Hi Everyone

I'm just in the research stage of diorama making, and resently bought the Verlinden book on 'Super Dioramas' . I was reading the article by 'Bob Letterman', and he mentions using Artist paste on the surface of his building to carve in detail.
I searched art supply stores but cannot find any such product.
What is it? Is there another name for it, or is it just plaster?

Thanks,
Bill