Using plaster of paris for molds11/19/2023 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. When the plaster has set, turn the whole thing over, peel the clay out of the plaster, and let the plaster continue to dry. Then you just pour the plaster over the slab (or other object). 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. 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. First you need to surround your slab with something to hold the plaster. 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. You can pour plaster on top of leatherhard clay. Every time you pour, you will be making a reverse of the object you are pouring into or around. POURED PRESS MOLDSĪnother way to use plaster is to pour it into and around existing objects. Stamps and sprigs are especially nice when used around a border, or across multiple pieces to achieve a repetitive design element. If you want a recessed design, read the next section. 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). You can make a small press mold into a stamp for signing your work, or to impress into your piece and make designs. 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 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. Use the same clay a few times, then discard. Your clay will pick up the loose plaster particles and also allow you to see what the pattern you have carved looks like. While carving, periodically take some clay and test your design by pushing the clay into the surface. ![]() Now you can carve designs into the plaster, and use it for press molds. If you wait too long and the plaster sticks, you can always cut the plastic off. Normally after the plaster has set (about a half hour) it will release by itself, even without any mold release on the plastic. Pour plaster into plastic containers such as margarine tubs and cottage cheese containers, about 1-2 inches thick. One way to use plaster is to carve your shape into it. So mold release should be cleaned off as much as possible. 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. ![]() It can also be useful to use mold release on leather hard clay, prior to pouring the plaster. ![]() Or using a thin clay slip the plaster absorbs the water and leaves a film of clay as a barrier. Some more unusual methods of mold release are using shaving cream which is said to leave a lovely waxy film when dry. Once this happens, repeat twice more for insurance. You will see the water start to bead up on the plaster surface. 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. You need mold release in this case, or the plaster will stick. ![]() You can pour plaster into another plaster mold to make the reverse of it. Vaseline is thick and stiff, and thus tends to leave brush marks behind. 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. The purpose of mold release is to form a barrier between objects so you can separate them later. WAYS TO USE PLASTER MOLDS MOLD RELEASEįirst it is useful to understand mold release. The last tip was on mixing and pouring plaster. This is the second in a series of 3 tips on Plaster and Molds.
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