pat gilmartin on tue 21 aug 12
A while back I asked if it made sense to use Rock Hard to make press molds =
with; I had made a mold using Plaster of Paris which turned out to be quite=
soft and lost its detail after just 2-3 uses. The feedback I received told=
me that Rock Hard was not absorbent enough to use for molds - that clay wo=
uld not release from it. I tried it anyway just to see what would happen, a=
nd the Clayarters were right again: the Rock Hard mold did not want to turn=
loose of my clay, and is not a satisfactory material for mold-making.
Having said that, I found that in my particular case, I was able to use the=
mold anyway - only because the pieces I am molding are small (a little ove=
r an inch in length). So I can leave a glob of clay on the back of it and p=
ull it back out of the mold pretty well. I also found that if I dust the Ro=
ck Hard mold lightly with baby powder before pressing the clay into it, it =
releases much more easily. All in all, though, Plaster of Paris, correctly =
mixed, is the better choice.
Robert Harris on tue 21 aug 12
If you look at the MSDS sheets for Hydrocal and the other various "harder"
plasters, all are at least 85% Plaster of Paris, with 0-5% crystalline
silica and/or 0-10% Portland cement added.
For those "cutting" their Hydrocal with Pottery plaster, all you are doing,
in effect, is reducing the amount of hardening agents (i.e. Silica and
Cement).
I am unsure how "crystalline silica" differs from the silica we use in clay
bodies, but it seems to me that it would be fairly easy to make your own
"house blend" plaster that optimises hardness versus absorbancy for your
particular need.
Robert
On Tue, Aug 21, 2012 at 8:24 PM, pat gilmartin wrot=
e:
> A while back I asked if it made sense to use Rock Hard to make press mold=
s
> with; I had made a mold using Plaster of Paris which turned out to be qui=
te
> soft and lost its detail after just 2-3 uses. The feedback I received tol=
d
> me that Rock Hard was not absorbent enough to use for molds - that clay
> would not release from it. I tried it anyway just to see what would happe=
n,
> and the Clayarters were right again: the Rock Hard mold did not want to
> turn loose of my clay, and is not a satisfactory material for mold-making=
.
>
> Having said that, I found that in my particular case, I was able to use
> the mold anyway - only because the pieces I am molding are small (a littl=
e
> over an inch in length). So I can leave a glob of clay on the back of it
> and pull it back out of the mold pretty well. I also found that if I dust
> the Rock Hard mold lightly with baby powder before pressing the clay into
> it, it releases much more easily. All in all, though, Plaster of Paris,
> correctly mixed, is the better choice.
>
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