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Should I use clay or ceramic to make my own mold? I want to make sure that my expantion is right.

What type of Clay or Ceramic should I use. last ime I was at the store, they had 120 different kind of clay. Obviously they had no clue which clay I should use for glass slumping

Thanks

Call Laguna Clay.They are located in the town of City of Industry, California. They sell high fire clay that can be used to create reuseble molds. Ask for the salesperson who has this knowledge, I spoke with him before, I don't recall his name.
Pat

Low-fire earthenware clay is suitable for making molds; this clay fires at cone 04, around 1900F. Any local pottery supplier will carry it. High fire clay is not necessary, in fact, a lower fired clay mold is less rigid than a high fired one, and can better withstand expansion in the kiln.

You can also use plaster slip cast molds for casting mold shapes. You may be able to find used ones quite cheaply. In this case, you would buy low fire earthenware casting slip. Of course, you would have to read up on slip casting pottery, but it's not difficult.

Cheers,
Suzan

I use cone 4 lowfire clay to make my molds. I fire it to 1650 (like most glass kilns my kiln isn't supposed to go over 1700) and the molds work fine, though they do break if not handled with care. The most important thing about making clay molds is to make sure they are completely dry bofore firing or they will explode (like mind did a few days ago). Marc

On the topic of drying, how long should a clay mold dry before being fired? I too had one explode in my old kiln, and it scared me so badly, I do not want it to happen again! LOL! My molds are about 1/2 inch thick. How long should I let them dry before firing them??? Thanks.

Molly, if you are making your mold from solid clay at 1/2 " it should dry a week or two. If the mold feels cooler than the table it is sitting on it is still wet. If you are slipcasting 1/2" is way too thick. Try more like 3/16" for cast molds.

Best wishes,
Tom in Texas

Tom: I do not know much about pottery. Can you explain slip casting? I'd really like to learn more about pottery and ceramics, among other things, but glass is my "love" right now! Any help with these unfamiliar terms will be most appreciated....LOL!

Even castings from slip-cast molds shrink. We made tableware plaster slip-cast molds and the castings fired about 1/2" smaller than the original plates. Good process for production tho. - Barbara

Molly, slipcasting is the method used to produce many ceramic items. A plaster of Paris mold containing from one to as many as ten separate pieces is made from an original sculpture. The mold pieces are assembled and held together with large rubber bands and sometimes locking web straps. A liquid suspension of clay in water with the proper chemicals to keep it suspended, called "slip", is poured into the plaster mold. The plaster mold absorbs the water from the slip like a sponge soaking up water. When the water goes into the plaster mold it pulls the suspended clay particles against the inside wall of the mold and leaves it there because the solid clay cannot follow the water into the plaster. Over time this process builds up a solid wall of clay against the inside of the mold. When this wall is the desired thickness the excess liquid slip is dumped back into the holding tank for use later. The mold with the casting inside it is set aside for a while to allow the very wet clay inside to firm up by more water going from the clay into the mold and some evaporating into the air from the inside surface of the item. When the clay has hardened enough to keep its shape outside the mold the bands are removed and the mold is carefully opened and the casting removed from the mold. The casting is soft and somewhat flexable at this time so care is needed to avoid distorting it before it drys enough to become stiff enough to hold the desired shape. If the item is something like a figurine or animal additiional legs, arms, and horns are assembled to it at this time using the casting slip as glue. Molds for glass slumping do not have add ons. After drying completely the casting is smoothed and fired to 1600 - 1800 F for slumping molds.

Best wishes,
Tom in Texas