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Stains on Rubber molds

Jewelry Discussion
#1

Greetings all.

I have been making some rubber molds with the Castraldo Gold
material. The metal masters were cast in Manganese Bronze. The
pieces need to be in bronze and that is what I had on hand. They
will be clasps for heavy wool capes for the period reenactment. I
found that there is a stain on the rubber at the point of contact
with the metal master. This stain keeps the wax from releasing from
the mold, even with the silicon release spray. I finally nickle
plated the masters and made new molds. These release just fine and
have no stain on the rubber.

I am curious as to what is causing the stain on the rubber? It is a
brown color. I notice that I get the same stain from the sprue
formers which may be brass. Is the copper in the alloy causing this
reaction with the rubber?

No more metal masters in bronze, one lesson learned.

I am also open to suggestions on a bronze choice.

When you have a Siberian Husky, one of the first things you learn is
that the weather is NEVER to cold not to walk the dog.

Bill Churlik
@Bill_Churlik
www.earthspeakarts.com

#2

Bill, the stains on your molds are caused by sulfur in the rubber
reacting with the metal. Plating will solve the problem, but I
recommend silicone rubber such as Moldex from the Contenti Company
in Providence, RI. No special alloys or plating are needed, and
built in release agents provide effortless release from the molds.
Very fast and easy material to work with, with slightly higher
shrinkage than natural rubber.

For bronze I swear by Everdur, available from Belmont Metals in
Brooklyn, NY.

Mark Moretti
Alexandria, VA

#3

Hi Bill and Merry Christmas to all, It is the sulfer in the rubber
that is reacting to the copper in the bronze and brass. It will also
react with sterling silver because of the small amount of copper in
it. Don’t throw away your stained molds just yet. Clean the surface
with a Q-tip and some Citra-Solv. This will remove the surface "dirt"
that is causing the problem with your waxes. When I want the best
results I have the model rhodium plated along with the sprue and
former. You should also try Castaldo’s “Gelato” silcone mold rubbers,
very colorful and no need for the spray.

John, J.A.Henkel Co., Inc., Moldmaking Casting Finishing, Producing
Solutions For Jewelry Artists, Our long haired German Shepherd
"Ruby" loves the sub zero weather here in Maine.

#4

Dear Bill,

The cure system used with our CASTALDO Gold Label, White Label,
Titanium Label and No Shrink Pink jewelery molding is sulfur-based.
That means that the curing process liberates free sulfur, which
reacts with the copper in your brass pieces and forms copper
sulfides. That’s the stain that you see. Same thing happens with
silver models – silver sulfides.

Most people don’t have a problem with wax sticking to the discolored
rubber – I can’t say why you did.

Another thing to consider is that in some cases brass will actually
BOND to natural rubber – it’s one way used to make plumbing parts,
etc., with rubber seals in brass housings.

So, yes, stay away from brass unless it is plated first, preferably
with rhodium.

Another approach is to use our Super High Strength Silicone Molding
Rubber, which does not discolor and with which there is no bonding
issue. You can read about it at:

http://www.castaldo.com/english/products_eng/0800gelato_eng/0800gelato_eng.html

Michael Knight
CASTALDO

#5

Bill,

The stain that you speak of is an oxide that results when high
copper bearing alloys-- copper, bronze, brass, tumbago-- are
vulcanized. ) Sulfur in the rubbers reacts with the copper, I think.)
You can certainly use bronze masters (after all, the “button
formers” are bronze).

I sometimes do, and I find that the oxide’s ill effects eventually
settle down, especially after the mold has been “pounced” (dusted)
with talc or cornstarch a few times and shot with several waxes…
(Remember that talc is not good to inhale…)

What I often do when using bronze masters is silver plate them.
Just a flash plate will do. This solves the problem pretty much
entirely. One concern here: make sure the master is quite clean prior
to plating. If not, the silver plate will flake off the model,
remain attached to the inner mold surface and – at the most
inopportune time-- come loose and find its way into the wax. This
doesn’t happen often but you can’t discount Murphy’s Law.

Take care and hope this helps.
Andy Cooperman
coopermanjewelry.com

#6 
    The stain that you speak of is an oxide that results when high
copper bearing alloys-- copper, bronze, brass, tumbago-- are
vulcanized. ) Sulfur in the rubbers reacts with the copper, I
think.) You can certainly use bronze masters (after all, the
"button formers" are bronze).

Does this also apply to silicone mold making material?

Elaine
Elaine Luther
Metalsmith, Certified PMC Instructor
http://www.CreativeTextureTools.com
Hard to Find Tools for Metal Clay

#7 
        The stain that you speak of is an oxide that results when
high copper bearing alloys-- copper, bronze, brass, tumbago-- are
vulcanized. ) Sulfur in the rubbers reacts with the copper, I
think.) You can certainly use bronze masters (after all, the
"button formers" are bronze). 

   Does this also apply to silicone mold making material?

Elaine, Silicone rubbers use a different curing system. They are not
based on sulfur compounds. thus you don’t get discolorations due to
sulphide forming on the metal from reacting with the rubber’s sulphur
content.

Peter

#8

Dear Bill,

I use Zero D products. Bill Mull, the owner of the company, says
that this product has quite a few chemical differences with that of
Castaldo, therefore, you won’t find this problem with the Akron Mold
Rubber.

If you need more technical about this or other topics in
reference to rubber or silicone, you can communicate with Bill
(Jwryrubber@aol.com).

Best regards,
Juan Pablo Martenez Mansilla
CEO
Grupo Rex Nisi, S.A.
Calzada Roosevelt 33-86 Z.7 Edificio Ilumina Of. 801.
Guatemala Ciudad, Guatemala, Centroamerica.
Tels.:++(502) 2439 7003 - 2439 7007
Fax.: ++(502) 2439 7005