02.01 Slip Casting

Slip is clay suspended in water. Plaster absorbs water. When slip is poured into a plaster mold, the plaster sucks the water out of the slip that is closest to the plaster. This slip starts to become more dense and transforms into a clay shell. There is still wet slip inside the shell. After a specified amount of time when the shell is at the desired thickness, the remaining wet slip is poured out of the mold for use in another slip casting. The clay shell is still very moist and soft. Molds can be flipped upside down to allow all excess slip to drain out. After waiting for the shell to dry and harden more, it can be removed from the mold. Then the slip cast clay object can be cleaned up and or manipulated before firing in a kiln.

Process

1. Secure the pieces of your mold together with large rubber bands and or clamps.
2. Use clay along the seam as needed to fill any large gaps in the mold to prevent slip from pouring out. The slip will lose moisture after touching the plaster and become less fluid so small gaps will be plugged by the slip.
3. Gently pour the slip all the way to the top of the mold to allow all air from the mold cavity to escape.
4. Allow the slip to sit in the mold for a predetermined amount of time, generally 10 - 20 minutes. This will change based on the moisture content of the mold, the size of the mold and mold cavity, the consistency of the casting slip and other variables.
5. Pour the slip out into the spare slip container.
6. Leave your mold upside down so all of the slip drains out.
7. Wait until the clay is “leather hard” before taking it out of the mold. The clay will fell less sticky and should release from the plaster mold. If it does not release easily, then wait a bit longer.
8. Take notes about everything that you do, especially on the early casts. Write down:
   • the time the slip was poured
   • the time left in the mold
   • moisture content of mold
   • how long since previous slip cast pull
   • how thick the slip walls became
   • any other observations
9. After the cast is demolded, the lip can be trimmed and slightly smoothed with a wet sponge. Be careful not to use too much water.
10. To keep your slip casts from drying out, wrap them in plastic or put them in a sealed plastic food container.

Example Slip Casting Log Sheet

How often can you pull casts from a slip cast mold?

Once your mold is dry, your can fill it with slip, and de-mold a cast three times in a row. After the third cast, place your open and empty mold in front of a blowing air fan for at least an hour. Then you can fil#l with slip and pull a fourth and final cast for the day. Then leave your mold opened up to dry overnight. Then you can repeat the 4 cast process the next day. ¹

Tips

• Similarly to preparing paint for spraying, it can be a good idea to pour slip through a screen or sieve to catch any solid particles or debris that would make a poor casting surface
• If your the slip level in the pour resevoir drops, you can add a bit more slip to ensure even wall thickness. The resevoir will be cut off so this should not be a problem unless the resevoir is very small.
• Keep the lid on the slip container so it does not evaporate.
• Pour excess slip back into the container through a wire mesh to catch any debris.
• If you have places in your mold that are prone to capturing air bubbles then first pour some slip in and tilt the mold to coat all surfaces. Then continue to ad dslip and occasonally tilt and rotate the mold to allow all trapped air to escape.
• If you want to modify your slip casts later, or combine the clay into modular built forms, then you need to work with clay that is the same level of wetness.

References