Back to Ganoksin | FAQ | Contact

Best Type or Brand of RTV Rubber?

For strength, fine detail and long lasting molds, what brand of RTV
is the best? I am aware that vulcanized rubber is considered the
"best for strength and longevity, but for my purposes I need
suggestions for RTV. Also, how do you preserve and store RTV molds to
keep them from decomposing.


I have always preferred the Silastic RTV by Dow Corning. It has
however become difficult to find and expensive. I have since
switched to Polytek. They can be reached at 800-858-5990 they have a
very informative catalog and are very helpful folks to find the
material with the properties you need. (usual disclaimer
goes here)

Frank Goss

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.

John Dach

Hi Elkka, I’ve used all the RTV’s over time and found the best way to
go for stregnth and longevity is Akron RTV-RP made by Zero-D
Products. It is clear, so bubbles show and can be removed. Silicone
RTV molds will not decompose. Just place them in a drawer away from
Sunlight. There are some RTV’s that are made from Polyurethane that
will decompose. These are less costly than Silicone, but you really
save nothing if the mold decomposes.


Just wanted to thank you three fro the replies. I may try the clear
compound from Zero D - looks interesting.

Just wanted to thank you three fro the replies. I may try the
clear compound from Zero D - looks interesting. 

If it is the same as the material I tried, it is very hard. I ruined
the piece I tried it on that my wife had sculpted. It was done in a
pretty hard oil clay (2" long baby ). I didn’t like it at all but I
am sure there are users of it that love it. Soooooooooooooo many

John Dach

I am just wondering. Has any one tried the Freeman Jewel-Sil Kits of
2 part RTV mold rubber.

Andy The Tool Guy Kroungold

Hi John,

If it is the same as the material I tried, it is very hard. I
ruined the piece I tried it on that my wife had sculpted. 

I’m not sure you tried Akron RTV-RP silicone either, as it has a
hardness of 40A, which is a standard hardness found in natural rubber
and most heat cure silicones made for jewelers. I am happy to have an
opportunity to address an issue that folks often don’t think about.
That is “what if I break the carved wax?”

If you cannot for some reason afford to break the wax or model,
don’t use this method, but please remember something. If you break
the wax, you still have the mold you created from it. Just inject a
new wax.

It is a good idea to do a small test with any new material before
risking loss of a valuable model. If it is too hard, you will know
ahead of time.

If the RTV is too hard at 40A, we offer other choices down to 30A

Happy New Year,

Bill Mull
Zero-D Products, Inc

Hi Andy;

I am just wondering. Has any one tried the Freeman Jewel-Sil Kits
of 2 part RTV mold rubber.

I’ve tried it, I’m not crazy about it. It’s a bit easy to tear, the
texture of it’s a little grainy, but it’s ok for the price. It’s
stiff enough not to deform under clamping. Oh, I hate to pick on you
guys, but what the heck has happened to Stuller’s in house brand of
saw blades? I got some Pike blades, which I’ve used for years, and
they broke just looking at them. So the next order I got some of
Stuller’s Premium brand. No better. They say “Swiss made” but I’m
suspecting somebody is jobbing these out to you-know-where. I had the
same thing happen with Stuller’s “Robinson Brushes” (what we used to
call the bristle rotary brushes). One day, they were just crap. I
figured it was a fluke, but the next box were the same, they just
flew apart immediately. I called Stuller and, yep, outsourced the
manufacture. I’m hoping Stuller will get it that we jewelers will be
happy to continue to buy their products as long as the quality is
there, and would rather pay more so that Stuller can profit rather
than get less for out money and start shopping elsewhere.

Please don’t read this as a threat of a criticism. I love doing
business with Stuller, but the day they became a publicly traded
company, I’ve been hoping they wouldn’t end up joining the race to
the bottom. I really want the company to keep it’s “edge”. They’ve
always been a model to follow.

David L. Huffman.