John H. Rodgers on thu 11 dec 97
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
This was started some time ago but due to interruptions was not completed
until now. It's a bit long but perhaps it will be of help to those working
with molds.
Hi Greg!
I assume from your post that you were doing relief work in waterbase clay
as opposed to oilbase clay. I am going to address moldmaking in regards to
both mediums.
OK, if there are undercuts present in the relief or if any portion of the
work lies in more than one plane, a multiple section mold will have to be
made. Also, often as not, the original work will be damaged or destroyed in
the process. How to deal with it is addressed later on.With that said, lets
move on.
If your are going to do work involving molds,you should consider that having
a copy of Donald Friths "Moldmaking for Ceramics" an absolute must. In
addition you should not be without a copy of the materials catalog from
Polytek, Inc., a molding materials manufacturing company. Their catalog has
a world of information about how to set up for rubber molds. Find them at
WWW.Polytek.com/ Phone is 610-559-8620. ( I have no connection with this
company).
The first step is to preserve the design. To do this, a first off mold is to
be made directly on the original design.
Wet Clay:
Build a fence or mold form around the piece. The plaster mix can be poured
directly over the clay. No further surface preparation is necessary. Be sure
you have at least 1inch thickness over the entire surface. The plaster can
be applied with a spatula, if it is allowed to reach a plastic state, but it
is tricky, because it is hardening all the time, and you could damage the
surface detail of your piece, so pouring is much better.
In the setting process, the plaster will pull water from the clay,
toughening the clay surface and causing it to shrink slightly. When the
plaster sets, remove the mold forms, and try lifting the new plaster mold.
It may pull completely free of the clay. if it does not, and all or any part
of the clay stays in the mold, not to worry!. Simply, gently, pull the clay
out. Examine the mold and note where any clay remains. Remaining clay points
to the possibility of undercuts. They need to be worked out before
proceeding. Any clay that remains can be washed out with warm water.
Now you have a first mold, commonly called a waste or throw-a-way mold. This
mold can be used as is, but once it is used up, the design is gone forever
unless a master is made to preserve it. So, as a next step, lets preserve
the design by making a master mold. This will be discussed a little further
along.
Dry Clay(unfired):
Again, the plaster can be applied directly to the clay. Just follow
instructions for wet clay.Expect the original relief work to be destroyed.
Use of water will be necessary to remove the clay work. Warm to hot water
works best. A blast of compressed air right at the juncture between the clay
and the plaster can force the clay out. The plaster mold, when dry, can be
used directly for reproductions as above.
Fired clay(and hard dry clay):
This work requires a careful inspection of the original to ensure
absolutely that there are no undercuts in the original work. If any are
present, you will be unable to separate the mold from the relief without
damaging one or the other. Eliminate the undercuts before proceeding.
Make a mixture of 10% petroleum jelly dissolved in naptha. I put the
petroleum jelly in a stainless mixing bowl and melt it with a heat gun. Then
I pour in the solvent and stir. While warm, I paint the surface of the work.
This solution can be sprayed with a spray bottle if kept warm. Follow the
same procedures as for wet clay for pouring of the plaster. Use of this
method precludes the direct use of the mold because the presence of the
petroleum blocks water absorption by the plaster. But it will give a mold
from which a master mold can be made.
Oil Clay:
Plaster may be poured directly over the clay. The oils in the clay may be
absorbed by the plaster, and prevent direct use, but the mold can be used to
make a master mold.
There are two approaches to building master molds. One is rubber, the other
is "Stone" ( any hard cementious material- preferably Ultracal 30). The
master that is easiest to use is a rubber master. It is also the most
expensive to make. I use a polytek two-part polyurethane rubber of Shore
hardness 55 that when set has a built-in lubricant.
Up to this point the discussion of making the mold has be based on using #1
Pottery plaster. Pottery plaster allows you to use that first mold almost
immediately. But it is not the best if you are going to make a rubber master
mold from which to make more plaster molds. So, I want to change horses at
this point, because next is a description of making a rubber master mold.
For making a rubber master mold a throw-away mold of a more dense material
works best. If that is the goal, substitute Ultracal 30, a US Gypsum product
(cementious) for the #1 Pottery plaster. Use a batch calculator(slide rule)
that you can get from USG. Make your throw-away mold from this material.
Once set, but while still wet, seal per instructions below. Allow to dry.
Because of the density of the material, expect at least a week drying time,
two weeks or more would be better, depending on how big the mold is.
Rubber Master:
Begin first by thoroughly drying the plaster/UltraCal throw-a-way mold. Be
absolutely certain the mold is dry through and through, because any moisture
will react with the long-chain polymers in the rubber mix and cause it to
swell beyond belief.
Buff the surface with a dry brush natural bristle brush.
#1 Pottery Plaster and UltraCal 30:
Seize the surface ( all over - inside and out) with a 50-50 mix of water and
tincture of green soap. Use a paintbrush to apply. Natural bristle is best.
Nothing else works as well. The natural bristle absorbs the water and leaves
the soap.Stir vigorously on the surface until a bubbly froth is formed.
Allow to dry. Then with a dry Natural Bristle brush, brush the surface
thoroughly. China bristle(which is boars bristle) is what I use. Since I do
this stuff professionally, I use a cheap throw-away brush from HQ for
applying the soap, but for drybrushing and polishing I use the $10-$14 china
bristle brushes from the art supply store. 4-1/2" size. After use, clean
brush with alcohol, then water, then alcohol, slinging vigorously between,
and allow to dry between projects.The second application of alcohol speeds
the drying process. I actually put them under a fan to speed the evaporation
process. Treated this way they will last a very long time.
Apply three coats of soap mix, drying and brushing between each. The soap,
which, if I still remember my chemistry from so long ago, is a stearate
compound. When applied to the surface of the mold, some of the stearate
combines with the calcium in the plaster to form calcium stearate, which is
not soluble in water. Therefore the surface of the plaster develops a very
thin, but waterproof layer. Subsequent applications of soap, when brushed
smooth and dried, simply form a water soluble layer that aids in removal of
any molding material subsequently applied. To test whether or not the
surface is sealed adequately, allow two or three drops of water to fall on
the surface. The water should bead up and run off like water on a waxed
surface. If not, apply another coat of soap ant test again.
After the last coat is dry-brushed, spray the entire mold, inside and out,
with mold release agent, ( from the mold rubber supplier) that is compatible
with the rubber to be used. This is typically a silicon based release agent
that does not react to the polyurethane or plaster. It can be applied over
the soaped surface as long as the surface has been adequately brushed dry.
Mix the rubber and apply according to the instructions.
Once the rubber has set, the rubber and the plaster can be separated, and
the rubber will be a duplicate of the original relief design. Now plaster
can be poured over this rubber model repeatedly to replicate that first
plaster mold. To get a really good face surface on the new plaster mold,
just before pouring the plaster, spritz the rubber mold surface with a mist
of handwash dish detergent mix comprised of 1 tablespoon detergent to a
quart of water. The detergent serves as a surfactant to break bubbles that
might tend to form on the surface of the plaster as it is poured onto the
rubber. Also, to aid in eliminating bubbles, bounce the plaster bucket on
the floor the way one would shake a cakemix to remove bubbles. This will
cause any entrained air bubbles to rise to the surface. The mist the surface
with alcohol and this will break the bubbles. Now you are set for a clean
pour.
"Stone" master:
A stone master can be made by soap treating the #1 pottery plaster throwaway
, mold as described above, and then pouring a properly mixed batch of
Ultracal 30 into it. When the Ultracal sets, the UltraCal becomes the master
mold.
Caution!!!!!. Do not make an UltraCal throw-a-way mold and attempt to make a
stone master from it. If an undercut is present anywhere, the plaster cast
can be broken off, leaving the stone master intact, and the undercut can be
cleaned up. But if the throw-a-way is UltraCal, it will be impossible to
separate the two without damaging them both.
After drying, the UltraCal master must be treated with three layers of soap
EVERY TIME a plaster mold is cast.
Rubber/Ultracal Combo:
Be sure your model is perfectly dry. Acquire a brushable rubber mix, such as
can be acquired from Polytek or others, and apply to the surface of the art
form per the instruction. First treat the surface of the art form as
described in their catalog, spray with silicon mold release and apply rubber
.. When, set do not disturb the rubber. Brush on a coat of petroleum jelly
dissolved in naptha. Then mix plaster and allow to reach a plastic state.
Use a spatula and apply to outer surface. Make at least 1 inch thick. This
is the "Mother" mold or shell. To strengthen it strips of cheesecloth can be
layered and more plaster applied. This is necessary if the mold is large. I
use hemp fiber available through Polytek. Once this is set. Remove the
shell, then the rubber. Place the rubber back into the shell, mix a batch of
Ultracal and cast a master mold of your art piece. This then can be used to
make plaster production molds.
So far as the use of the production molds is concerned, the purpose of using
the plaster molds is to have a medium to give form to the clay, and remove
water. Unless the detail of the relief is simple, you may expect to allow
the clay to remain in the mold until sufficient water has been extracted by
the plaster to cause the clay to shrink away from the sides of the mold. I
feel that many get impatient and try to rush the process. Having worked with
molds and slip-cast clay for years, I have learned that you can't rush the
process. To do so is to invite a lot of extra work if not outright grief.
The clay will release when it is ready and not before. True, you can use
oily sprays, etc,sometimes, but you ruin the absorptive quality of the mold
when you do that. If time is of the essence, make enough molds of any one
design to be efficient. Don't expect one mold to make a days production run,
unless you are into using ram-press molds with air release. That is a horse
of another horsefeather.
When casting plaster in the rubber master, the new production mold can be
removes as soon as the plaster sets. The plaster will get hot when it
reaches set. This is the heat of hydration, ie the heat released when the
water present is chemically locked up. The plaster will consume as much
water as is needed in the process to produce a crystaline structure. The
remaining water will be free to migrate to the surface through capillary
action and evaporate. When I can feel the plaster begin to cool, that is
when I remove the plaster mold. Sharp edges of the mold are trimmed to a 45
degree bevel about 1/8th to 1/4 inch wide to eliminate chipping.
When casting plaster on a stone master, I pour the plaster immediately after
the master has been polished smooth with the brushes. I monitor the
temperature very closely and remove the plaster mold just after peak
temperature. There remains a layer of free water between the stone master
and the pottery plaster. The heat, working on softening the dry, brushed
soap line, and the water, work together to allow separation. If separation
is difficult, immerse in a bucket of hot water. The difference in material
density between the to molds will result in differential expansion, and
should cause a separation line to open up. Once that happens, more water
works its way into the increasing space between them and aids separation. If
you can see the line opening up, but it still won't separate, give the mold
a modest whack along the edge with a RUBBER mallet. As an absolutely last
resort you may drive a WOODEN wedge into the separation line with WOODEN.
Anything more and you could damage the mold.
When working with a stone master, do not allow a new mold to stay on the
master for more than a few minutes after set. The longer it stays the harder
it is to remove. If it stays on over a few hours it may become impossible to
remove without damage to the master.
The more a stone master is used the better it gets because the surface
becomes burnished with use.
Comments before closing:
Molds are intended to give form to the clay. They are porous and
absorptive. The work best when those attributes are allowed to work and do
their job. Removal of water from the clay makes it shrink and pull away from
the mold for ease of removal. A properly designed mold will do exactly that.
Using oils or grease will ruin the mold surface, and interfere with its
performance. Oils and grease works best on impervious surfaces, like glass
or glazed surfaces.
Some sources are:
Mold Rubber: Polytek, P.O. Box 384, Lebanon, NJ 908-534-
5990
Smooth-On, Inc. 1000 Valley Rd,
Gillette, NJ 07933
International Coatings, Cerritos, CA
Mold Soap Tincture of Green Soap -- this is my choice.
The other two can kill the plaster or
cause alligator hide effect if care is
not exercised.( any drugstore)(by the gallon is cheaper)
Mix 50-50 with water
Crystal Soap Co. Landsdale, PA 800-
525-8842
English Crown Soap
Milligan Hardware and Supply Co
East Liverpool, PA 216-385-
2000
Plaster Batch Calculator
US Gypsum Company
Industrial Gypsum Division
101 south Wacker Drive
Chicago, Ill 60606-4385
Sorry if this has rambled a bit and is a bit longish. Hope it helps.
John Rodgers
It's COLD in Alabama (but not so cold as Umiat, Alaska, which is -19F) -----
--- REPLY, Original message follows --------
Date: Tuesday, 02-Dec-97 10:22 AM
From: Greg Skipper \ Internet: (skipper@aracnet.com)
To: CLAYART LIST \ Internet: (clayart@lsv.uky.edu)
Subject: Cast Questions?
----------------------------Original message----------------------------
I've been sculpting reliefs and someone showed me how to make plaster
casts. They suggested that I could efficiently replicate my work and
that the copies would be as good as the originals. The one I did with
initially my "teacher" used dental floss to suspend the dried clay
relief face down into the plaster. The result was not very impressive.
To remove the original we had to use a hose with water and the original
was destroyed (which I'd spent many hours making - ugh). I have done a
couple of these and I tried putting the relief face up at the bottom and
poured plaster on top. This was better, with less air bubbles, but
still was not great. I'm using dental lab plaster.
Any help I could get would be appreciated:
1. Is there any way to not loose the original when making a cast?
(Putting WD40 on it or something? Letting it dry more before removing
it from the cast?)
2. Is there a better way to treat the cast before pressing clay into it
so that the finished product comes out with less distortion? I end up
spending a fair amount of time "finishing" the final version because
it's not perfect.
3. Any more ideas on what I might could do better?
Thanks
Greg
-------- REPLY, End of original message --------
| |
|
