Making Porcelain Slip for Slipcasting

Slipcasting is a popular technique for creating ceramic objects. (Here are some examples of my work, which was all done in slipcast). The process involves pouring a liquid clay mixture, called slip, into a plaster mold. As the plaster absorbs the water from the slip, a solid layer of clay forms on the interior of the mold. Once the slip has hardened, the mold can be opened to reveal a cast ceramic object. Making your own casting slip is an economical way to produce large quantities of uniform objects. In this article, I will guide you through the steps involved in making casting slip for slipcasting with a high fired porcelain clay body.

Step 1: Choose your clay body Selecting the right clay body is crucial for slipcasting. Porcelain clay is a popular choice for slipcasting as it has a low absorption rate and is very fine-grained, which results in a smooth surface finish. Porcelain clay bodies are typically fired to high temperatures (cone 10-12), making them ideal for functional ware such as tableware, vases, and sculptures.

(These dinosaur sculptures were made out of porcelain clay)

Step 2: Prepare your materials To make your own casting slip, you will need to gather the following materials:

  • Porcelain clay body
  • Ball mill or rock tumbler
  • Water
  • Deflocculant (sodium silicate or Darvan)
  • Sieve (80-120 mesh)
  • Mixing container (5-gallon bucket)
  • Stirring tool (drill with a mixing attachment)

Step 3: Mix the clay and water Begin by weighing out the dry porcelain clay powder. For every 1000 grams of dry clay, you will need approximately 2000-2500 grams of water. Slowly add water to the dry clay, stirring continuously until the mixture is smooth and free of lumps. Mix the clay and water for at least 20 minutes to ensure that the clay is fully hydrated.

Step 4: Ball milling Ball milling is the process of grinding the clay and water mixture in a ball mill or rock tumbler. This process breaks down any large particles and ensures that the clay particles are evenly distributed throughout the slip. Ball milling should be done for at least 24 hours for best results.

Step 5: Sieving After ball milling, the slip should be sieved through an 80-120 mesh sieve. This will remove any remaining large particles and ensure a smooth slip. If necessary, add water to the slip to achieve the desired consistency. The slip should have a specific gravity of around 1.75.

Step 6: Deflocculation Deflocculation is the process of reducing the water content of the slip without compromising the fluidity. This is achieved by adding a deflocculant, such as sodium silicate or Darvan, to the slip. The amount of deflocculant needed will depend on the specific clay body and the desired consistency of the slip. Typically, 1-3% by weight of the dry clay is added. Stir the slip thoroughly after adding the deflocculant.

Step 7: Aging Aging the slip allows the deflocculant to fully disperse throughout the mixture and results in a more stable and consistent slip. Store the slip in a covered container for at least 24 hours before using it for slipcasting.

In summary, making your own casting slip is a straightforward process that can save you money and allow for greater control over the quality of your ceramic objects. By following these steps, you can create a high-quality porcelain casting slip that is suitable for slipcasting at high temperatures. With practice and experimentation, you can customize your slip to achieve different textures and finishes. Happy slipcasting!

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