I'm planning to make a great number of these sorts of shapes for an installation I'm thinking of doing. To help speed up the process, and because some of my students were curious about the process, I thought I would make some molds for casting the shapes.
I started by throwing an example of the shape I wanted. For the first mold I made, I fired the form in the bisque kiln. For later molds, I used a leather-hard form I made the day before.
|wet clay form in half a plaster mold|
I made the first side of the first mold at school so I could show my students, but didn't take pictures of the process. I later broke the bisque form I was using and had to replicate it in wet clay to make the second half of the mold.
|leather-hard clay forms in wet clay bases|
To make the first half of any of the molds, I inset the leather-hard or bisque form into a flat base of wet clay. I tried to place the form so that widest part was exactly lined up with the top edge of the wet clay. I did this so that the mold won't have any undercuts that make it difficult to remove the cast form.
|ready to cast the first half of this mold (except for that puddle of oil soap in the key)|
I then pressed the end of a round tool into the wet clay base to create indents that will eventually be keys for the two-part plaster mold. I rolled out a slab of clay and wrapped it around the wet clay base with the inset form to create a well into which I would later pour plaster.
|making a mold for slip casting|
I want to try slip casting with one of the molds, so I created a hump of clay to take the place of the opening through which I will eventually pour the slip. I haven't created a slip casting mold before, so I'm not sure if my "sprue" will be wide enough.
|second half ready to cast|
For the second half of the mold I created earlier, I wrapped a new wet clay slab around the already-set first half of the plaster mold. I then brushed both the plaster and wet clay surfaces of all three molds with Murphy's oil soap so that the new plaster will come apart from the old plaster and clay.
Earlier in the weekend, I had left my oil soap at school and decided to use baby oil as a replacement. That is apparently a terrible idea, as it didn't act as a release at all. The baby oil experiment led to an incredibly strong attachment between the fired clay and the set plaster. Even smashing the bisque form with a hammer didn't break it apart from the plaster mold.
|bisque form stuck in the mold|
After the clay molds for the plaster were made and coated with an effective release agent, I mixed the plaster. I found that coating a bucket with a plastic bag is a nice idea and helps make cleanup easier only if one uses a bag without a hole in it. An unnoticed hole results in water dripping mysteriously onto the poured plaster and confusing the mold maker for a while until she noticed the puddle under the bag.
|easy plaster clean-up|
The plaster sets fairly quickly and then the clay can be peeled off easily. Ideally, the plaster and fired clay both come apart easily, too.
|plaster setting in molds|
The plaster needs to be thrown away after use and any clay with plaster stuck to it should also be thrown away since plaster looks like clay then bursts in the kiln, leaving a hole in the side of whatever was being fired.
|the first half of the slip casting mold, set and almost ready for the second half of the mold|