Baking Clay/firing

[thinklogically]

I am a complete noob to pottery, honestly I'm only interested in clay casting with slip. I want to make a olla, I just have a couple of questions.

For that thickness,1/8 of a inch, how long will it have to sit for casting?

Is it possible I can bake this in my own oven? There is a pottery place near me, Dallas, but it is far away. I will be using earthenware clay. I need it to be porous still.

Also, does the mold for casting have to made out of plaster? I know how to make molds- can it be made out of plastic or silicone?

Also, there is a place, what temperature do I need to fire it at?

Thanks, Timothy

[Benzine]

Welcome to the forums.

 

To start off, no you cannot harden your clay in an oven.  To remove the chemical water, that makes clay become ceramic, you have to get above 900 F.  Any lower than that, and the clay will be simply air dried, and will still absorb water.  

There are air dry clay and oven hardened varieties, but they are not functional.  

 

Plaster is ideal for casting, because the plaster absorbs the water from the slip, so it sticks to the walls of the mold, and dries quicker.  Things like plastic and silicon work well enough for press molds/ sprigs, but not for casting.  

 

I haven't done much slip casting, so I am not sure how long it would have to sit, before removing, in the mold.

 

Best of luck in your ventures.  

[bciskepottery]

When slip casting, the purpose of the plaster is to draw away the water in the slip, leaving the clay form behind. As neither silicone or plastic are absorbent, they will not be suitable for slip casting. How long to get to 1/8" . . . depends on the thickness and dryness of your plaster mold, also the thickness/viscosity of your clay slip. You'll need to experiment to get the timing down right.

 

To be used as an olla, you'll need to fire the raw clay vessel at a higher temperature than your oven can go.

[Callie Beller Diesel]

The temperature you'll need to fire to will depend on your clay, but that information should be available from wherever you buy your slip, or your casting mix. Also, you will hear us talk about firing to a "cone", not usually a temperature. Cones are a tool that measure time as well as heat. Both are needed to create ceramic from clay. Orton is the brand most used in North America. If you google orton cones, or orton pyrometric cones, you can easily find charts that will give you approximate corresponding temperatures.