Casting Slips

University of the Arts ^6/7 Casting Slip

For a 5 gallon bucket of slip:

Material                                   Weight in Grams

Water                                               10,000

Darvan #7                                              80

Bentonite                                             100

Grolleg (English Kaolin)              11,000

Nepheline Syenite                         4,600

Silica (Flint)                                       4,400

Step 1
 Blunge water and Darvan with a heavy duty drill with a squirrel cage or jiffy mixer attachment. Add the bentonite and continue blunging until thoroughly mixed.

Step 2 
Add dry materials in order while continuing to mix thoroughly between each addition. It’s important that you add the materials in the order listed! As you mix, try to avoid creating air bubbles from mixing at too fast a speed. The bubbles can get trapped in the slip and reappear as pin holes (or larger) in your castings.

If the mixture gets too thick to mix properly, you can add up to 18 grams more Darvan 7. Do not add any extra water until you have checked the specific gravity of your slip.

Step 3 
Check the specific gravity of your slip. Specific gravity is the measure of the density of a substance. For liquids, we compare the density to water. Weigh a specific volume of slip and divide that weight by the weight of the same volume of water. I do this using a large 60 mL syringe that I picked up at a farm supply store. I weigh 50 mL of water and 50 mL of slip. The ideal result of your test should be 1.7 – 1.8. This means that your slip is 1.7 – 1.8 times as dense as water. Most good slips are in the 1.75- 1.78 range.

If the specific gravity of your slip is above 1.8, you can add water (no more than a half-cup at a time) to bring the specific gravity down, but I’ve never had this problem.

Step 4 
Let the slip stand overnight (or continue mixing overnight if you have the equipment to do so). Mix once again the next day and recheck the specific gravity before using. It is possible to use the slip right away, but waiting insures that all of the particles are thoroughly wet.

Your slip is ready to use! Make sure you thoroughly mix the slip with a blunger before you begin pouring your molds. I use a 2 quart pitcher, and fill my molds with that unless they are exceptionally large. You want to make sure you fill the mold in one pour, so that you don’t have any lines in the finished cast.

Here’s the clay body recipe. If you want to combine slipcast and hand built or wheel thrown pieces, this could come in handy.

University of the Arts Clay Body

Material                                 Percentage

Nepheline Syenite                        23%

Grolleg (English Kaolin)               55%

Silica (Flint)                                      22%

Add: Bentonite or Macaloid         1%

*For a cone 10 clay body or casting slip, use Custer Feldspar instead of the Nepheline Syenite.

 

^6 Porcelain Slip (from Dylan Beck)

For a 5 gallon bucket of slip:

Material                                  Amount

Water                                     2 Gallons

Darvan 7                                68 grams

OM 4 Ball Clay                       10 lbs

EPK                                         5 lbs

Grolleg                                    5 lbs

G200 Feldspar                       5 lbs

Nepheline Syenite                 5 lbs

Silica (Flint)                           7.5 lbs

Begin with 2 gallons of water. Add Darvan 7 deflocculant, and mix thoroughly. Add dry materials in listed order, blunging between each addition. Weigh 50 mls in a syringe, should weigh between 80-90 grmas. If too heavy, add more water & reweigh.