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This is the second in a series of 3 tips on Plaster and Molds. The last tip was on mixing and pouring plaster.
First it is useful to understand mold release. The purpose of mold release is to form a barrier between objects so you can separate them later. There are commercial mold release agents available, but people often use Vaseline, liquid hand soap, Pam Cooking Spray, Crisco, Vegetable Oil, liquid soap, or Murphy's Oil Soap. Vaseline is thick and stiff, and thus tends to leave brush marks behind.
Plaster to Plaster
You can pour plaster into another plaster mold to make the reverse of it. You need mold release in this case, or the plaster will stick.
When using Murphy's Oil Soap, sponge on the soap with a sponge full of hot water, rinse the sponge in hot water, and rub again. Repeat this process at least 5 times. You will see the water start to bead up on the plaster surface. Once this happens, repeat twice more for insurance.
Some more unusual methods of mold release are using shaving cream which is said to leave a lovely waxy film when dry. Or using a thin clay slip; the plaster absorbs the water and leaves a film of clay as a barrier.
Plaster Over Wet Clay
It can also be useful to use mold release on leather hard clay, prior to pouring the plaster. This is often not necessary.
One problem with various methods of mold release is they can clog the surface of the plaster, making it less porous, so clay sticks to it more. So mold release should be cleaned off as much as possible.
One way to use plaster is to carve your shape into it.
Pour plaster into plastic containers such as margarine tubs and cottage cheese containers, about 1-2 inches thick. Normally after the plaster has set (about a half hour) it will release by itself, even without any mold release on the plastic. If you wait too long and the plaster sticks, you can always cut the plastic off. Now you can carve designs into the plaster, and use it for press molds. While carving, periodically take some clay and test your design by pushing the clay into the surface. Your clay will pick up the loose plaster particles and also allow you to see what the pattern you have carved looks like. Use the same clay a few times, then discard. You do not want to fire this clay, or mix it into your other clay, as plaster mixed in clay is likely to cause an explosion in the kiln.
These molds can now be used as press molds (press clay into it, peel it out), and to make sprigs (clay that is attached to your piece for raised decorative effect).
You can make a small press mold into a stamp for signing your work, or to impress into your piece and make designs. This works best if you want a raised design on your piece (since it is easier to carve into plaster than it is to carve away and leave a raised profile behind). If you want a recessed design, read the next section.
Stamps and sprigs are especially nice when used around a border, or across multiple pieces to achieve a repetitive design element.
Another way to use plaster is to pour it into and around existing objects. Every time you pour, you will be making a reverse of the object you are pouring into or around.
You can pour plaster on top of leatherhard clay. For example, say you carved a relief design into a slab of clay, and you want a mold of it so you can make exact duplicates of that design. First you need to surround your slab with something to hold the plaster. There are many ways to do this. You could put the slab of clay in a box, or you could build a moat around the slab with clay, or you could build a wooden frame around the slab of clay, or use a piece of linoleum or flexible metal rolled into a circle around it. Whatever you use, make sure it is attached well so it doesn't come apart (with string or strong tape), and make a strong connection at the bottom with a roll of clay so the plaster doesn't run out. Then you just pour the plaster over the slab (or other object). When the plaster has set, turn the whole thing over, peel the clay out of the plaster, and let the plaster continue to dry. When pouring these types of molds, it is best to pour enough that your objects are covered with a couple inches of plaster, and have at least 2” around of plaster around the outside.
You can do the same thing as above, but with an object such as a commercial tile edge piece, or a light switch plate, or numerous found items. Just remember that the item cannot have undercuts. That is, it can't have areas underneath where the plaster can flow under, or you won't be able to pull the item back out. If you have an item with undercuts, use clay to fill the undercuts. Another useful trick is to take several found objects, push them into a slab of clay (imbedding any undercuts into the clay), then pour the plaster.
To make a stamp with a raised profile (as described above), it is easiest to first carve your design into clay or plaster. Then form a moat around this with clay and pour plaster over it, making the negative. The advantage is that what you are carving is the same as what will eventually be on your piece (not the negative of it.)
It is very useful to have molds that you can put slabs of clay over (hump molds) or into (slump molds) for making platters and dishes. The benefit of working with a hump mold is you have access to the back (bottom) of the clay, for adding feet or decorating the back side. The advantage of working with a slump mold is you can work on the top surface while it is still wet. But you have to remove the item before it is completely dry to attach any feet or work on the back side.
Plaster makes great slump and hump molds because the clay doesn’t stick. And you can make them in a variety of ways. Remember to use mold release on the object, before pouring the plaster.
You can pour plaster into objects you have around the house such as bowls and platters.
You can pour plaster into a bisque item.
You can make your own shape out of clay by pressing balls of clay together in roughly the shape you want. For example, you could rough out a rectangular casserole dish. A wooden paddle is useful for getting your clay close to the right shape. Then cut a template out of cardboard which is the profile of the cross section you desire. For a rectangular shape, you will need two pieces of cardboard, one for each direction. Pull this cardboard across the soft clay pieces to finalize the shape. One this shape is made, burnish the edges so they are very smooth, then build a moat around it (as above) and fill with plaster.
A round hump mold can be made out of clay on the potters wheel. Just center a lump of clay, use a rib or trimming tool if desired to alter and smooth the shape. Then plaster can be poured around this to create a slump mold.
Once you have a slump mold, you can then pour plaster into it to make a hump mold. Because it is plaster on plaster, use a release agent. Likewise, you can make a slump mold from a hump mold by sitting the hump mold on the floor, making a moat (see above) and filling the space with plaster.
Cool Trick! You can make a nice slump mold by centering a tub of plaster in its liquid stage on your wheel and spinning it at constant speed till it sets up. Spinning a liquid in a tub produces a parabolic surface and the faster it spins, the deeper the curvature is. If the speed is fast enough for the edge of the parabola to climb over the edge it will spill plaster all over the place! Try this with a tub of water until you are confident you won't spill it over the edges. Disclaimer, I have not tried this myself yet, but I can’t wait.
You can modify the profile of a plaster mold after it has been made, and before it is dry. Say you want a rounded bottom hump mold, but only have mixing bowls with flat bottoms. Pour the plaster into the mixing bowl, and when it is has set but is still rather wet, put it on your pottery wheel (flat side down). Center, and trim the bottom into a round shape with a trimming tool.
Mushroom mold. It is useful to have hump molds elevated off the work surface so your clay can extend past the edge. This also makes it easier to trim the bottom if you want the clay even with the plaster surface. You can do this by adding a foot to the hump mold (making a mushroom mold). One way is to take a piece of cardboard tubing (the stuff that posters come in) and imbed it into the backside of the plaster before it sets. A more common way is to make a plaster foot. Before the mold has set, scratch and roughen up the middle of the plaster, then take a thick coil of clay and place it where you want the foot. Make sure your area is large enough that the mold won’t be tippy. Fill the coil with more plaster and let set.
Remember that clay shrinks, so your plaster molds should be about 10% larger than the desired size of your final piece (depending on the clay used.)
You can use cheesecloth between the plaster mold and the clay to facilitate removal, especially as the plaster starts to get wet.
Let air circulate around your plaster mold as it dries. You can put it on top of your kiln or over a radiator to speed drying, but keep elevated so it dries evenly.
Browse our selection of plaster molds.