to ignore the next paragraph.
Since the cottle boards are adjustable, I can easily alter the mold form to the size of my tile. I place the cottle boards around the tile, abutting the end of one cottle board to the side of its neighbor. I then use 1.5-inch clamps to secure the boards to each other, ensuring the sides of the form are strong.
Beginning to assemble the cottle boards around the model tile. You can see residual plaster on the side of the board from previous pours (use your mold release, kids).
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A closeup of the abutment between two cottle boards. Note the corner bracket which will allow a clamp to secure a cottle board to its neighbor.
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The cottle boards are assembled. The end of each board meets the side of its neighbor, forming a rectangular box which can easily be resized.
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Clamps have been added to the mold form in order to secure the cottle boards to each other. Corner brackets on one end of each cottle board allow it to be attached to its neighbor.
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Step 4: Seal the Sides of the Mold Form
♦ Use clay to seal around the edges where the form sides meet the bottom.
If you are working with a mold form that separates from its bottom, the edges of the walls should be sealed where they meet the bottom of the form. Any other gaps between the mold form walls will also have to be sealed to ensure that no plaster leaks out of the form. I have forgotten to seal the edges once or twice and have inadvertently created plaster lakes on my floor. Not fun to clean up!
If you are working with a mold form that separates from its bottom, the edges of the walls should be sealed where they meet the bottom of the form. Any other gaps between the mold form walls will also have to be sealed to ensure that no plaster leaks out of the form. I have forgotten to seal the edges once or twice and have inadvertently created plaster lakes on my floor. Not fun to clean up!
Using a snake of clay to seal the edges between the cottle boards and the bottom of the mold form.
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After rolling a snake of clay between my palms, I firmly press a continuous bead around the inside bottom edges of the mold form where the wood meets the glass. I make sure that the clay is firmly attached to both the sides and the bottom of the mold form.
The edges between the cottle boards and bottom of the mold form have been sealed with clay.
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Closeup of the clay-sealed edges between the cottle boards and the bottom of the mold form.
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Step 5: Volume Calculations
♦ Calculate the appropriate amounts of plaster and water.
Now for the math!
The size of your mold form will determine how much plaster to mix. In order to determine how much plaster you will need, calculate the volume of your mold by multiplying the finished length, width and height (for rectangular molds) and then subtracting the volume of the model tile. (Note: If your mold form or model is non-rectangular, use the appropriate formula to calculate volume. The formula for a cylindrical volume is provided below.)
Now for the math!
The size of your mold form will determine how much plaster to mix. In order to determine how much plaster you will need, calculate the volume of your mold by multiplying the finished length, width and height (for rectangular molds) and then subtracting the volume of the model tile. (Note: If your mold form or model is non-rectangular, use the appropriate formula to calculate volume. The formula for a cylindrical volume is provided below.)
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For this tutorial, I am making a mold for one of my Knot Number Tiles, which is about 4 inches wide, by 5.25 inches long, by ½ inch thick. As mentioned above, I like to leave at least a 1.5 inch border around the tile, with an extra 2 inch thickness. My mold, therefore, will be about 7 inches wide by 8 inches long, by 2.5 inches thick (see the diagrams below). This results in a total volume of 140 cubic inches (7" * 8" * 2.5"). Subtracting the volume of the model tile (10.5 cubic inches) will result in the exact volume of the mold: 129.5 cubic inches, or 0.56 gallons (thanks, Google Unit Converter).
A sketch of the measurements for the tile mold (looking down from above).
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A cross section sketch of the measurements for the tile mold.
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Now that we have determined the total volume of the mold, we can calculate how much plaster and water are required to fill that volume. For the best results in terms of strength and performance, the United States Gypsum Company (USG), the main manufacturer of Pottery Plaster #1, recommends that the plaster and water be mixed by weight in the ratio of 100 parts plaster to 70 parts water (or 11.4 pounds of plaster to 1 gallon of water).
To determine the volume of water needed, subtract about 25 to 30 percent from the calculated volume of the mold to account for the plaster volume. In order to have a bit more plaster to play with, I will subtract 25 percent of the volume (0.56 gallons * 0.25 = 0.14 gallons).
Water Volume = 0.56 gallons - 0.14 gallons = 0.42 gallons water
Now, since the plaster to water ratio must be calculated by weight, convert the gallons to pounds by multiplying the water volume by 8 (there are about 8 pounds in a gallon of water).
Water Weight = 0.42 gallons * 8 pounds (per gallon) = 3.4 pounds water.
After finding the weight of the water, find the weight of the plaster using the 100 to 70 plaster to water ratio.
Water Volume = 0.56 gallons - 0.14 gallons = 0.42 gallons water
Now, since the plaster to water ratio must be calculated by weight, convert the gallons to pounds by multiplying the water volume by 8 (there are about 8 pounds in a gallon of water).
Water Weight = 0.42 gallons * 8 pounds (per gallon) = 3.4 pounds water.
After finding the weight of the water, find the weight of the plaster using the 100 to 70 plaster to water ratio.
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Plaster Weight = 3.4 pounds water * 100 / 70 = 4.9 pounds plaster.
Tip: For those of you aching for more information, USG provides an instruction sheet for the plaster , which includes more technical details about the effects of temperature, mixing time and drying temperature and humidity on the mold.
Step 6: Weigh Components
♦ Using a scale, weigh out the required amounts plaster and water.
Based on the calculations, we will need:
Based on the calculations, we will need:
- Plaster: 4.9 pounds
- Water: 3.4 pounds
Before beginning, it's a good idea to check your plaster to make sure it is good, especially if you have been storing it for a while. Plaster that has been exposed to moisture is not suitable for mold making. Roll some plaster between your fingers. If it has any hard lumps that do not fall apart easily, this is a sign that it has gotten wet. Throw the plaster away and get a new batch.
Add your water to the mixing bucket, and keep your plaster in a separate container for the time being. Mixing buckets are easy to come by, and there is really no need to buy anything special. Currently, I am using an old spackle bucket and an old paint bucket for my containers.
Add your water to the mixing bucket, and keep your plaster in a separate container for the time being. Mixing buckets are easy to come by, and there is really no need to buy anything special. Currently, I am using an old spackle bucket and an old paint bucket for my containers.
Tip: If you usually pour molds that are similar sizes, mark the sides of your buckets with the volumes you need. In this way, you will not have to weigh out the plaster and water for each mold.
When I first began making plaster molds, I did not own a scale, and this presented a problem. In order to ensure that the mold has the maximum strength of 1,000 psi and the correct porosity, exact ratios of plaster and water obtained only by accurate weighing are required. But, if you are okay with a mold that may not be quite as durable, then there is another way: the Island Method, discussed in Step 7b.
Step 7: Dissolve the Plaster
In order to achieve proper mixing, the plaster should always be added to the water, and not the other way around. Adding the plaster slowly decreases air bubbles and clumps in the final mixture. As you add the plaster, you will notice that in the beginning the plaster dissolves very quickly. As more plaster is added, its rate of dissolution decreases.
Slowly sifting the plaster into the bucket of water. The plaster dissolves in the water fairly quickly at this point. I find it is easier to use a smaller scoop to add the plaster rather than the plaster bucket.
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Continuing to sift the plaster into the water. A fair amount of the plaster has been added at this point, and it is dissolving more slowly, remaining at the surface for a moment before combining.
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Step 7b: The Island Method (Optional)
♦ If you do not have a scale (or are feeling lazy), replace Steps 5-7 with the Island Method.
In the beginning of my dabbling, I was taught to mix plaster using the quick and dirty Island Method. The volume of water is estimated, but the amount of plaster needed is not calculated beforehand.
Begin by filling a bucket with water to about ¾ of the total volume needed. As above, slowly sift plaster into the water. Enough plaster had been added once the water is saturated and small, slow-to-dissolve “islands” of plaster form in the center of the bucket.
In the beginning of my dabbling, I was taught to mix plaster using the quick and dirty Island Method. The volume of water is estimated, but the amount of plaster needed is not calculated beforehand.
Begin by filling a bucket with water to about ¾ of the total volume needed. As above, slowly sift plaster into the water. Enough plaster had been added once the water is saturated and small, slow-to-dissolve “islands” of plaster form in the center of the bucket.
"Islands" of plaster forming on the surface of the water. After a sufficient amount of plaster has been added, the plaster will no longer dissolve quickly in the water and will remain on the surface.
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Pros: There are no detailed calculations of volumes and weights (the main selling point for some). Also, there is no weighing of materials in this method. Not only is it quicker and easier, but it does not require a scale.
Cons: The Island Method is not consistent, and the ratio of water to plaster may not be exact. This leads to molds that may not have the optimal durability or porosity.
Cons: The Island Method is not consistent, and the ratio of water to plaster may not be exact. This leads to molds that may not have the optimal durability or porosity.
Step 8: Rest the Plaster
♦ Let the plaster and water sit for 1-2 minutes before mixing.
Letting the mixture stand unagitated for a couple minutes allows the plaster to absorb the maximum amount of water, leading to a stronger mold. And I am convinced that the rest period is the key to a smooth plaster slurry. Forego this step at your own peril (I have, and then found lumps in my plaster).
Letting the mixture stand unagitated for a couple minutes allows the plaster to absorb the maximum amount of water, leading to a stronger mold. And I am convinced that the rest period is the key to a smooth plaster slurry. Forego this step at your own peril (I have, and then found lumps in my plaster).
Step 9: Mix the Plaster Slurry
♦ Mix the plaster, either by hand or with a mechanical mixer.
Since I mix relatively small batches of plaster at a time (~ 1 gallon), I mix by hand (wearing rubber gloves). For hand mixing, a plastic spoon or paint stirrer will also work. For larger batches, use a drill with a paint mixer attachment at slow rpms. USG prefers mechanical mixing, stating it leads to a more uniform and thus durable plaster mix.
Since I mix relatively small batches of plaster at a time (~ 1 gallon), I mix by hand (wearing rubber gloves). For hand mixing, a plastic spoon or paint stirrer will also work. For larger batches, use a drill with a paint mixer attachment at slow rpms. USG prefers mechanical mixing, stating it leads to a more uniform and thus durable plaster mix.
The plaster should be the consistency of a milk shake or heavy cream, smooth without any chunks of unmixed plaster. Ideally, the mixture should not have any air bubbles in it, but I have yet to find a way to stir the plaster that does not incorporate bubbles! Tapping the side of the container after the plaster is mixed seems to help expel some (but not all!) of the air from the plaster.
Sporting a pair of nifty yellow rubber dish gloves, I stir the plaster gently by hand, attempting to thoroughly mix the slurry without adding too many air bubbles.
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Still mixing. I stir the slurry for 1-2 minutes until it is the consistency of heavy cream and there are no noticeable chunks of undissolved plaster.
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Tip: Keep a bucket of rinse water in you plaster pouring area. Once you are finish mixing the plaster, immediately rinse your hands and any tools in the bucket. Do not rinse plaster down the drain!
Step 10: Pour the Plaster
♦ Filling from the corner, pour the plaster slowly into the mold.
Try not pour the plaster directly over the tile. I have found that with intricate designs some of those pesky air bubbles from the plaster get trapped in small crevices, creating voids in the finished mold. It seems that pouring the plaster slowly and evenly reduces the amount of air bubbles. After the plaster is poured, I also like to tap the sides of the mold form to release any remaining air bubbles.
Try not pour the plaster directly over the tile. I have found that with intricate designs some of those pesky air bubbles from the plaster get trapped in small crevices, creating voids in the finished mold. It seems that pouring the plaster slowly and evenly reduces the amount of air bubbles. After the plaster is poured, I also like to tap the sides of the mold form to release any remaining air bubbles.
Beginning to pour the plaster. To avoid pouring the plaster directly on the model, start the pour from an empty corner of the mold form.
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The plaster has surrounded the tile model and is beginning to flow over the surface of the mold. I have had problems with air bubbles getting trapped in the details of the tile face before, so I am pouring the plaster extra slowly.
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Still pouring. The plaster has almost covered the face of the model tile.
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Nearing the end of the pour. The model tile has been covered and I am getting to the bottom of the plaster bucket.
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After the mold is poured, clean out your plaster mixing bucket by adding a bit of water to the bucket and swishing it around. A sponge helps to remove the plaster from the sides of the container. Pour the waste water into a slop bucket (the plaster will solidify at the bottom. The excess water can then be drained off, and the solid plaster can be thrown away). Again, do NOT pour plaster down the drain!
Tip: Forgot to clean out your mixing bucket when the plaster was still wet? I actually allow the residual plaster to dry in the mixing container. Since my mixing bucket is plastic and somewhat flexible, the dry plaster easily flakes off when I squeeze the sides of the container and I can just throw the chips away.
Step 11: Finishing the Mold
♦ Remove the plaster mold from the mold form.
Plaster sets up in about 45 minutes. The reaction of plaster and water is exothermic (heat producing), so I usually wait until the plaster is cool before taking it out of the mold form. My cottle form makes removing the plaster mold quite easy by merely removing the clamps and pulling the individual cottle boards away from the mold.
Plaster sets up in about 45 minutes. The reaction of plaster and water is exothermic (heat producing), so I usually wait until the plaster is cool before taking it out of the mold form. My cottle form makes removing the plaster mold quite easy by merely removing the clamps and pulling the individual cottle boards away from the mold.
The new mold. The plaster has hardened, and I have removed the cottle boards from around the mold. The tile model is still in the mold cavity, and I have not yet removed the sealing clay from around the edges.
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♦ Remove the clay from the mold and clean the mold.
Once the plaster has been taken from the mold form, I remove the clay model tile from the mold cavity and the sealing clay from around the edges. Since this clay has been contaminated with plaster, I keep it separate from the rest of my clay in a clearly marked plastic container. I can use it again for creating other molds, but I will not use this clay for my other projects since any plaster in the clay can ruin a finished ceramic piece.
Once the plaster has been taken from the mold form, I remove the clay model tile from the mold cavity and the sealing clay from around the edges. Since this clay has been contaminated with plaster, I keep it separate from the rest of my clay in a clearly marked plastic container. I can use it again for creating other molds, but I will not use this clay for my other projects since any plaster in the clay can ruin a finished ceramic piece.
Removing the sealing clay from the edges of the mold. Most of it comes off easily, but there will be little bits of clay left stuck to the plaster that should be wiped off with a damp sponge.
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Removing the tile model from the mold cavity. Since the mold is still quite wet, the clay will not release as easily as if the plaster were dry. Therefore, I use a metal tool with a small scoop at the end to pry up a corner of the model. It is then easy to use my fingers to lift the model the rest of the way out.
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Note: You will most likely loose the original tile in the casting process, so hopefully the casting goes smoothly! You may be a bit nervous casting your first tile, since many hours goes into making it – I know I was. But never fear! I have had very few abject failures where the mold was beyond reclamation. And, you may mourn your lost original tile. (The broken bodies of my first tiles littered my studio for too long before I turned them back into clay.) But that is just part of the process!
♦ Using metal tools, clean the edges of the mold and any irregularities in the plaster.
I then finish the edges of the mold by rounding them with a knife. This helps to keep the corners from chipping as I use the finished molds. At his point I also smooth any burrs or other irregularities in the tile cavity that may have been produced from the plaster pour.
I then finish the edges of the mold by rounding them with a knife. This helps to keep the corners from chipping as I use the finished molds. At his point I also smooth any burrs or other irregularities in the tile cavity that may have been produced from the plaster pour.
Using the back edge of a small metal saw I had lying around, I smooth down the edges and corners of the mold so they are easier to handle.
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Cleaning up a few tiny plaster burrs on the design with a metal carving tool.
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♦ Wash the mold.
Finally, with a damp sponge I clean off any clay or plaster dust that is still stuck to the mold. This final cleaning is good practice for keeping your studio dust free, since any residual clay will dry with the mold and eventually flake off.
Finally, with a damp sponge I clean off any clay or plaster dust that is still stuck to the mold. This final cleaning is good practice for keeping your studio dust free, since any residual clay will dry with the mold and eventually flake off.
Cleaning off the residual clay from the mold using a damp sponge.
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The plaster is still soft at this point, so be carful with the mold. While plaster hardens quickly, it takes days or weeks (depending on the conditions) to cure completely and reach its full strength. USG suggests that drying the mold quickly (possibly increasing air flow or decreasing the humidity of the surrounding air) increases the strength of the mold.
I set my molds on two furring strips to allow the air to circulate under the molds while they dry. I also try to speed up the drying by placing a box fan by the molds.
I set my molds on two furring strips to allow the air to circulate under the molds while they dry. I also try to speed up the drying by placing a box fan by the molds.
And there you have it. Once the mold is dry, it is ready to be pressed!
Health and Safety Notes:
1.) Always wear a dust mask while mixing plaster or generating plaster dust. Plaster contains silica, and breathing the dust can lead to respiratory problems over time. See USG’s Pottery Plaster #1 MSDS.
2.) When mixing plaster by hand, wear rubber gloves. Plaster can dry out the skin and cause irritation.
3.) Do not pour any plaster down the drain or clean out plaster equipment in the sink. The plaster will harden in the drain and ruin the pipes.
4.) Keep plaster away from the good clay in your studio. Even the tiniest piece of plaster can ruin a finished ceramic piece.
