Celluclay - Archival quality?

I am planning to make a rather large bas relief. I need for it to be
light, since it will hang. I'm thinking about sculpting the relief out
of clay, molding it, then pouring the mold full of something like
Celluclay. This will then be covered with canvas and painted. There
will be detail, but not fine detail. The Celluclay itself is just a
filler. I'm concerned about the lasting value of Celluclay. I can't
find any specs for it. Any experience with this stuff? Any suggestions
for alternatives?

Many thanks for any help.

Raymond