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Plaster Mould of a ball, what to use for separation?



 
 
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  #1  
Old June 8th 08, 09:59 AM posted to rec.crafts.pottery
Gap
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Posts: 2
Default Plaster Mould of a ball, what to use for separation?

Hello,
I want to make a plaster mold of a ball which is why I have 2 halfs of
polystyrene balls.
I got them sent to Australia from my german pottery friend and I made
one mold already approx 20 cm diameter which worked so far so good.
The only problem was to get the halfs polystyrene balls out of the
dried plaster mould.
Because the plaster shrinks my ball was in there pretty tight and I
had to break it to get it out.
I have 4 more balls bigger ones that the one I made before and wonder
if anyone here know what I can use to seperate my balls from the
plaster so that I can re-use the balls if anything goes wrong with the
mold.
Last time I have mixed dishwashing liquid with oil and little water
which didn't work too good and I think I will have more trouble with
the bigger balls.

If you have any ideas or suggestions please let me know.
Thanks in advance
Gabi
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  #2  
Old June 8th 08, 03:00 PM posted to rec.crafts.pottery
Bob Masta
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Posts: 96
Default Plaster Mould of a ball, what to use for separation?

On Sun, 8 Jun 2008 01:59:08 -0700 (PDT), Gap
wrote:

Hello,
I want to make a plaster mold of a ball which is why I have 2 halfs of
polystyrene balls.
I got them sent to Australia from my german pottery friend and I made
one mold already approx 20 cm diameter which worked so far so good.
The only problem was to get the halfs polystyrene balls out of the
dried plaster mould.
Because the plaster shrinks my ball was in there pretty tight and I
had to break it to get it out.
I have 4 more balls bigger ones that the one I made before and wonder
if anyone here know what I can use to seperate my balls from the
plaster so that I can re-use the balls if anything goes wrong with the
mold.
Last time I have mixed dishwashing liquid with oil and little water
which didn't work too good and I think I will have more trouble with
the bigger balls.

If you have any ideas or suggestions please let me know.
Thanks in advance
Gabi


I have also had problems when trying to use dishwashing liquid,
which is actually a detergent. I think you really need a true soap,
which has always worked well for me. (Liquid hand "soaps" are often
really detergents, unfortunately, though it may not be obvious. Too
bad, since they are so readily available!)

Here in the US, the "old time" recommendation was Fels Naptha soap or
(I think) Murphy's Oil Soap. I have also used old Ivory soap flakes
with good results, but I don't know if they even make those any more.
(My box is about 30 years old!) If all else fails, and you don't want
to use commercially made parting compound, you could probably shave a
bar of hand soap into a powder and mix it up with water.

Plaster is generally not supposed to shrink. More likely is that the
plaster is just *really* close to the original, with no airspace
between. That means that atmospheric pressure (14.7 pounds per square
inch of surface) does a terrific job of holding the surfaces
together... in fact, many products sold as glues do nothing more than
fill up the air space between surfaces to produce this effect.

The most important thing is to make sure your parting line is
*exactly* at the diameter of the sphere, or you will never get it out
intact. Also, you mention that the balls are polystyrene. If this is
the kind with a pebbled surface like Styrofoam, you can also expect
problems because there are lots of little pockets that the plaster
will fill and lock things tight... it's really the same as an undercut
mold, on a smaller scale but multiplied many tmes over. You might
want to apply some sort of finish to the balls first to make them
glassy smooth. The question is *what*, since many things will dissolve
polystyrene. Maybe paste wax? Try on a scrap of the ball you already
destroyed.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
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  #3  
Old June 8th 08, 07:18 PM posted to rec.crafts.pottery
D Kat
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Posts: 55
Default Plaster Mould of a ball, what to use for separation?


"Bob Masta" wrote in message
...
On Sun, 8 Jun 2008 01:59:08 -0700 (PDT), Gap
wrote:

Hello,
I want to make a plaster mold of a ball which is why I have 2 halfs of
polystyrene balls.
Gabi

(sorry - all good advice snipped for space sake)
Plaster is generally not supposed to shrink. ..... Bob Masta



Any plaster I have ever worked with expands! I mean that in a powerful way.
I have seen it push well nailed frames apart.

I would think you would want to cover the Styrofoam in something soft enough
that it is not going to damage the delicate structure, that hardens so some
degree and yet can be removed from the plaster when it is set. You can't
heat up plaster without damaging it but I don't know how high of heat is
involved for that to be an issue or I would suggest dunking the ball in
melted wax. In fact if you put it out in a hot room I bet that would be
warm enough to peel the wax away without damaging the plaster... Just a
thought.

Donna


  #4  
Old June 9th 08, 10:30 AM posted to rec.crafts.pottery
Gap
external usenet poster
 
Posts: 2
Default Plaster Mould of a ball, what to use for separation?

Bob and Donna,
thanks for all your input!
It is not a problem to make the parting line exactly at half of the
ball because they do come in 2 halves.

I also thought it is because of the porosity of the polystyrene which
is tini balls melted together and I
understand what you mean with undercut.
That is probably not a good surface finish to start with..

I have asked my pottery teacher in Germany who has made a lot of these
balls and he said clay slip would be also a good medium for
separation, so I might try that..
Someone else said I should put gladwrap over the balls before I put
the plaster on. That would make the surface smooth but would give me
lines where the gladwrap is "folded".
I have also thought about painting the ball with some paint or even
fluid latex which I have here.

I guess I just have to try different things and see which works best.

Thanks again hand happy pottering and/or mould making!
Cheers from Gabi
  #5  
Old June 9th 08, 02:56 PM posted to rec.crafts.pottery
Bob Masta
external usenet poster
 
Posts: 96
Default Plaster Mould of a ball, what to use for separation?

On Sun, 8 Jun 2008 14:18:51 -0400, "D Kat"
wrote:


"Bob Masta" wrote in message
...
On Sun, 8 Jun 2008 01:59:08 -0700 (PDT), Gap
wrote:

Hello,
I want to make a plaster mold of a ball which is why I have 2 halfs of
polystyrene balls.
Gabi

(sorry - all good advice snipped for space sake)
Plaster is generally not supposed to shrink. ..... Bob Masta



Any plaster I have ever worked with expands! I mean that in a powerful way.
I have seen it push well nailed frames apart.

I would think you would want to cover the Styrofoam in something soft enough
that it is not going to damage the delicate structure, that hardens so some
degree and yet can be removed from the plaster when it is set. You can't
heat up plaster without damaging it but I don't know how high of heat is
involved for that to be an issue or I would suggest dunking the ball in
melted wax. In fact if you put it out in a hot room I bet that would be
warm enough to peel the wax away without damaging the plaster... Just a
thought.


Plaster doesn't convert back to gypsum until something like 250-300F
or more, depending on how long you hold it at temperature, etc . So
molten wax at the boling point of water or less should be OK.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
  #6  
Old June 9th 08, 03:01 PM posted to rec.crafts.pottery
Bob Masta
external usenet poster
 
Posts: 96
Default Plaster Mould of a ball, what to use for separation?

On Mon, 9 Jun 2008 02:30:34 -0700 (PDT), Gap
wrote:

Bob and Donna,
thanks for all your input!
It is not a problem to make the parting line exactly at half of the
ball because they do come in 2 halves.

I also thought it is because of the porosity of the polystyrene which
is tini balls melted together and I
understand what you mean with undercut.
That is probably not a good surface finish to start with..

I have asked my pottery teacher in Germany who has made a lot of these
balls and he said clay slip would be also a good medium for
separation, so I might try that..
Someone else said I should put gladwrap over the balls before I put
the plaster on. That would make the surface smooth but would give me
lines where the gladwrap is "folded".
I have also thought about painting the ball with some paint or even
fluid latex which I have here.

I guess I just have to try different things and see which works best.

Thanks again hand happy pottering and/or mould making!
Cheers from Gabi


Clay slip is a good idea, if it will stick to the balls. I often use
no parting compounds at all when casting objects made of soft clay,
since the clay can be deformed a bit if needed to release it.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
  #7  
Old June 10th 08, 05:55 PM posted to rec.crafts.pottery
DKat
external usenet poster
 
Posts: 99
Default Plaster Mould of a ball, what to use for separation?


"Gap" wrote in message
...
Bob and Donna,
thanks for all your input!
It is not a problem to make the parting line exactly at half of the
ball because they do come in 2 halves.

I also thought it is because of the porosity of the polystyrene which
is tini balls melted together and I
understand what you mean with undercut.
That is probably not a good surface finish to start with..

I have asked my pottery teacher in Germany who has made a lot of these
balls and he said clay slip would be also a good medium for
separation, so I might try that..
Someone else said I should put gladwrap over the balls before I put
the plaster on. That would make the surface smooth but would give me
lines where the gladwrap is "folded".
I have also thought about painting the ball with some paint or even
fluid latex which I have here.

I guess I just have to try different things and see which works best.

Thanks again hand happy pottering and/or mould making!
Cheers from Gabi


I love the idea of a thick coat of latex paint! But I think the clay slip
if you can manage not to have brush marks sounds the most practical (again
you can dip coat it...). It would be easy to remove from the plaster after
it was set.

I assume you are building a box that the half sphere sits in, with the flat
of the half sphere laying on the bottom of the box. I would think you would
want to at least glue around the edge of the flat half so that plaster does
not seep under or shift the sphere out of shape. If you had a router the
size of a dowel you could easily build in your key. Router out the shape
and in one pour leave the dowel out and the next pour put the dowel in.

Ok, I have obviously gotten carried away here but it was fun the think
about. Good luck and please let us know how it worked out.

Donna


  #8  
Old June 10th 08, 05:57 PM posted to rec.crafts.pottery
DKat
external usenet poster
 
Posts: 99
Default Plaster Mould of a ball, what to use for separation?


"Bob Masta" wrote in message
...

Plaster doesn't convert back to gypsum until something like 250-300F
or more, depending on how long you hold it at temperature, etc . So
molten wax at the boling point of water or less should be OK.

Best regards,


Bob Masta


I had images of taking a blow torch to the piece to burn the wax away - not
so good for plaster I would think.... What can I say - I like the flame and
it didn't occur to me to be practical and use boiling water...


 




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