Now I found some time to start the mold for this panel, hopefully I will finish it tomorrow, but I did get a late start on the first application of silicone mold rubber, and it takes about 6 applications total, each an hour or so apart to build up sufficient thickness. Then it cures in about 6 hours, or in this case, over night before the plaster supporting shell can be made.
Here’s the origial clay model laid flat on a board with the first “detail coat” carefully applied, very thinly over the whole surface to eliminate as much air bubbles as possible when it’s brushed on.
Subsequent applications have a thickener added so it’s more of a mayonaise consistancy and will stay put on vertical surfaces as well as build up thickness rapidly.
Without the thickener the material is more like honey, wanting to flow in a slow and sticky way to seek a level like water does.
The final measurements of the panel is about 17″ x 21, meaning the original clay as it was has shrunk about one inch in width and about 1-3/4 inches in length.
All clay shrinks as it dries, and this is about 6% shrinkage, so in order to gain a specific size that has to be allowed for in the original design.
This also shows how damaged or missing terra cotta components on buildings being restored cannot simply be replaced by making a mold off an existing or undamaged element to make replacements- the final casts would wind up about 6% smaller.
The alternative then is concrete “replacements” cheap fiberglass, or having a sculptor sculpt a replacement model about 12% larger if using water clay for the model due to the double shrinking, about 6% if using an oil based clay or other material such as carved and shaped plaster.