Mix Plaster for Molds
Whether you are using plaster to make a mold or to fill an existing mold, how your plaster is mixed is a determining factor on how strong and bubble-free your resulting pour will be. Mixing plaster incorrectly may result in cured plaster that is soft, brittle or has pockmarks from trapped air bubbles. Flaws in a plaster mold will transfer to the poured item, while pouring incorrectly mixed plaster into a mold will produce a flawed finished product. Learning mix plaster properly helps turn out perfect products and reduces the need to do multiple pours.
Instructions
1. Prepare the items that will be in contact with the poured plaster. Depending on if you are making a mold or pouring an existing mold, items can include the mold box, the item to be molded or the inside of an existing mold. Prepare the areas that will be in contact with the poured plaster by coating them with mold soap or other mold release agent. Use the appropriate size container for the amount of plaster you are mixing. The plastic container should hold twice as much water as the amount you need.
2. Mix up only enough plaster that you can pour within 10 minutes. If you are doing multiple pours, this may mean mixing several batches of plaster.
3. Pour clean, room-temperature water into your mixing container. Use tap water or distilled water only, as well water may contain minerals that affect the finished plaster.
4. Measure out the correct amount of dry plaster mix into a dry plastic container. Depending on the brand of plaster, you may need to weight it on a scale or measure it by volume. If your dry plaster mix does not provide mixing ratios, start with 2 lb. 14 oz. dry plaster for 1 qt. water. You may end up using more or less than this amount.
5. Sprinkle the dry plaster mix over the surface of the water by gently shaking the container with the measured dry plaster mix. Work quickly, but do not dump the dry plaster mix into the water. Sprinkle the dry plaster mix over the whole water surface so that the mix doesn't form a mound in one area.
Continue sprinkling the dry plaster mix until the dry mix is fully saturated and the water will not absorb any more dry mix. You may end up with extra dry plaster mix. Do not force extra dry plaster mix into the water. Do not take more than three minutes to pour the dry plaster mix into the water.
6. Pour any excess dry plaster mix back into its container. Allow the saturated plaster to rest for one minute.
7. Stir the saturated plaster with a metal spoon or your hand. Use gentle motions to avoid stirring bubbles into the mix. Stir the mix to fully incorporate the dry plaster mix with the water. The finished mix will resemble heavy cream or pancake batter, depending on the plaster brand. You need to complete the stirring within two minutes.
8. Tap the mixing container against the counter or other hard surface to remove trapped air bubbles. Rotate the mixing container and tap it from several angles until you no longer see air bubbles. The plaster is ready to pour.
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