Silastic J

This conversation went so quickly from Silastic to Liquicast I
wanted to point out that RTV rubbers can be made of silicone,
polyurethane or polysulfide. If you search the internet, you’ll find
infinite formulations and applications, most of them geared towards
big stuff. Jewelry-oriented RTV is silicone or polyurethane. There
are clear and opaque versions of both.

If you use polyurethane RTV rubber (like Liquicast), you need to
keep it away from moisture, sulphur and, get this, polystyrene.
Jack, I suspect the tray you welded your mold to was made of
polystyrene. Also, depending on what brand of polyurethane RTV
rubber you use, you may have to be careful about exposing it to light
as some kinds can yellow and stiffen.

If you use silicone RTV rubber (like Silastic, Ditto, Akron), you
don’t have to worry about anything. Moisture, sulphur, polystyrene
and light don’t do anything to silicone.

Dana Carlson

But be sure to make a “barrier” layer/coating on your model,
especially if it contains sulphur, especially if you are using many
of the silicons. Many will not set up in the presence of sulphur
bearing clays for example, and you end up with a layer of goo next
to the model. Best to do a trial with any new product.