How to mix plaster for molds

Written by drue tibbits
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How to mix plaster for molds
(Head of plaster image by Pontus Edenberg from Fotolia.com)

Whether you are using plaster to make a mould or to fill an existing mould, 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 mould will transfer to the poured item, while pouring incorrectly mixed plaster into a mould will produce a flawed finished product. Learning how to mix plaster properly helps turn out perfect products and reduces the need to do multiple pours.

Skill level:
Moderate

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Things you need

  • Mold soap or other mould release agent
  • Plastic mixing container
  • Dry plaster mix
  • Measuring scale or cups
  • Metal spoon

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Instructions

  1. 1

    Prepare the items that will be in contact with the poured plaster. Depending on if you are making a mould or pouring an existing mould, items can include the mould box, the item to be moulded or the inside of an existing mould. Prepare the areas that will be in contact with the poured plaster by coating them with mould soap or other mould 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. 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. 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. 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 0.907kg. 397gr. dry plaster for 1 qt. water. You may end up using more or less than this amount.

  5. 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. 6

    Pour any excess dry plaster mix back into its container. Allow the saturated plaster to rest for one minute.

  7. 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 double cream or pancake batter, depending on the plaster brand. You need to complete the stirring within two minutes.

  8. 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.

Tips and warnings

  • A quick way to calculate how much water to use for a batch of plaster is to pour water into the mould box or into the existing mould, and then pour that water into a separate container. Measure the water and deduct 20 per cent of the volume. This is not an exact calculation, but it will give you a starting point.
  • Do not use dry plaster mix that contains hard lumps. Hard lumps are a sign of dry mix contaminated by water or humidity. Discard the dry mix and use fresh mix.
  • Do not rinse your mixing equipment in the sink. The liquid mix will harden and block plumbing pipes. Instead, line a large bucket with a garbage bag. Fill the garbage bag with water and rinse all mixing equipment in the bucket. Wait for the plaster to settle and harden in the bottom of the garbage bag, and then dump out the water and throw away the garbage bag and hardened plaster. For small plaster batches, you can rinse all items in a plastic container filled with water. Wait for the plaster to settle and harden in the bottom of the container and dump out the water. Turn the plastic container upside-down and tap the bottom to release the hardened plaster.

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