Almost every fossil presents us with some obstacle which must be overcome in order to produce a usable cast. In this instance the fossil, Manchurochelys liaoxiensis from Liaoning, China, resided on a layer of shale which was only about 3/16's of an inch thick. The shale and the fossil appeared to be fairly stable. The problem was that the cast needed to be about 3/4's of an inch thick in order to attach the hanger on back with screws.
To address this depth problem we added a roll of clay to the back edge of the fossil.
The fossil is then pressed onto the clay roll and trimmed to the shape of the edge of the original matrix.
A custom frame is made for the mold which allows for at least 3/4's of an inch for the mold sides and at least 1/4 inch of depth over the highest point on the fossil.
This is the point at which you are absolutely commited to making the mold. Once the silicone goes onto the fossil you must wait at least 24 - 48 hours before the fossil and mold can be separated. It takes that amount of time for the mold to be adequately cured. You can expect that some small pieces of the fossil or matrix may be pulled off when the mold is separated from the original. This is more common with fossil fish, which typically loose a few scales or bones in the molding process. Another fear with a piece this thin is that the slab will break when the mold is pulled off.
After the last of the silicone is poured onto the specimen, the mold must cure for 24 to 48 hours (depending on the volume of silicone used). This mold was fairly thin and required only 24 hours to cure.
Manchurochelys showing edge thickness
Clay ring to increase cast thickness
Excess clay trimmed from edges
Manchurochelys and mold frame
Silicone for mold poured on fossil
Manchurochelys mold curing