Kathy Forer on sun 27 apr 08
I have an unfired piece I'd like to use to make a rubber mold. I don't
want to lose the original but am not sure how to seal it against the
rubber. I guess whatever I use will burn off, but what?
I'm ready to experiment but wonder what hard-earned experiences there
are in clayland.
An archive search was helpful but I couldn't find specific detail on
rubber molds from greenware. Will it work or is the original safer
with a plaster piece mold? Do I even need a parting agent with rubber
or will the powdery surface suffice? (Doesn't seem likely but I'd try
it.)
Kathy Forer
--
www.kforer.com
Snail Scott on mon 28 apr 08
On Apr 28, 2008, at 12:00 AM, Automatic digest processor wrote:
> Date: Sun, 27 Apr 2008 22:45:27 -0400
> From: Kathy Forer
> I have an unfired piece I'd like to use to make a rubber mold. I don't
> want to lose the original but am not sure how to seal it against the
> rubber. I guess whatever I use will burn off, but what?
I have done this.
But first, why do you want to make the mold from
greenware? Its chances for survival are vastly
higher if you bisque it first. Seriously, I can't
imagine any reason not to bisque first, especially
if you want to preserve the pattern to fire later,
because the odds of destroying greenware are
really high.
Assuming you want to go forward, the optimum
process would be the same for greenware or
bisque: shellac. Spray it, if the form will allow it
onto all surfaces. If not, it will need to be painted
on. If the form has an interior volume (like a vase)
I would close it off with plasticine (oil-based) clay.
If it is an open form, making a mold of both sides
is fine. If it is a closed form already, no problem.
In other words, if the inside would be an undercut,
close it off.
You will need to attach a sprue, which can be made
of almost anything. Plasticine or wax will work well.
After you shellac the surface, use a silicone mold
release spray all over.
For the mold itself, I would use a paint-on urethane
rubber like those sold by Polytek or Smooth-On.
Apply about two thin coats, then if it is to be a multi-
part mold, set shims to make a parting line. Cut-up
soft drink cans are nice and thin, as are other
materials, like litho-plate metal and some plastics.
Use the release on the shims, too, but don't get any
on the rubber that's already applied.
Coat the shims along with the whole piece up to
a thickness of at least 1/8 of an inch, and thicker if
you need to fill small undercuts or deep texture.
(That's the nice thing about rubber - undercuts are
not much problem. The backing mold will have to
have none, though.)
Then you need to make the backing mold of a rigid
material. Some people like fiberglass, but I prefer
fiber-reinforced plaster. The plaster can have no
undercuts, so it may end up having more parting
lines than the rubber. Use vaseline or soap to coat
the rubber, and as a parting agent between plaster
sections.
So far, so good, but the real hazard to greenware
will come when you try to take this all apart, prying
the plaster backing off and using a knife to cut open
the seams of the rubber. If the form has no delicate
parts and is thick and sturdy, it may survive, but I
would still strongly consider bisquing the clay first.
Re-bisquing will remove all the sealants and goop
from the mold process, and leave you set to do
whatever surfaces you have in mind for the piece
before the final firing.
I'm assuming you know a lot of this already, but if
you would like more detail, let me know.
-Snail
John Rodgers on mon 28 apr 08
Kathy,
Have you made molds before?
Have you made rubber molds before?
What kind of rubber mold do you want? Flexible latex, Silicone,
Polyurethane? Flexible? Rigid?
How do you intend to use the mold?
Heep in mind that when you make a mold, there is always the risk of
destroying the original model.
You will need to seal the greenware.with a number of coats of spray-on
urethane varnish. When done, wax the surface like furniture. Next
lightly spray a coat of
Polyease 2300 urethane mold release agent (from Polytek.com on the
internet). Then, according to the instructions, brush on a coat of
2-part brushable rubber compound. When it is set, Cast a plaster
"Mother-mold" over it to give support to the rubber. When set, remove
the plaster mold pieces, carefully cut a single cut through the rubber
mold and then remove the model. Put the rubber mold back into the mother
mold, then cast a new model in wax or plaster. From these you can make
as many molds as you wish without worrying about losing the original model.
Good luck,
John Rodgers
Chelsea, AL
Kathy Forer wrote:
> I have an unfired piece I'd like to use to make a rubber mold. I don't
> want to lose the original but am not sure how to seal it against the
> rubber. I guess whatever I use will burn off, but what?
>
> I'm ready to experiment but wonder what hard-earned experiences there
> are in clayland.
>
> An archive search was helpful but I couldn't find specific detail on
> rubber molds from greenware. Will it work or is the original safer
> with a plaster piece mold? Do I even need a parting agent with rubber
> or will the powdery surface suffice? (Doesn't seem likely but I'd try
> it.)
>
>
> Kathy Forer
> --
> www.kforer.com
>
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Kathy Forer on mon 28 apr 08
On Apr 28, 2008, at 8:35 AM, Snail Scott wrote:
> But first, why do you want to make the mold from
> greenware? Its chances for survival are vastly
> higher if you bisque it first. Seriously, I can't
> imagine any reason not to bisque first, especially
> if you want to preserve the pattern to fire later,
> because the odds of destroying greenware are
> really high.
Ever-practical, your question is helping me reconsider my desired
plans. I was hoping to make a mold before the piece went through final
firing shrinkage.
It's a fairly simple low to medium-relief piece with minimal undercuts
-- basically a blanket mold. It seems some of the silicon rubbers
don't require sealant, theoretically, that is.
But really, why take such a chance, why not fire the piece first? Good
question, thanks.
Kathy
--
www.kforer.com
Kathy Forer on mon 28 apr 08
On Apr 28, 2008, at 9:29 AM, John Rodgers wrote:
> Kathy,
>
> Have you made molds before?
>
> Have you made rubber molds before?
>
> What kind of rubber mold do you want? Flexible latex, Silicone,
> Polyurethane? Flexible? Rigid?
>
> How do you intend to use the mold?
>
> Heep in mind that when you make a mold, there is always the risk of
> destroying the original model.
Hi John,
I've made molds before, all kinds. But not for a while and it's not
quite as easy as getting back on a bicycle. I've done dozens of
plaster waste and piece molds and am quite happy working with plaster.
Rubber or polyurethane always makes me think twice.
Long ago I had a job in a Queens, NY sweatshop, or dustshop, as
resident prototype tool-and-die person for production of polyester
resin models. I worked in the office area, a semi-clean enough place
removed from the filthy factory floor which had no ventilation
whatsoever, a few dirty paper masks for captive illegal immigrant
workers who produced and finished thousands of ugliness-incarnate
models amidst piles or resin dust and spilled catalyst and ate at
their tables.
The spot which I cleared to work, mostly patching and grinding, had a
nice window onto the working floor below but, again, no ventilation
other than the office A/C. The dust bothered me but I wore my hair in
a kerchief or hat, wore a mask, bathed quite often and tried to ignore
it. One summer day I went swimming in a chlorinated pool and within a
few hours I found I couldn't see a thing. I was terrified but guessed
that something in the dust or catalyst fumes had infiltrated my system
and reacted to the chlorine in the pool. My eye doctor gave me drops
and I called in after missing an anxious Monday to say I wouldn't be
in to work for a few days. I was told by the "boss" that I'd better
get back soon, he wouldn't tolerate malingering. Man I should have
sued his slavemaster mentality to the next century but I dutifully and
blessedly healed and went back to work, staying only a few months more
to wrap up what I'd been working on and because the other owner was a
nice guy. Nice enough considering what was happening on the floor
below, though these were jobs indeed where there would otherwise be
none. OSHA was nowhere to be found, except maybe with some extra
pocket money. I can only surmise.
They had a vacuum de-airing machine, some kind of rocking table, a hot
box of some kind, lots of compressed air tools (their oh so blase and
stupid method of cleaning the work spaces was blowing the dust onto
the floor) and the gentle working partner had the wonderful talent of
being able to cut a glove mold properly with a surgical knife, in
zigzags so they fit together exactly later. I learned a little about a
lot but it's always a struggle to set up anew each time.
There are so many new materials out all the time. I'm looking at
silicon rubbers from both smooth-on and polytek, brush-on and pour,
and also Resilpom, a putty-like mold material, but there are too many
options for me at the moment.
I would like to cast a variety of materials, though probably not more
than nine or twenty: plaster, wax, press-clay, maybe concrete,
aggregates and paper. I don't need to cast slip, slip shrinks far too
much in my experience, though maybe I'm not doing it right. I'm not
particularly mad for resin and I'm happy to avoid epoxy for the same
reasons.
It's probably most prudent to fire the piece and then make the mold,
even if it does shrink another few percent.
Thanks,
Kathy
--
www.kforer.com
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