Sibelco Casting Slip: Terracotta

Hot Clay’s new range of powdered casting bodies are the perfect introduction into the world of slip casting. The 1.5kg Introduction Kit will make 1.2 Litres of casting slip, enough for a medium-sized cast.

Clay body: A smooth terracotta that starts out yellow but fires to an orange/red. Ideal for slip casting small to medium sized wares.

When making the casting slip refer to the measurements and ratios in instructions tab below as each casting slip is different.

For additional advice on understanding casting slips, see our pdf under the product attachment tab.

There is a helpful video can also be found by scanning the QR code on your kit.

Once you are confident in the process then simply reorder the 5kg amount along with some deflocculant, to make as much or as little as you want.

We recommend using earthenware glazes for this slip.

In your bucket will be 1.5kg of powdered slip and 40ml of a deflocculant called Dispex. You will need only a small amount if this.

Deflocculant: This can be liquid or a powder and when added to a clay slip or slurry it makes it runnier i.e., less viscous without adding more water.

Health and Safety: When mixing the powder wear a FFP3 mask and gloves.

Suggested additional equipment:

• Extra bucket 3 Litres or larger
• Large jug with measurements
• No splash glaze mixer or similar
• Precision scales if using deflocculant
• Drill
• Sieve

Wearing your mask, remove the lid from your bucket. Measure out 550ml of water into another bucket and add just 3.6g of Dispex, when measuring this out please use precision scales as you will need to be accurate. Mix the water and Dispex together. Begin adding a little powder and at the same time mix continuously with a ‘no splash glaze mixer’. Continue this until all the powder has been added. This should take 5 minutes or longer as the mixture should be mixed very thoroughly. After mixing the casting slip run the mixture through a sieve, this will help separate clay particles. We would recommend 100 mesh size.

When purchasing the 5kg powder, use the ratio 1kg of powder to 367g of water and 2.4g of Deflocculant. When making larger quantities of casting slip it is better to first get your specific gravity right (1.75 to 1.8) before adding the deflocculant. For information on how to measure specific gravity, see product attachment: How to Mix Casting Slips.

*We have noticed that domestic scales and level marks on jugs can be a little inaccurate and vary in design. Therefore when weighing the water content against powder in small volumes, we allow a tolerance of + or – 30g on the scales and we suggest keeping measurements simple by weighing the water in grams, but first zero the jug from its calculation.

There are certain conditions that can affect the casting time. When casting the slip in your mould consider how damp or dry your plaster is and the size or the shape of your mould. As you use a mould multiple times, the plaster will begin to soak up water, making the drying time longer.

Keep your moulds in a dry environment to avoid them being damp. We also recommend drying plaster moulds after 5 casts.

Recommended Timings:

Our findings in a small one part drop out mould (100mm x 50mm)

Timings for first cast

Cast time in mould: 3 minutes (aiming for a 4mm thickness)

Time drying in mould: 30 minutes

Timings for second cast

Cast time in mould: 3.5 minutes (aiming for a 4mm thickness)

Time drying in mould: 35 minutes

Timings for third cast

Cast time in mould: 4 minutes for (aiming for a 4mm thickness)

Time drying in mould: 40 minutes

Our findings in a small multiple part mould (60mm x 50mm)

Timings for first cast

Cast time in mould: 4 minutes for (aiming for a 4mm thickness)

Time drying in mould: 50 minutes

Timings for second cast

Cast time in mould: 4.5 minutes (aiming for a 4mm thickness)

Time drying in mould: 1 hour

Our findings in a medium one-part mould (135mm x 100mm)

Timings for first cast

Cast time in mould: 5 minutes (aiming for a 4mm thickness)

Time drying in mould: 1 hour 10 minutes

Timings for second cast

Cast time in mould: 6 minutes (aiming for a 4mm thickness)

Time drying in mould: 1 hour 20 minutes

Our findings in a medium multiple part mould size: (100mm x 115mm x 80mm)

Timings for first cast

Cast time in mould: 5 minutes for (aiming for a 4mm thickness)

Time drying in mould: 50 minutes

Timings for second cast

Cast time in mould: 6 minutes (aiming for a 4mm thickness)

Time drying in mould: 1 hour

Recommended Firing

Bisque fire - 950 °C to 1100 °C 

Glaze fire - 1040 °C to 1060 °C

Hot Clay Tip

When slipware is fully dried, you can fire to the above temperatures with faster climb rates as the walls of the wares are even and usually thin.

• Terracotta Casting Slip - SDS (PDF)
• Terracotta Casting Slip – Technical Data Sheet (PDF)
• How To: Make a Casting Slip (PDF)