How slip casting and plaster molds work

James Pagni

Plaster molds work in a simple enough way so let us explain. First of all, plaster is made from ground gypsum, a mineral consisting of hydrated calcium sulfate. Gypsum is one of the main ingredients but it is also mixed with other chemicals to provide different outcomes. For example, many arts make use of plaster molds including glass for slumping glass in the kiln. Metal for pouring molten metal in, and as we are going to cover, for casting liquid clay, also known as slip.
See the source image Image result for glass slumping molds in the kiln
Now for slip casting we have to pick a specific type of plaster called "Pottery Plaster #1". As you can imagine, it is for us potters who are going to be casting with slip. The cool thing about our plaster is that after it has been mixed (7 parts H2O and 10 parts plaster) and made into a mold, the plaster then is always in a constant state of thirst. So when we make the mold it will be wet and needs to dry for 5 to 10 days depending on warmth and air movement to help speed drying up. But after that, it will want to absorb any moisture that directly comes in contact with.
Image result for pottery plaster #1 50lb bag
Lets just say our 2 part mold is sealed, and we have our slip prepared, so we are going to then pour the slip into the plaster mold. As the slip sits in the mold, it begins drying on the inside surface of the mold. As time passes, the mold continues to suck out the water from the slip and begins forming a thickness of clay on the mold wall. We normally time how long the slip is in the mold so we can have a consistent thickness every time we cast.
After say 25 minutes has passed, we would drain out any extra slip into a bucket so we can reuse the slip. Then we let the slip dry in the mold for about 3 times as long as we left it in the mold. So that would be 75 minutes to let the slip dry to be leather hard. Then it will be strong enough to open up the mold to remove the clay. If it is not dry enough, then the clay piece inside will rip in half at the center line of the mold.
See the source image
That is it! Plaster isn't to confusing on how it works. Do note that if you are trying to buy plaster #1, then go buy it locally at a pottery supply store or a foundry supply store. Shipping for this product is a silly idea. Lastly, keep in mind that plaster molds have life span of about 200 to 300 uses before the will not function properly and will need replacing. Go check out our molds and get to casting.

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