Casting slips I’ve known and loved…. Part One

I recently had a request for my casting slip recipe from an old friend, and also promised the recipe to my students at Glassell Studio School, where I just finished teaching a workshop on making and altering slipcast forms.

 

The process for making slip is fairly simple. If you can successfully follow a baking recipe, you can make your own casting slip. The key is to make sure you have all of your ingredients on hand, measure carefully, and add them in the proper order.

This recipe is from the University of the Arts in Philadelphia, and was shared with me by Del Harrow when I assisted at his workshop at Penland School of Crafts in 2011. It is a very white cone 6 casting body, but not plastic at all, which means that it has a tendency to develop cracks if you’re not careful when joining pieces.  In my sculptural work I have been using it with a Grolleg terra sigillata, and firing to cone 04 for a beautiful, gleaming white surface. This casting body is translucent when thin and fired to cone 6/7, but will also warp a bit.  It’s a good base for creating colored casting slips by adding approximately 10% body stains, and has an accompanying clay body recipe, although I haven’t tried that yet.

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.