Throwing Molds on the Wheel

by A. Blair Clemo

Making molds is an investment of time, material and space. Big blocks of plaster quickly pile up in every conceivable storage space a promise that a day in the studio is also a great upper-body workout. Because I use molds in nearly every pot I make, I am always looking for ways to make more efficient molds that utilize less plaster, take up less space and save my back. Throwing mold coddles, rather than using traditional wooden coddles is a great way to get your molds out of the cumbersome cube shape. It also saves plaster, lots of it.

Because most of the prototypes I make are thrown on the wheel, it makes sense for me to make molds on the wheel too. These wheel thrown molds are round and can be made to fit the contours of almost any thrown shape, wasting almost no plaster.

Always keep plaster away from your clay. I have a designated bat strictly for making plaster molds on the wheel, which helps to reduce contamination. The bat is made from wood that has been treated with several coats of lacquer. This will prevent warping and keep the bat easy to clean.

Designated Bat for Plaster

Designated Bat for Plaster

Here I have a prototype that was thrown on the wheel, has been carved, cleaned and is ready for the mold-making process. I will be pouring this mold with the form upside-down. I measure the width of the lip with calipers, this will give me a gauge for how large to throw the coddle.

Measuring the Prototype

Measuring the Prototype

Blair Clemo

Next, on my clean designated plaster bat, I center and throw a cylinder without a bottom, open all the way to the bat. I can compare the width of this cylinder with my caliper measurement to ensure that I have the desired wall thickness I need for the poured mold. Here, there is an inch on either side of my calipers, so my mold will be one inch thick all the way around. This is plenty thick enough for press molding, slipcasting may require a thicker mold.

Coat the bat with mold soap before centering the prototype and casting the plaster. This will help the plaster come cleanly off the bat and will inhibit plaster from leaking under the prototype.

Coat the bat with mold soap before centering the prototype and casting the plaster. This will help the plaster come cleanly off the bat and will inhibit plaster from leaking under the prototype.

Now I center my carved prototype upside down. It is important that the prototype is stuck down very well to the bat. To accomplish this, I make sure that there is plenty of mold soap on the bat and I wet down the rim of the piece with a sponge. When the piece is centered, I press down very firmly and wiggle it side-to-side a little to make sure it is fully adhered to the bat. I am careful to ensure that both the thrown coddle and the prototype are perfectly centered. This is important to making a mold with even wall thickness. Next I compress the cylinder so that it will follow the curve of the prototype, remembering that I am looking for an even thickness of 1” throughout the mold.

Blair Clemo
The coddle is compressed so that it follows the contour of the prototype.

The coddle is compressed so that it follows the contour of the prototype.

Now I am ready to pour the plaster. I have found that letting the wheel spin at a slow/medium speed allows me to pour the plaster more evenly over the piece and helps reduce air bubbles. I fill the coddle so that there is 1” of plaster over prototype, keeping the bottom consistent with the walls.

Blair Clemo

That's it. I'll let the plaster set and clean the mold up after it is solid, which takes about an hour. Because both the coddle and the prototype were centered, this mold will be very easy to center on the wheel, perfect for throwing inside of for quick serialized production.

A cut away of the thrown coddle and the set plaster mold.

A cut away of the thrown coddle and the set plaster mold.

The finished mold, with prototype removed. There are no air bubbles on the inner surface due to the movement of the wheel upon pouring the plaster. The surface will be cleaned with a sponge and allowed to dry before use.

The finished mold, with prototype removed. There are no air bubbles on the inner surface due to the movement of the wheel upon pouring the plaster. The surface will be cleaned with a sponge and allowed to dry before use.