clay for planting in crevices?

I just watched a video by Harvey Wrightman of Wrightman's Alpines where he uses clay as a growing medium for alpines planted in crevices. He spreads the clay on o piece of tufa stone, lays the bare root plants on the clay, and then places the whole thing underneath another flat stone, kind of like a stone sandwich. What is this clay material he is working with? the consistency is not like any clay that I have seen before. Does anyone know what it is and where to get it or how to make it?

Why not ask at the Wrightman's site? I'm sure someone there would be happy to supply the answer.
http://www.wrightmanalpines.com/about/contact

I guess that was too obvious...

Did they get back to you? I'm assuming its water soluable clay you use to make pottery?
(not the modelling clay that does not "dry out")

Thanks
Tam

I never did ask , but I think you are right about using clay intended for ceramics, with extra water added to give it a spreadable consistency.

I talked to a speaker at one of our chapter meetings who said he added greensand to the clay to add fertility.
I haven't tried it myself yet but I may this year. Will post here if I do.
Tam