Paul Taylor on tue 24 oct 00
Dear All
One of my students is an excellent modeler and wants to make models for
casting.
I explained that making molds is a trade in its self but am persuaded to go
through it once with her so that she understands the process so when she
designed more she will have the casting in mind .
However It is nearly thirty years since I made any molds and going through
the process in my mind there are a few things which I can not remember. Like
how do you pour the plaster without getting air bubbles or was there some
secret in the mixing that avoids them. Do I have to use soft soap between
the molds or will a coat of Palmolive do for the three piece experiment. I
am not making master molds so what weight of plaster would be best.
Are there any other tips. I can remember how to set up the model for
casting.
I have been honest with the student that I am at the end of my technical
tether on this one. I wish she had less faith in me.
Regards from Paul Taylor
http://www.anu.ie/westportpottery
Snail Scott on tue 24 oct 00
Paul-
Frith's book on mold making is about the best out there. It
used to be outrageously expensive, too, but the new paperback
edition is quite reasonably priced.
At 06:57 PM 10/24/00 +0100, you wrote:
>Dear All
>
>One of my students is an excellent modeler and wants to make models for
>casting.
>I explained that making molds is a trade in its self but am persuaded to go
>through it once with her so that she understands the process so when she
>designed more she will have the casting in mind .
>
>However It is nearly thirty years since I made any molds and going through
>the process in my mind there are a few things which I can not remember. Like
>how do you pour the plaster without getting air bubbles or was there some
>secret in the mixing that avoids them. Do I have to use soft soap between
>the molds or will a coat of Palmolive do for the three piece experiment. I
>am not making master molds so what weight of plaster would be best.
>
>Are there any other tips. I can remember how to set up the model for
>casting.
>
>I have been honest with the student that I am at the end of my technical
>tether on this one. I wish she had less faith in me.
>
>Regards from Paul Taylor
>http://www.anu.ie/westportpottery
>
>___________________________________________________________________________
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
BobWicks@AOL.COM on wed 25 oct 00
Paul:
I have been in education all of my career and now retired. I know what you
are experiencing regarding the end of your tether technically. I always
toiled my students if I don't know the answer that I can surely resort to
others that know more about it.
Regarding air bubbles: I always stir the plaster mix by hand and to avoid
doing it too vigorously. When pouring I always allow, the stream of plaster
to run off the palm of my hand or off my fingers. This slows down the speed
that the plaster hits the surface. It always helps if you apply a very thin
coating all over the surface and allow it to set a little before pouring the
final amount necessary for the mold. In essence the thin coat allows any air
bubbles to come to the surface. It is the surface that touches the pattern
that you are interested in, not the bulk that makes up the balance of the
mold. For a separator I find that just ordinary liquid hand soap rubbed to a
shine work very well to stop the parts from sticking. I have tried other
things such as Vaseline, Pam spray, Crisco and they all work.
If you write to USG (United States Gypsum) ask for the specs on the plasters
that they sell. They are very generous and have a fine folder dealing with
all the characteristics of USG products. They even describe how you
determine the exact amount of water to use to achieve certain results.
You might try doing a small casting yourself and even discover something that
you overlooked before.
Good Luck.
Bob Wicks Prof. Emeritus HACC
Jeff Lawrence on thu 26 oct 00
Paul Taylor was asking about casting plaster...
Hello Paul,
Here are couple of pertinent items:
Get the mfr's recommended water:plaster ratio and use it.
Guess the amount of plaster per pour and add 10% -- mixing up another bit to
finish it off never works very well -- you'll have a hard spot in your mold,
and possibly cracks.
Even worse, it is an aesthetic defeat...
Mix with an electric drill and paint mixer for the recommended time, less 30
seconds (I mix 2 min 30 sec. with the drill). Use a elbow-length rubber
glove and hand mix the plaster for the remaining 30 seconds -- vigorously
but without entraining more air. The electric mixing ensures a good set and
the hand mixing helps de-bubblify the mix. Watch the mix and pour when you
can just start to see finger tracks in the surface.
When pouring, pour slowly to avoid adding more bubbles. Some people
recommend pouring down the outside of a stick or pipe. This never works for
me because I always need both hands to hold my mix and I lack a prehensile
tail. Perhaps if you are more dextrous than I or better equipped in the
appendage department, this might be something to try.
Jeff Lawrence ph. 505-753-5913
Sun Dagger Design fx. 505-753-8074
18496 US HWY 285/84 jml@sundagger.com
Espanola, NM 87532 www.sundagger.com
Ross Hartman on thu 26 oct 00
Paul;
IMHO you cannot avoid getting air bubbles in plaster when mixing.
Here are some tips and tricks that I have used over 30 years plus of mold making.
First of all I always mix plaster with a machine, i.e. a mixer (I have a simple
one that works well for large mixes). For small batches of plaster the hand scoop
technique works ok, but what we try to do is to leave a small pour for part of a
larger one. We prepare multiple smaller pours to make a larger mix and pour
several at one time.
It is important to let the plaster slake. Slaking is the process of leaving the
plaster in the bucket with the water to let the plaster itself eliminate air
bubbles by soaking. I would usually wait a couple to several minutes depending on
the water temperature. The cooler the water the longer it can slake but the water
temperature should probably always be in room temperature range.
When I mix the plaster I put the mixer blade (small props (propellors) work best,
stainless steel is preferable, on the end of a long shaft) very near the bottom of
the bucket and then I cup my hand in the mix near the prop to provide a sort of
'direction vane' to produce the best results. The best results being a compromise
between air bubbles and the plaster blowing out of the bucket .
Use Isopropyl alcohol (rubbing alcohol) in an old spray bottle. When the time is
nearing completion for mixing shoot in a couple of spray bursts. This has the
effect of breaking the surface tension of the bubbles at the surface and you'll
see them just disappear before your eyes (it's kinda neat)...
When pouring the plaster I shake the table with my hip and pour into a corner of
the mold cavity that will not be threatened as much by bubbles that don't make it
up in the shaking process. After the pour is made shaking the table a little more
sharply and/or vigorously will help bubbles up. After the pour another spritz of
Isopropyl will help eliminate surface bubbles again.
There are other things to consider but I think this covers the majority, from my
perspective..
FWIW...
Let me know off list if I can be of any further assistance.
L8R/Ross
Paul Taylor wrote:
> Dear All
>
> One of my students is an excellent modeler and wants to make models for
> casting.
> I explained that making molds is a trade in its self but am persuaded to go
> through it once with her so that she understands the process so when she
> designed more she will have the casting in mind .
>
> However It is nearly thirty years since I made any molds and going through
> the process in my mind there are a few things which I can not remember. Like
> how do you pour the plaster without getting air bubbles or was there some
> secret in the mixing that avoids them. Do I have to use soft soap between
> the molds or will a coat of Palmolive do for the three piece experiment. I
> am not making master molds so what weight of plaster would be best.
>
> Are there any other tips. I can remember how to set up the model for
> casting.
>
> I have been honest with the student that I am at the end of my technical
> tether on this one. I wish she had less faith in me.
>
> Regards from Paul Taylor
> http://www.anu.ie/westportpottery
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Susan Fox Hirschmann on fri 27 oct 00
Hi!
for your info: Donald Firth's book "Moldmaking for Ceramics" is a gem. It
goes thru all the steps, shrinkage variables (with appropriate measuring
devises) and is a first rate book for step by step ways to take your original
and create a plaster mold.
Good Luck!
Susan
Susan Fox Hirschmann
Annandale, VA
happy to finally have the copyright and trademark (another post!) to my new
designs.
You can
See my new work, combining my love for clay and running:
Feat-in-Clay
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