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

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

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

(sorry - all good advice snipped for space sake)

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

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

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

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

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

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

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

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...