A 1-lb block of non-hardening, sulfur-free modeling clay. Good for sculpting as well as reusable caulk for mold boxes and mold shells to make them liquid tight. The clay that we use is sulfur-free. This is important because sulfur can cause cure inhibition in platinum-cured silicone rubber. It is best to use an oil-based, sulfur-free clay, especially when working with platinum silicones. 
 
You can find more uses and examples here HERE 
 
USES :
Sealing Seams
For the purposes of sealing, clay can often be easier to work with than hot glue or silicone caulking (and clay is recyclable!), especially when it’s warmed for easier application. We always keep a batch of clay in our oven, set at 140°F, so it’s ready to go at any time.
 
Clay as Mold Boxes & Containment Areas
Not only is plasticine clay great at sealing mold boxes, it can can actually serve as a “mold box”, or containment area, itself
 
Two-Part Block Molds
Clay is an integral part in the making of two-part block molds, which are constructed by pouring one piece of the mold, allowing it to cure, and then pouring the second piece.