. . And The Answer Is . .

The guesses were (thanks everyone by the way!!!!):

Sponge
Styrofoam
Oasis

Plaster

Plaster of Paris

Coral

Cement

Rubber 

Clay

Couch Stuffing

And the answer was:

Porcelain!

Yes, the strange spongy looking thing is made from fine porcelain and is incredibly delicate! 

Because porcelain has to be fired at such a high heat, pretty much anything you put in there with it will be burned into nothingness. So, by putting a porcelain covered something in a kiln, you will only end up with the porcelain with an imprint of whatever you put in there with it (badly written sentence but you get the idea). This can give some really weird results with stuff coming out looking soft, spongy or even wet! Good fun!