Making Simple Molds for Slabware

Making Styrofoam Molds for a Simple Platter!

simple molds

My son is finishing up fifth grade this year (how could that possibly be?!), so the rapid pace at which this school year has flown by is on my mind quite a bit. This might also be the case for all of those school teachers out there.

So, I thought I’d share a project that would work great as a lesson plan from a technique in Pottery Making Illustrated. It would also work great for all of you non teachers who are just looking for new ways to streamline your processes in the studio. – Jennifer Poellot Harnetty, editor.

What you will need:

1 inch blue Styrofoam, available at home centers in 4×8 ft. sheets (one sheet can make many molds)

Felt-tipped markers

Straightedge or ruler

Measuring tape

Jigsaw with an adjustable base plate.

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The first step is to make a template for the shape you wish to make in clay. This can be an oval, a square, a rectangle, or any other polygon shape. Draw an oval on a piece of matte board, Masonite, or thin plywood.

A simple beginning; limitless potential!

Despite its flat beginnings, a slab has limitless potential. Daryl Baird shows you everything you need to know to get started with slab building and stay inspired for years in From a Slab of Clay. You’ll find everything from the tools you need (including plans for building a slab roller!) to 16 demonstrations that you can try out to help develop skills.

Read an excerpt!

Then extend a rectangle around its perimeter as shown in figure 1. This will give the template some rigidity. Make sure to oversize the mold to account for the shrinkage of your clay body. Lay the template on the Styrofoam sheet. Measure an additional 1 inch around the perimeter of the template to make the mold sturdy and rigid and, using a felt-tipped marker, draw the perimeter of the template as well as the inside shape (figure 1).

Then, using a sharp utility knife and a ruler, cut the Styrofoam along the outside perimeter. Once this basic shape is finished, it’s time to cut the interior shape, which will then be used to form the inside of the oval platter. Using a jigsaw set at a 20° angle cut out the oval (figure 2). Remember that the saw blade needs to bevel the oval outward, so that the top of the cut is wider than the bottom. This is called ‘draft’ and allows for the easy removal of the pressed clay piece.

Remove the cut oval (figure 3) and using a piece of medium-grit sandpaper, sand the cut edge smooth (figure 4). Then place this cut piece on a larger piece of Styrofoam and cut a rectangle to correspond to the outside edge of the first mold piece. This becomes the bottom of the mold. Once cut and squared up, use duct tape along all the edges and the face to secure the bottom of the mold to the top piece with the cut oval (figure 5). Your mold is now complete! If you wish to make different shapes, now is a great time to make additional molds so that you have a variety of shapes to work with.

Making an Oval Plate

Using a slab roller or a rolling pin, roll out a slab. Macy uses a selection of SlabMats with a table top slab roller for smaller pieces. Canvas can also be used as an alternative to the SlabMats. The slab is rolled with a SlabMat on the top and the bottom of the clay. This allows the finished slab to be moved without distortion. Macy uses the outside perimeter of the Styrofoam mold to estimate an approximate size to cut the slab.

Once the slab is prepared, he uses a small rib, moving in all directions to smooth out any marks in the slab and to compress the surface. Remember that the slab is just slightly thicker than what is desired for the finished ware since it will become thinner once it’s worked into the mold. Removing the top SlabMat, the mold is positioned upside down over the freshly rolled slab. The SlabMat is then wrapped over the mold and the entire “sandwich” is flipped over in one smooth move. The mat is removed revealing the slab as it begins to slump and conform to the cut Styrofoam oval.

Now comes the critical step of pressing the slab into the mold so that it is fully supported. Using a slightly damp sponge and his fingers, Macy deftly follows the oval cut out around its circumference and presses the moist clay into the mold. After that, he uses the same sponge to press the clay onto the flat bottom of the mold and compressing it fully around the inside perimeter and flat bottom. You can see how there is now excess clay on top of the mold. By using a needle tool or potter’s knife, the outside edge is cut away leaving an oval rim. Macy uses his fingers to trace the inside edge of the oval while cutting the excess clay away. This insures that the rim follows the contour of the interior oval (figures 6–8).

After the piece has set up, it can be safely removed from the mold by gently but swiftly flipping it onto a ware board (figure 10). Lastly, the piece is flipped over, the outside edge can be detailed and smoothed with a sponge or rib. To make sure the bottom does not dome up during the drying, he presses the bottom of the platter so that it stays flat.

Following this very simple process, you can now easily experiment with making multiples of different shaped platters, plates and bowls.

Macy Dorf, a well-established potter in Denver, Colorado, makes a colorful line of high-fired functional stoneware. He sells work out of his storefront in the Santa Fe Arts District as well as to galleries and craft shops in many western states. Jonathan Kaplan is a potter, designer, and gallerist living in Denver, Colorado.

**First published in 2015
  • Carol H.

    Thank you for the tips I will get the blue styra foam and start making some plates

  • Denise M.

    Thanks very much Eric for those warp avoidance suggestions. I make slabbed coasters and after trying all sorts of things, still loose about 40% to warpage which is unacceptable and I was going to give them up. I already turn the clay during hand rolling and weigh them down during very slow drying but didn’t think of weighing down during firing. The idea to take them to glost temperature weighed down and them giving them another glost firing with glaze is a brilliant idea which I will definitely try before giving them up. Won’t I get sticking issues as the clay starts to flux (I fire to 1280C stoneware)? Also the idea about sheets of newspaper sounds very sensible.

  • Subscriber T.

    Alice Eakin-Malicki mentioned “lots of foam debris will be flying around the room while cutting and sanding the molds.”

    They make a knife-like blade for jigsaws that cuts through foam insulation products without making lots little bits of debris. Much easier to use than a regular saw blade. Check any good hardware store or online.

  • Peter M.

    The styrofoam’s an excellent idea. But does it allow the absorption of water out of the clay item while it sits in the mold? Is the styrofoam porous? I do a lot of slab rolling (roller table or by hand) for my large stoneware sundial and birdbath pedestals. I slab the birdbath basins too – too broad and shallow for effective throwing, in my limited throwing experience. I use up a lot of plaster – heavy, messy, takes a very long time for large molds to dry before using. So I’m going to try making styrofoam molds for the basins.

    I haven’t experienced much trouble with warping. Trick is to remember that the less handling the better, as folk say above, to avoid the clay ‘memory’ effect. And patience. And, with largish items, a long soak in the kiln between 80 and 100 centigrade, before ramping up.

  • Eric W.

    Marie – to minimise warping in any flat slab work, a few tips.

    * Roll out in many different directions. “Clay memory” will make the clay want to shrink back in the directions it was rolled. Think of a stretched-out pizza dough. That’s why I rarely use a slab roller. Rolling pins and thickness-guide sticks do it for me. It makes it easier to do incremental turns and flips of the clay while rolling. Most people using a slab roller MAY give the clay a 90-degree turn if any at all. Slab rollers are good for very large/heavy slabs or lots of slabs, but one or a few slabs, I prefer to hand-roll.

    * Minimize stress. Roll your slab on a mat or cloth of some sort. When moving or flipping the slab, handle it by the mat/cloth. Avoid picking up the raw slab, which will cause stress and may lead to warps and cracks. (That pesky memory thing again.)

    * Dry slooooowly. Don’t rush it. Let the piece dry very slowly and very evenly, the slower the better. Once more, reduces clay stress and helps avoid warping.

    * Don’t work too thin. The thinner the slab, the easier it is to warp. Try to keep the slab as thick as your desired end results will allow. Just know that the thinner you need to go, the harder it will be to avoid warpage.

    * Dry with weights/blocks. Weigh down the flat portion to keep as stable as possible and avoid stress. Also, put a couple layers of newspaper between the clay and other surfaces — on both sides. The paper layers allow for slippage as the clay shrinks during drying and reduces the drag that can cause, also keeping down the stress on the clay. The paper also aids in evenly wicking moisture out if the clay for slow and even dying.

    * Bisque with weights/blocks. If possible, bisque fire with weights — a kiln shelf fragment, ceramic weights, whatever will survive in the kiln. This also to help hold flat during the firing. (No paper needed for this stage.) And if you have the option, finish-fire the piece raw with the weights, then glaze and refire for the actual glaze. Might give you slightly different glaze results, but you won’t get so much “glaze drag” as a fluxing glaze han have on an unvitrified bisque body.

    Will you still get warping? Probably. But as many of these steps you can implement the better in keeping it to a minimum.

    Good luck!

  • Nicky R.

    If you are a grammar school or junior high teacher it is very unlikely that you will have the space or slab roller to do this. You can achieve very similar results, however by using the terra cotta dishes that go under red clay planters. These have the added advantage that the bisqued dish is absorbent so the piece dries from both sides. Many families have stacks of these planters so they are easy to get for free. They are also usually smaller and more appropriate for kids. You will also need rolling pins and paint sticks (the larger ones) as thickness guides to make the slabs. Works well for all ages. Also great for applying stamps and textures.

  • Thank you Alice Eakin Malicki! You’ve answered the questions I had when I read this article.

  • ALICE E.

    Have made several molds like this, after reading an article about this artist. Three important details are left out: 1) you must securely clamp down the foam to a workbench with open center area, to keep the foam from shaking furiously and for the jigsaw blade to pass through the foam. The shortest blade I found was 2 3/4″, which required clamping over an open space, so the blade would cut through. 2)wear a mask and vacuum the area thoroughly afterwards, because lots of foam debris will be flying around the room while cutting and sanding the molds. 3)If you live in a more humid climate (like I do), you may need to drill a series of small holes in the bottom of each mold to allow the center of the tray or platter to dry evenly with the rim.

    To avoid the need for a jigsaw, I started using a hand saw specifically made for foam cutting, which can be found at any craft store carrying foam craft supplies. No need to clamp down the foam and, if you can hold the knife at a steady angle, you can still cut a fairly uniform 20 degree or greater angle all the way around.

    RE warping: After I remove the tray from the mold, if I can’t dry it on its rim due to added handles or altered rim shape, I weight down the middle of the tray with ceramic pie weights or wood blocks to keep it from warping while drying. I also move the tray several times during drying, from one drywall board to another, to keep the bottom drying uniformly with the rim, and cover the rim area with plastic during the early stages of drying.

    Love my foam molds!

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