Plaster Mould of a ball, what to use for separation?
NewsGroups | Search | About
Login Password
Register Now! Lost Password?
 rec.crafts.pottery    Post an article   get this group's latest topics as an RSS feed add this group's latest topics to your My MSN content add this group's latest topics to your My Yahoo content
Subject Author Date
Plaster Mould of a ball, what to use for separation? Gap 06-08-2008
Posted by Gap on June 8, 2008, 4:59 am
Please log in for more thread options
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

Posted by Bob Masta on June 8, 2008, 10:00 am
Please log in for more thread options
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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

Posted by D Kat on June 8, 2008, 2:18 pm
Please log in for more thread options

> 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



Posted by Gap on June 9, 2008, 5:30 am
Please log in for more thread options
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

Posted by Bob Masta on June 9, 2008, 10:01 am
Please log in for more thread options
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!

Similar ThreadsPosted
what now with my plaster man? September 15, 2006, 8:43 pm

The site map in XML format XML site map
 Contact Us | Privacy Policy