Silicone for Plaster & Porcelain Plaster | Making Plaster Molds

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Silicone for plaster molds

Plaster is the easiest material to cast in a silicone mold. It doesn't attack silicone, releases without a release agent and picks up every detail of the mold. With a good silicone mold you can cast hundreds of pieces in porcelain plaster or model plaster. What does matter with plaster: the right hardness for your model, a mold that keeps its shape and a silicone that flows well enough for fine details.

  • ✓ For porcelain plaster, model plaster and other types of plaster
  • ✓ Usually no release agent needed
  • ✓ Tin and platinum silicones from Shore A 5 to 45
  • ✓ For ornaments, cornices, figurines, reliefs, jewelry and master molds for ceramics

Which silicone should you choose for plaster?

Condensation or addition cure

Condensation cure silicones (tin) are economical and not sensitive to cure inhibition. You can safely make molds of clay, wood, plaster or a 3D print. They are a good choice for most plaster molds. Addition cure silicones (platinum) shrink less than 0.1%, cure faster and stay stable on the shelf for years. Choose platinum when dimensional accuracy matters, for example master molds for ceramics or parts that must fit exactly. With platinum, watch out for cure inhibition from sulfur-based clay, latex and some 3D prints.

Hardness

  • → Shore A 5 to 15: very flexible, for deep undercuts, faces and figurines. Use a support shell, or the mold will deform under the weight of the plaster.
  • → Shore A 20 to 30: the all-round choice for ornaments, reliefs, figurines and jewelry.
  • → Shore A 30 to 45: firm and dimensionally stable, for cornices, tiles, large flat pieces and master molds.

Viscosity

The thinner the silicone, the better it flows into fine details and the more easily air bubbles escape. For poured molds without a vacuum pump, choose a low viscosity silicone. For a brush-on mold that you build up in layers, thicken the silicone with a thickener for addition cure or for condensation cure.

Our silicones for plaster and porcelain plaster

Condensation cure silicones

Addition cure silicones

How to make a plaster mold from silicone

Block mold

For smaller models, fix the model in a mold box and pour the silicone over it, with at least 10 mm of silicone all around. Pour from one point and let the silicone flow over the model by itself. A low viscosity silicone or vacuum degassing prevents air bubbles.

Brush-on mold with support shell

For large statues, reliefs and cornices, build up the silicone in a few layers to a thickness of 5 to 10 mm. Over it, make a support shell of plaster bandage, plaster or acrylic resin. This saves a lot of silicone and keeps the mold dimensionally stable.

Two-part mold

For round objects without a flat base, make the mold in two halves. Cover one half of the model with clay, add registration keys, pour the first half, then pour the second half after removing the clay. Use a release agent between the two silicone halves, or they will bond together.

Tips for casting porcelain plaster

  • → Mix porcelain plaster 4 parts powder to 1 part water, always by weight. Sprinkle the powder into the water and let it soak briefly before stirring.
  • → Porcelain plaster has a working time of about 6 minutes and can be demolded after 30 to 45 minutes. Work quickly and don't mix more than you need.
  • → Gently tap or vibrate the mold after pouring so air bubbles rise.
  • → Let the cast dry fully before painting or finishing it.
  • → Color the plaster throughout with pigment for plaster and acrylic resin.

Frequently asked questions

Do I need a release agent for plaster?
Usually not, plaster doesn't stick to silicone. With a porous model, such as wood or unsealed plaster, it is wise to seal or coat the model itself before pouring the silicone.

How many plaster casts can I get from a silicone mold?
With plaster a good mold lasts hundreds of castings. Mold life mainly depends on the tear strength of the silicone and how deep the undercuts are.

Can I use a silicone mold to make plaster molds for ceramics?
Yes. With a silicone master mold you can cast identical plaster molds for slip casting every time. Choose a firm, dimensionally stable platinum silicone from Shore A 30.

Can I also cast acrylic resin or concrete in the same mold?
Yes, most silicones on this page are suitable for that too. See also silicone for acrylic resin and silicone for concrete molds.

Related categories

Advice for your plaster mold

Tell us what you want to mold, how large the model is and how many castings you need. We will then recommend the silicone that fits best. Email info@siliconesandmore.com.