Looking for clay casting recipe

[LBV]

Hi, I am relatively new to ceramics (only a few years) and am looking for a casting recipe for a specific purpose. I am wanting to make ollas, so I need a recipe that will allow the final product to be porous. Most of the recipes call for redart  which I do not have access to in NZ. I can find plenty of articles about the making and using of ollas, not much about the composition of the clay. Many mention using terracotta, but the terracotta I have access to is a stoneware, not earthenware and solid.  Some also talk about a dding sand to the mix, although I do not know whether it helps with porosity or not. As the ollas will be buried in the ground, they need to be quite strong The club where I have access to a kiln typically fire bisque at 900C (1652)F, and glaze at 1200C (2192)F.

[liambesaw]

Any bisqued stoneware will weep water. Does not have to be terra cotta for that.

You can add grog, sawdust, paper pulp, etc. to increase porosity as well.  I'd probably go with a stoneware and add paper pulp if I was going to be casting slip, just because it won't settle like sand or grog, and won't be TOO porous like it would with sawdust or rice husk, etc.

You'll want to experiment because I am guessing you'll need to shoot for a specific flow rate for something that is supposed to slowly water surrounding plants.  

I would first see if your normal bisqued stoneware would be strong enough though, you may need to do nothing.