Castaldo makes some RTV’s and they may pipe up here with
If long library life is the most important thing, I would probably
go with a platinum cure silicon. This is essentially the most
expensive RTV but one with almost unlimited library life. Platinum
cure silicons are touchy with proper catalyst mix and outside
contamination (do not mix equipment used on tin cure with platinum
cure, ideally don’t even have them in the same area!). Tin silicons
should give a 10 year+ library life. The base component of both types
will settle and need stirring if stored for a time and the catalyst
materials have about 1 year shelf life. Pretty good tear strength,
great wax release and wonderful wax surface finish. Best if you can
vacuum the rubber either before pouring or after pouring into the
mold (poor bubble release).
Other RTV’s would be:
poly sulfides (black tuffy) Good strength, good library life (see
cold creep below), mold material and catalysts have great storage
life (years), good wax release, good smooth wax surface, but some
are pretty smelly while curing, easy to mix (good tolerance to
slightly wrong catalyst amounts, and all have the problem of cold
creep (the mold rubber will move after being catalyzed if not
supported by a wax or if put together incorrectly and stored for a
while). Good bubble release and if careful, you can get bubble free
molds without vacuum.
urethanes Good mold strength, pretty good library life, not as
flexible as the polys or silicons, really need to be well released
to keep the wax from sticking to the mold or to the master (it is
great glue), poor wax surface finish. Can be made “clear” by
vacuuming before pouring (sort of honey colored). Relatively good
shelf life for the mold materials, especially if stored with a
nitrogen gas cap.
There are other materials available but these are the ones I have
used and know about. If it were me and for jewelry and needing long
library life, I would migrate towards the platinum silicons. I have
used all of these (also vulcanized rubbers) for jewelry, but now I
mostly make large molds for bronze casting. These are not poured
(they are painted on the masters) and currently I am using mostly
tin silicon as 10 year library life is fine, no cold creep problems,
great wax finish and master/wax release.
Hope this helps a bit rather than confuses you.