Slip Casting Ceramics Using Colored Casting Slip

Creating Interesting Slipcast Surfaces with Colored Clay Casting Slip!

slip casting ceramics

Andrew Gilliatt’s work combines colored porcelain slip, brightly colored glazes and resist patterns to create pots that convey the fun he has working in the studio.

Slip casting ceramics is a forming technique that has endless potential. Andrew Gilliatt adds a whole new level of fun to the slip casting process by using multiple colors of casting slip. In today’s video, an excerpt from his video Layers of Color, Andrew shares how he uses a turkey baster to add an organic colored pattern to a slip cast piece. We’ve also included how he makes his custom shaped molds and another way he layers color in the slip casting ceramics process. – Jennifer Poellot Harnetty, editor.

PS. New to slip casting ceramics? Check out this post on how to make pottery molds in the Ceramics Monthly archives!


Ceramic Mold Making Techniques

Learn to make and use bisque molds and plaster molds, plus get casting slip recipes when you download this freebieCeramic Mold Making Techniques.



More on Andrew Gilliatt’s Method of Slip Casting Ceramics

Identifying ways of working that successfully support your ideas can be just as critical and expressive as the ideas themselves. With my functional pots, I’m designing pieces that, with the use of color and imagery, are expressive, visually inviting, and easily accessible for domestic use.

The process I developed includes sketching, using drafting software, making models with MDF, then making plaster molds from those models. The forms can then be repeated, and each one individualized through surface decoration and glazing.

Making Prototypes

slip casting ceramics

Fig.1 Finished wooden prototypes of various vessels sealed with polyurethane.

Each new piece begins with a prototype, generally made of wood or MDF, from which I create a plaster mold. The prototypes can be made from clay, but I prefer using wood for its durability. I’m not the savviest mold maker, so if at some point I have an accident during the mold-making process, the prototype is safe and intact. I’ve also found that making prototypes from wood is great for achieving sharp, transitional lines and edges (figure 1).

Once I’ve settled on a design, I produce two scale drawings—one illustrating the side view or profile, which includes the number of stacked pieces of MDF I will need to make the model, and one illustrating the top view. Using the first drawing as a blueprint, disks of MDF are cut, glued together, stacked, and turned on a lathe to make a solid round form whose shape is close to the side profile of the finished piece (figure 2). Tip: You can use a Surform tool to shape the MDF if you do not have a lathe. The second drawing works as a cutting template that is glued to the top of the form (see figure 2).

slip casting ceramics

Fig.2 Turn a solid, laminated MDF form on a lathe to get close to the right profile.

Using a band saw, I cut into the shape of the form, carefully following the outside edges of the glued-on template. The sides of the form are then sanded smooth to erase any irregularities from sawing. Finally, the prototype is sealed with one coat of Minwax Sanding Sealer and two coats of polyurethane.

The casting slip I use has a 16% shrinkage rate so the prototype must be made appropriately larger to accommodate the final size of the pot (see the reverse shrinkage equation for help with the math). Always test the shrinkage rate of your casting slip before making the prototype.

Making the Mold

slip casting ceramics

Fig.3 Finish shaping the prototype on a band saw and draw seam lines.</p>

When making molds, it’s important to remember that casting, like any other building method, is strictly a means to a desired end. It doesn’t have to be an overly technical venture and, depending on the form, can be quite easy. I’ve learned to make molds simply by reading books on the subject, and by asking for help from others.

The biggest trick to making molds is figuring out the number of parts to cast. Most of my molds are made with four parts—a bottom, two sides, and a top piece used as a pouring gate or slip reservoir. Before I make a mold, I take my prototype and draw seam lines on it with a black marker so that I know how many parts I will need for the mold (figure 3). Then I add a clay slab to the top of the prototype for a pouring gate (see figure 4). By making my pouring gate just a little taller than need be, I can control the quality of the rim after the piece has been cast.

slip casting ceramics

Fig.4 Add a clay slab to the sealed prototype to create a pouring gate or slip reservoir.

Next, I embed the form into a block of clay up to the seam lines marking off the first section of the mold, set up cottle boards, seal the seams between the blocking clay and the cottles, and pour the plaster. Parts of the blocking clay are removed as I’m ready to cast successive sections. The image shows the mold halfway through the casting process, with the bottom and first side cast, and the second side and slip reservoir or pouring gate still to be cast (figure 4). Note that the location of the seams has been planned so that they correspond to edges or places where planes and curves shift, rather than flat faces of the form. This makes them easier to clean up, and makes them less noticeable in the finished form.

Mixing the Casting Slip

slip casting ceramics

Fig.5 Pour colored casting slip into the plaster mold first.

Most of my pots are cast using two different slips—a colored casting slip for the exterior of the piece, and a white casting slip for the interior, both are made from the same base recipe. The colored casting slips are tinted using Mason stains. Using only colored slip would be more expensive, and, lining the colored slip with a white slip allows me to get different color effects on the interior and exterior of a form using only one glaze. The colored slip is essentially a decorative coating, much like an engobe applied to a thrown or handbuilt form, but in this case, the coating is laid down first.

To make the colored slip, ball mill 100 grams of stain per gallon of white casting slip and let them mix for two hours (14 lbs. of casting slip is roughly equivalent to one gallon). Ball milling gives a more consistent color saturation than blunging and the stain mixes in with the wet slip more easily. If you do not have a ball mill, use a kitchen blender and mix in small batches before combining.

Test shrinkage rates when using more than one slip in the same cast, even if they are made from the same base recipes. If the different slips have different rates of shrinkage, they will crack.

Check out this cone 6 casting slip recipe on Ceramic Recipes!


slip casting ceramics

Fig.6 Drain the excess colored casting slip from the plaster mold.

Reverse Shrinkage Equation

1 – shrinkage rate = X.
Fired dimension of piece divided by X = prototype dimension.

For example:
My bowl needs to be 4 inches high when fired and my clay shrinks 16%
1 – .16 = .84
4 ÷ .84 = 4.76
My prototype needs to be 4.76 inches tall.


Casting the Pieces

slip casting ceramics

Fig.7 Cut away the pouring gate. Keep the blade flat on the top of the mold.

Wet the mold with a sponge. Pour the colored casting slip into the mold and let it set up for approximately ten minutes (figure 5). Then pour the colored slip out of the mold and let it drain (figure 6). Once the slip has stopped dripping from the mold, immediately pour in the white casting slip. Leave the white slip in the mold for about 30 minutes before draining. The longer you leave the slip in the mold the thicker the piece will be. I prefer to make my pots just a little on the thicker side.

Finishing the Cast

Remove the top piece of the mold (here the pouring gate section is removed first) when the slip is no longer glossy or tacky. Using an X-Acto knife, cut away the excess clay. Hold the blade flush with the top of the mold as a guide, After piercing the form in one spot, angle the blade in the same direction of your cut so that you’re always cutting the interior wall first, then moving through to the exterior. By doing so, you press the form back into the mold walls as you cut, and avoid warping the form by pulling the walls away from the mold (figure 7).

slip casting ceramics

Fig.8 Finish the rim with a red rubber rib. Note the striations of contrasting colored slip.

Smooth the rim with a damp sponge and a soft, flexible rib (figure 8). Let the piece dry sufficiently before removing it from the mold (figure 9). Once the piece is bone-dry, remove any seam lines with an X-Acto blade, fettling knife, or metal rib. Smooth away any inconsistencies using drywall sanding mesh and a sponge. Caution: Always wear a respirator when sanding pots.

Glazing and Firing

I leave patterned areas on the outside of my vessels unglazed to expose the colored clay underneath. Stickers and masking tape work great as a glaze resist and give a far crisper and better line quality than anything I can achieve using wax and a brush.

Clean the bisqued ware with a damp sponge. Using a pencil, outline the areas you want to leave bare. Follow the lines with masking tape (I use quarter inch masking tape because it is more flexible than the wider tapes). For curved lines, focus on laying down just the outer edge of the tape rather than trying to lay down the whole width of the tape all at once (figure 10). With the resist pattern complete, dip or spray to apply the glaze. Peel away the tape and stickers as soon as the glaze is dry enough to handle (figure 11). Make sure to peel away the stickers entirely. Any remaining residue will leave a noticeable blemish even after firing.

One upside to using different colored casting slips is that the glazes you use will have a different color quality on glazed and unglazed areas, depending on the color of the clay underneath, and the translucency of the glaze.

slip casting ceramics

Fig.9 Cast bowl, dried and ready to remove from mold.

slip casting ceramics

Fig.10 Decorate the bisqued bowl using masking tape and stickers before glazing.

 

To see more of Andrew Gilliatt’s work visit his website at www.andrewgilliatt.com.

**First published in 2010.
Comments
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  • Jamie B.

    I have slip that fired to bisque at 04 and then glazes that fire to 06. Could I fire the decals on my 06 glaze at say 07 or 08? Or are they not compatable with glazes that fire that low?

  • Susan M.

    The link that was posted is expired. What is the name of the decal paper? Is it the bake on decal paper or something else?

    Thanks!

  • Very interesting love the results.I have used kids stickers with slip which has worked pretty well thanks for the mould making tutorial i find the whole process tedious I like the alternate use of wood keep up the great work.

  • Heather G.

    Thank you. This is a very interesting article. I especially enjoyed the photos of the whole mold making and pouring process. Hey PC how about having a mold designing, making and using workshop. I am very interested slipcasting. Having read several books and done some rudimentary plaster casting of a tile and a few basic forms, I find that I definitely need more hands on with an experienced plasterer in order to get it right. I have had some very messy plaster disasters.

  • Thanks again CAD for a great piece of information! I have just been working on a project where I need to cast molds for production and this is great! I am very impressed with the quality of the work and wish the artist the very best in future endeavors. I can honestly say this is the first one in these series that I can’t wait to order. Happy Holidays!

  • shawn f.

    I rarely enjoy the post that don’t revolve around wheel throwing, stoneware, wood firing, etc… because that is my thing. This however, was great. Completely out of my comfort zone. Excellent !

  • I would recommend decal paper from Beldecal for over Pappilio’s paper. I found their product to be a tad thicker and thus easier to handle and there is no smell when printing. Every time I ran a sheet of decal paper from Papilio through the printer I had to walk away. Pappilio said the smell wasn’t toxic but it sure was offensive!

  • As a graphic designer turned potter, I have been interested in designing with decals, but have not been sure which product to use. Could you help clarify which Papilio product you use? Papilio’s website says that their papers CANNOT be used to make ceramic decals. Thanks for the article and any additional information you can share.

  • Subscriber T.

    Hi thaks for this irt is very interesting, hoping I can source the decal paper in New Zealand. Just wondering you state that you must use a lazer printer, but the decal paper appears to be for ink jet printing, I guess this is correct??? thanks Michelle

  • Subscriber T.

    Terry – to get the colour you need to have ceramic inks in the printer – so colour fire on decals are usually made commercially. It is possible to screen print your own colour decals but that’s a whole new subject.

  • definitely interesting. but I need a dictionary!
    Is there any way to print colored decals that will hold their color in firing?

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