John H. Rodgers on fri 15 aug 97
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
Followed this thread a while and here's more about this molds thing.
Press molds, typically made from #1 pottery plaster, but also made from
other grades, don't necessarily need any release agent to get them to work
properly. The key to release is the extraction of water by the mold from the
clay and the subsequent shrinkage of the clay results in the clay pulling
slightly away from the plaster mold. Even highly detailed work should come
out with relative ease after the clay filled mold sits a while. The
exception would be if there are undercuts in the design. Slight undercuts,
and I mean very slight undercuts, can , with shrinkage of drying, pull away
sufficiently to be removed from the mold. But anything more will definitely
hang up and probably damage the clay work upon removal. Of course if the
damage is minimal, the clay can be re-detailed with a tool, and it will be
as good as ever. The mold can also be worked on and re-tooled to reduce the
undercut. If this doesn't solve the problem, then there is a design problem
and its back to the drawing board.
Plaster mold surfaces should be free of any contaminants- no oil, no soap,
just clean, clean, clean. The use of some dry powdered materials like talc,
kaolins, etc, may help a bit but these materials may very well affect the
surface of the green clay piece, and make it difficult to finish and/or
decorate. And may affect the surface of the mold itself. Far better to try
to work with a squeaky clean mold, and let the dry plaster do it's job.
Other plasters availble for specialty mold-making include hydro-cal, ultra-
cal 30/60, moulding plaster, and hydrostone, to name a few. There are also a
whole range of dental plasters, all of which could have some use for the
artis/sculptor/moldmaker.
I personally use ultracal 30 for certain kinds of models and for building
case-molds, ie, molds made for making molds. The material was developed for
the tool and diemakers trades, and captures detail beautifully. It is very
hard and extremely dense. Too much so for use in power presses. Power
presses such as RAM, etc, need a hard but porous material through which
release air can be forced, to release pressed clay items. Hydrostone is
generally use for that.
Some mention has been made about the use of pam and other materials as
release agents. In my own experience over the last ten years, agents (called
dressings) for preparing surfaces for mold work were in two categories.
Dressings for rubber molds, dressings for plaster molds.
When preparing a model from which a plaster mold would be taken, care was
taken to ensure that the dressing used would not interfere with water
absorption in the plaster mold. A commercial dressing or shop made dressing
would be applied to a plaster model to size the surface, ie, seal it and
make waterproof. This typically is a soap - and the one I like is tincture
of green soap cut 50% with water. (dishwashing and laundry detergents are
not, IMH, soaps - and they don't work for this purpose). This soap is a
stearate and when applied to the plaster surface combines with the calcium
in the plaster to produce calcium stearate, a waterproof compound. When this
has been applied at least three times, and buffed between each application,
a water test is performed. Two or three water droplets are allowed to fall
on the surface. If it beads up and runs as on a waxed surface the sizing is
good. If not another application should be made, and the test tried again.
Once done, a final coat of soap is applied and brushed dry. One thing here,
where the soap was concerned, this is a case where if one is good, then more
is NOT better. To much soap will mke the plaster have "alligator hide".
Renders the mold useless. You will know it when you see it.
After the models will bead the water, then the plaster is poured or applied
to the surface of the model. When the plaster begins to set it will begin to
get hot. This is heat of hydration, when water molecules are locked up
chemically. The heat will soften the outer layer of soap, and just after
maximum heat is reached and the temperature begins to drop, the new mold can
be removed. The soap will not interfere with the surface of the new mold, as
it will adhere to the surface of the model. The new mold should be clean and
its surface porous.When completely dry, press in the clay.
If the model used for the mold is oil clay, the clay must be sealed with
orange shellac and then a couple of layers of dressing applied before
applying the plaster. Otherwise the oil in the clay, while against the hot
plaster, tends to penetrate the surface of the plaster and will ruin the
mold. If the model is pottery clay, the plaster can be applied directly to
the clay surface.
Rubber molds are another matter. Plaster models can be sized as for plaster
molds. Be sure the model is absolutely dry, because any moisture contacting
polyurethane rubber compounds will cause it to swell ten times its normal
size. When well dry, an overcoat of mold release sprayed on. If the mold
is to be urethane rubber, a silicone mold release is used. Other rubber
compounds have their own requirements. After the rubber is poured, set, and
removed from the model, plaster molding can be done. If the rubber that has
be poured is a master mold, and is for making plaster molds, the surface
should be misted ever so slightly with that 50/50 mix of green soap. This is
not to serve as a release agent, but serves as a surfactant to break any
bubbles that might form as the plaster is poured into the rubber mold. It
will not interfere with the water absorption qualities of the new plaster
mold.
Oil Base clays, except sulphur bearing clays, will accept rubber poured
directly against them. I avoid the problem by using non-sulphur clay. A
little more expensive, but worth it.
I hope this has been of help to those interested in working with molds.
David Friths book "Moldmaking for Ceramics" is the definitive work on the
the subject and has been my mold-makers bible. And though I have been
involved in moldmaking for production for 11 years I refer to it constantly.
An excellent guide and source book for rubber mold work is the catalog from
Polytek, in New Jersey. It illustrates how to set up different molds, and
how to build and pour a mother-mold, and a shell for it, among other tips.
John Rodgers
That Alaska Sourdough
vintagerose73@yahoo.com on tue 6 apr 04
Hi, Ive been trying to find out information regarding making your own
ceramic molds..ive found some stuff that tells you what you need to make
a 3d mold, but what I want to make are open pour molds..i like to use my
pieces in my mosaic projects and I need them to be 1-5 inches...they can
be flat and plain on the bottom or underside, but I would like the front
or top of it to have a lot of detail...I really dont know where to
start..would plaster of paris work w/ ceramic slip?? Right now I have
been pressing my wet clay into candy molds..but this is very time
consuming as the clay will crack as it dries and is sometimes no good as
it has cracked up so much...could someone lead me in the right direction
on how to get started? I am relativly new to ceramics and I still have A
LOT to learn..thank you in advance for any help
Cherie
Snail Scott on wed 7 apr 04
At 10:08 AM 4/6/04 -0400, you wrote:
>...what I want to make are open pour molds..i like to use my
>pieces in my mosaic projects and I need them to be 1-5 inches...they can
>be flat and plain on the bottom or underside...would plaster of paris work
w/ ceramic slip?? Right now I have
>been pressing my wet clay into candy molds...
Yes, Plaster of Paris works very well for this.
Even better is the stuff called #1 Pottery Plaster.
It's more absorbent and is designed for claywork.
Candy molds and other impervious materials allow
the clay to stick, making it hard to get out. Plaster
sucks just enough moisture out of the clay to free
is up from the mold and stiffen it enough to handle.
Plaster molds work well for both slip-casting and
press-molding.
Unlike candy molds, plaster molds won't be flexible,
so you need to design a shape without undercuts.
The relief can be pretty deep, if the design is not
too vertical. (In moldmaking, this is called 'draft' -
the angle that allows the casting to be pulled out
of the mold without dragging against vertical sides
in the design contour.) Hard to describe in e-mail,
but if you have read up on molds, you have probably
seen good pictures.
The backs of tiles for mosaic don't have to have any
particular texture. Flat is fine.
One way to make a tile mold:
Use wet clay to make a model of your design, flat on
a tabletop or board. Use wood or clay to build a solid
'corral' around the model, about an inch away or a
little more, and 1-2 inches taller than the highest
point of the design. Gently pour plaster into the corral
until it is full. Wait until the plaster is hard,
pull the clay out, and let it dry a few days. Then use
it!
This is very simplified, but it'll do for now, to get
you started. You can learn the finer points of the
process from the books you mentioned, or try the
Clayart archives. There's a lot of helpful info there.
-Snail
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