Q. I would like to know what type of clay is used in Vessel Core-Forming?

I'm forwarding a message from Bill Gudenrath describing his "core" method: 

"I've always done core-forming by trial & error, so there's no formula, per se. 

Generally speaking, it's a lot of dung and a little clay and water. 
The idea is for the dung to temporarily provide a framework for the clay. 

Then, as the clay is fired (at about 1,000 degrees Ferenheit), the organic matter burns and vaporizes away leaving a fine, strong - but ultimately easily crushable - matrix of tiny clay tubes onto which one creates the vessel. 

Next time I do it (this summer as a matter of fact), I'll attempt to describe it more thoroughly." 

I've attached a bibliography for core forming, including information about other artists versions. There are also a number of bead artists who have adapted modern methods to the form. Please let me know if you'd like more information.