Chris Stanley on sat 9 dec 00
OK. Here is the deal. Memory is weak as to where the specific information
is in the book. There is a picture of some tubes of clay and the artist is
from Norway (I believe) The book only refers to the technique in the small
print under the picture. I gained the information from experimentation and
because I also teach foundry. Both of my Nelson texts have gone missing so
I cannot pinpoint the exact place. It is probable that the picture is in
the first edition of the book. The process is so simple that it is almost
humorous.
Fact: Plaster mixed with water and then added to clay does not work.( It
will lime pop)
Fact: Plaster dry mixed as a fine powder (sieved) with a dry powdered clay
"body" (10-20-30% plaster per 100% clay) does work.
Fact: Bronze casters have used this for investment casting for years! Metal
casters actually increase the percentage of plaster and decrease the
percentage of clay!
Fact: From my experiments, the best clay body is a heavily groged one.
Once you have dry mixed the materials, you have about 10-15 minutes
depending on water temperature to get the mix into a mold. We began our
experiments using panty hose and canvas tubes (made on the sewing machine)
as the "mold". Survey says that the object can only be three inches in
diameter or width. (Dang good considering that the normal thickness of clay
is much less) Length was limited to 24" due to kiln shelf size. The
material was mixed as a thick slip and then poured into the tubes. My
students and I tied the tubes with string to create a somewhat intestinal
looking tube on one sample and then tied another one of the tubes in knots.
Both worked great! We fired the things to bisque (06) with no explosions
nor pops!
I cannot explain this phenomenon other than the fact that I have seen it
work. I am unsure if it would work in other casting processes. The idea is
intriguing. Imagine being able to cast a portrait bust from liquid to
leather hard in minutes instead of hours/days.
Here is a story that I tell my students in the clay and glaze course:
Plaster and Clay
The following is purely hypothetical. It is not intended as fact. It is a
story. Some of it is fact!
O.K.! You have all heard me say from time to time that you must keep
plaster and clay separate. While excessive amounts of lime, calcium, and
gypsum are widely known as a problem in clay bodies it is not necessarily
true that the two can never be used together. It is a commonly held belief
that the two materials plaster and clay cannot yield an archival product
when mixed together. I am not going to doubt that there is some shred of
truth to this superstition; however, if one would look outside of the world
of
ceramics for a moment then all would become clear.
Our first search must begin, well, back at the beginning of the first human
attempts at creating and controlling excessive amounts of heat to cast
metal. During the process of Bronze Casting, the Bronze Caster must find a
suitable material into which they can pour the metal.
Bronze Age Horns:
http://homepage.tinet.ie/~bronzeagehorns/casting/casting.html
Sand works. Sand and Linseed Oil works well. Sand, Sodium Silicate and a
little Carbon Dioxide works even better. Ah, but that is too far in the
future
for us to travel. Let’s stay with the sand for a moment. The early Bronze
Casters liked the sand as an investment material, but they needed some
way to keep it from falling apart when they moved it. Why do you think the
early Bronze Casters liked the sand?
Survey says that at some point one of the Bronze Casters borrowed some
clay from a nice generous local Potter and mixed it with the sand and tried
to make an investment from that mix. More than likely, the investment took
forever to dry and was still brittle and broke when it was moved. The Bronze
Casters of course blamed the Potter for the mishap and that explains the
little argument that has been going on for several thousand years between
the Potters and the Bronze Casters.
Anyway, back to the story, down the street from the pottery was the local
Alchemist. As far as Alchemists go, this guy was pretty good. His certain
passion in life was attempting to create rock from liquid and powder. While
all the other alchemists were running around attempting to change lead into
gold, or find the formula for eternal life, our alchemist spent his days
mixing
materials with liquids and watching what happened. The Bronze Casters,
knowing of the Alchemist's experiments, decided that they should go pay
him a visit. The Alchemist had been playing around with Gypsum and
Water. When mixed together, the material created an exothermic reaction
that caused the material to harden. The Alchemist, who was a little off
anyway, thought that the material could only be used as a hand-warmer on
cold days. The Bronze Casters were intrigued.
Now they had three materials from which they could make an investment for
their Bronzes: Plaster, Sand, and Clay. Mixing the three in equal parts
yielded a material that would be plastic enough to coat around a wax
positive yet at the same time set up and become rigid. The material would
also sustain an incredible amount of heat. The investment material worked
like a dream and could be reused. The Bronze Casters called this material
Luto. Since the Bronze Casters were still at odds with the Potters they
never told them of their success and thus, the legend of plaster and clay
not
being compatible materials was born.
Xris
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Millie Carpenter
Sent: Friday, December 08, 2000 4:32 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Plaster re-use
Chris
Okay I have two different editions of the Nelson book and can't find this
info
anywhere(I liked some of the ilustrations so decided to keep the older one)
looked under plaster, plaster clay, and under clay body recipes. what I
would
like to know is this a process to cast nonfunctional objects or is this a
process
to make molds, or both?
Millie in Md
> Does any one remember the experiments in Glen Nelson's text regarding
> PLASTER CLAY? They work! It is possible to mix plaster and clay
together
> and cast an object which sets in minutes. >>
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