After waiting for both test substrates to thoroughly dry I can report that the cellulose seems to have been quite successful, appearing smooth and free of cracks. However the surface is pretty fragile so I'll need to be very careful when applying the pigment so it doesn't flake away.
The silica and plaster mix has cracked in places unfortunately. However the uncracked areas have formed a solid, workable surface, so I want to persevere with these materials. Today I've remixed using different proportions of silica, plaster and water and again left them to dry. I've also applied pigment to the cellulose, and again am waiting for it to dry in order to gauge success.