Sulphur, silicon, and plasticine

[Kelly Godel]

I keep hearing about how sulphur in plasticine inhibits the curing of
silicon molds. But i have been using bathroom caulk variety silicon for
years--applied over roma plastilina with no sealer and it works fine!
The only time I ever had a problem with silicon was when I tried the
non-smell variety--it turned into gum. Terrible mess. Ruined the
sculpture too. :(
I havent tried the two part silicons--I always use caulk despite the
fact that you have to trowel it on in very thin layers--and its limited
strength. Love the way it picks up detail though.

Just my two cents.

[Battersby]

Whatever works for you is always going to be best for you. There's lots of
ways to skin a cat.
--
T. M. Battersby, stuccoist.
http://www.battersbyornamental.com
tbatt...@satx.rr.com

Kelly Godel <kgo...@idmail.com> wrote in message
news:38DE43EF...@idmail.com...

[sculpt...@my-deja.com]

Silicone caulk is an evaporative system- as the acetic acid boils out it
cures, but as you discovered it is limited to thin coats to enable that
evaporation.
Catalytic silicone systems operate differently, and the "gum" experience
you had was the result of sulfur inhibition of cure.
To avoid that- find a local supplier of Silicones Incorporated and buy
G.I. 1000- the most reliably sulfur immune product on the market. Then
you can pour your molds in one swell foop and save all that labor.
seal your sculpture with a lite spray of matte acrylic for a really good
release and clean mold surface.

Also- there is a two part silicone 'putty" that just hit the market. you
mix it up like A&B epoxy putty and press it on. It cures in 15 minutes
and will cure in any thickness- however- I have not tested it yet for
sulfur sensitivity, so beware.
christopher
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Sent via Deja.com http://www.deja.com/
Before you buy.

[WoN ereH]

I was a moldmaking disaster area till I discovered GI 1000. Great stuff and
very forgiving. Makes mold making fun even. Well, sorta.

Debra

[james Berrangé]

Any idea what its made from? I have used a silicon rubber called 585 in South
Africa which makes wonderful molds but is hell to use. It adheres strongly to
literally everything. If you sit down in some and it sets, you may never get up
again (not without a chair stuck to your bottom), or at least not with whatever
you are wearing.

[sculpt...@my-deja.com]

Sounds to me like what you have is a urethane rubber-they are famous for
sticking to everything.

One way to tell is by the catalyst.- Like polyester resin, RTV Silicone
undergoes true catalysis. Most systems use a typical 10% ratio of
catalyst to rubber.
This means if you start with 100 grams of rubber, you add 10 grams of
catlyst. As it cures, most of the catalyst boils out of the rubber and
the chemical does not combine with the rubber molecule. Although you
ended up with 110 grams of mixed material- after it cures for 48 hours,
you will find that it weighs 103 grams, The 3 grams remaining is mostly
pigment.

Urethanes, however, are a compounded system. You mix two chemicals
together and they combine in a reaction that results in a third, new
chemical. Urethanes typically are in a 1 to 1 ratio or a 60/40 ratio. If
the system you are using is like that, it is probably not really
silicone.

Some silicones, not intended for mold use, are formulated to be very
sticky- like the silicone caulk mentioned above.

Also, silicone rubber will stick to pourous surfaces. This is due to its
ultra-low surface tension- it will flow INTO pourous materials if they
are not sealed.
To facilitate release of silicone from a pattern you can use a teflon
spray, or you can spray the pattern with clear acrylic, or, for pourous
materials like plaster, you can use mold soap.

Be aware that some silicone formulations will stick like glue to certain
brands of spray laquer ( but not others!?) G.I. 1000 included. (in
truth, it doesn't stick to the laquer- the laquer affects the rubber in
such a way that when you try to cast resin into the mold, the resin
casting sticks to thesilicone like crazy glue )
Also- tin cure silicones can really stick to anything with a high tin
content- so, if you use metal shims, make sure its not tin, or that you
use a separtion agent on the tin.

The only way to get cured silicone out of fabric is with scissors.

Christopher
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