[Maker’s Note] Grouting Plaster Molds

by Yun-Han Lin

To build ceramic works, I have been trying to make plaster molds using plastic bottles. I have only tried small silicone molds before, so it took me some time to test with plaster, and the following are some conclusions I have come to.

Method 1: Floating

First, build a box in which you make plaster molds. Put an empty bottle inside the box, and then pour the plaster into the box to half full. As the empty bottle floats on the plaster surface, put something heavy inside the bottle, so that the plaster immerses half of the bottle. Next, press the bottle to keep it still before the plaster hardens. After drying, apply a layer of release agent to the plaster surface, before you pour in another half of the plaster. Lastly, remove the outer box, separate the two halves of plaster molds, and take out the bottle.

Problem: Once the molds were separated, there were traces on the surface of the first layer of plaster. Possible causes may be the bottle repeatedly floating and being pressed when it was in contact with the plaster. Also, the amount of plaster poured in was not measured well.

Method 2: Place a Stick

Place a stick in the plastic bottle and fix the stick in the middle of a concrete bottom, so that the bottle is fixed still in the center. Next, pour in the plaster separately. Do not forget to also apply the release agent between the two layers.

Problem: The surface of the first layer plaster was still uneven, while that of the second layer turned out better. My hypothesis is that when we pour in the plaster starting from the bottom, some air was mixed in, resulting in bubbles that lead to uneven surfaces.

Look up for tips

After two failed experiments, I looked up on the Internet how professionals make plaster molds. Some suggest using soil to fill the bottom space where the first layer of plaster goes. Then pour the plaster on top of the soil as the first layer. Remove the soil and pour in the second layer of plaster, while the bottom part can also serve as the third independent mold.

Method 3: Modeling Clay

Problem: Modeling clay can be too soft, which makes it difficult to flatten the surface. Therefore, the surface between the two layers of plaster turns out to be quite uneven, which may lead to crooked parting lines between the molds.

* The modeling clay is often used in clay art projects. It is harder material compared to the ones used in a classroom setting, but still very soft, if compared to that for sculpture.

Method 4: Clay

Finally, I used clay and try to follow the standard procedure by wrapping the bottle with the clay and smoothing the surface as much as I could. I separated the bottom layer as the third piece of molds, and voila! The surface turned out nice and smooth. The plaster was a bit brown on the surface though, since it got into contact with some clay.

As the plaster mold is done, we can move on to grouting. We are still working on that and will share with you the secret to it in the future!