How to Prepare Ceramics Slip for Casting

Slip is clay in a liquid form that is poured into plaster molds to make cast ceramic figures.

Instructions

Difficulty: Moderately challenging

Things You’ll Need:

• Clay Slip
• Molds
• Sodium Silicates
• Calculators
• Paint Stirrers
• Water
• Scales
• Thermometers
• Kitchen Timers
• Plastic Bowls
• Permanent Markers
• 16-oz. Plastic Water Bottles With Cap
• 1/8-inch Drill Bits
• Variable-speed Drills

1. Purchase dry slip mix from your ceramics dealer. It is usually packaged in 2-gallon cartons with plastic liners.
2. Add clean water, following the manufacturer's instructions, directly to the slip in its container.
3. Stir the mixture thoroughly with a wooden paint stick.
4. Test your slip's specific gravity ' this is how much more the slip weighs than water. At 70 degrees F, a gallon of water weighs 8.3 pounds.
5. Weigh 1 gallon of slip and divide by 8.3. As a rule, a result of 1.75 is right for casting. Try to get as close to this number as possible.
6. Add more dry slip material if it's too light (i.e., it has a specific gravity lower than 1.75), or add water a little bit at a time if it's too heavy (higher than 1.75).
7. Now check the thickness of your slip. This is its viscosity.
8. Drill a 1/8-inch hole in the bottom of an empty water bottle (16-ounce size).
9. Mark a fill line somewhere three-quarters of the way up on the bottle with a permanent marker.
10. Cover the hole with your finger and use a funnel to fill the bottle up to the line.
11. Put the lid on tight ' you don't want any air to escape through the top.
12. Hold the bottle right side up over a bowl.
13. Start your timer when you take your finger off the hole.
14. Time how long in seconds it takes until the flow changes from running to steady dripping. The longer it takes for the flow to slow, the thinner the slip.
15. Stir in a deflocculant by mixing equal parts of sodium silicate and clean water if the slip is too thick. Stir in 1/8 ounce of deflocculant per gallon of slip.
16. Add enough dry slip material to balance it out if the slip is too thin.
17. Once you have achieved the right specific gravity and viscosity, you're ready to cast a mold!

Tips & Warnings

• The viscosity you desire depends on what your project is: the larger the item, the thicker the slip you'll want so that it will stick to the sides of the mold. If it's too thin, the clay will just sink to the bottom of the mold, and the water will rise to the top.
• Ideally, the viscosity test should be run initially on slip you already know is ready to use so that you will have a reliable time to compare future batches against. To make it easy, write the time right on the test bottle.
• If you add deflocculant to the mix in step 15, go back to step 4 and measure the specific gravity again.
• For consistency, use the same bottle every time you do the viscosity test.
• Wipe out your molds before and after you use them.
• Avoid using slip that is too thin. Molds are made of plaster and will absorb moisture, making the plaster soft. Soft molds won't last very long.
• Add no more than 1/4-ounce sodium silicate per gallon of slip. You'll know you've used too much when you see black specks floating to the top. If this happens, stir in a little bit of dry slip material. Recalculate your specific gravity and viscosity.