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  #1  
Old 03-18-2009, 07:40 PM
11-11 11-11 is offline
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help! pinholes in my plaster castings

Help! I sculpted a bust for my client, made a silicone mold of it, and am to give him four hydrocal casts. These casts have been curing for the past week and I am polishing them up to deliver them to him tomorrow. I was blowing them off with compressed air and now I come to find all kinds of tiny pinholes!!??

I'm freaking out...does anyone have any suggestions on how best to fill them?

signed,
desperate
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  #2  
Old 03-19-2009, 01:53 AM
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Landseer Landseer is offline
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Re: help! pinholes in my plaster castings

Quote:
Originally Posted by 11-11 View Post
Help! I sculpted a bust for my client, made a silicone mold of it, and am to give him four hydrocal casts. These casts have been curing for the past week and I am polishing them up to deliver them to him tomorrow. I was blowing them off with compressed air and now I come to find all kinds of tiny pinholes!!??

I'm freaking out...does anyone have any suggestions on how best to fill them?

signed,
desperate
Spackle
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  #3  
Old 03-19-2009, 10:03 AM
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GlennT GlennT is offline
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Re: help! pinholes in my plaster castings

I was hoping Landseer would give an answer, since that is is expertise. I was going to suggest the kind of lightweight spackle such as "Lightenup" by the company Red Devil. It is lighter weight than the older type, eaiser to apply and clean up.
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  #4  
Old 03-19-2009, 12:57 PM
Giotto Giotto is offline
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Re: help! pinholes in my plaster castings

recast them....unless your painting the plasters the repair will show.

next time run your fingers along the inside of the mold to release any bubbles.
Also mold release works as does wetting the mold with water before casting. Finally, on small molds a hand held vibrator like the heavy duty kind for back massage will help release them too.

G
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  #5  
Old 03-20-2009, 08:43 AM
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Landseer Landseer is offline
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Re: help! pinholes in my plaster castings

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Originally Posted by GlennT View Post
I was hoping Landseer would give an answer, since that is is expertise.
Glenn, I don't GET pinholes and other defects in my casts in the first place, it's much easier to prevent them than to spend time fixing and patching!
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  #6  
Old 03-20-2009, 12:37 PM
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GlennT GlennT is offline
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Re: help! pinholes in my plaster castings

Right. I meant working with plaster was your expertise.
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  #7  
Old 03-20-2009, 10:32 PM
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Landseer Landseer is offline
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Re: help! pinholes in my plaster castings

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Originally Posted by GlennT View Post
Right. I meant working with plaster was your expertise.

Ok, cool.
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  #8  
Old 03-21-2009, 09:02 PM
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justme justme is offline
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Re: help! pinholes in my plaster castings

Hi,
Just wondering if you found a solution??? I've had the problem, and if there is a solution, I'd love to know.
Thanks
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  #9  
Old 03-22-2009, 12:39 PM
Giotto Giotto is offline
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Re: help! pinholes in my plaster castings

You can probably find lots of helpful information on the web. From my experience, there isn't a lot known about bubble formation but according to Henry's law, after initial mixing equilibrium is established with the gas in the plaster water mix, . When the mix is poured into a hotter container the pressure in the vapor phase suddenly changes. The thermodynamic equilibrium is changed, and to recover to a new stable thermodynamic state corresponding to the atmospheric pressure, the mix must degas. Apparently pouring low temperature mix into a hotter container causes bubbles. I have duplicated this with experimentation with wax. Also shocking the mix causes the mix to nucleate bubbles (like shaking up a water bottle). So

Mix plaster and water gently. I use a hand drill with a attachment sold in art supply stores that doesn't create bubbles. I use it for a short timed interval. Then Let the mix rest before pouring, I massage the mix by hand if necessary...the air caught in between the particles of plaster, now dissolved in the mix will rise.....
The temperature of the mix is also rising..I try to let it get to room temperature the same temperature of the mold.

When pouring into the mold pour carefully and gently..no splashing. Then do the other things I recommended to get rid of any remaining bubbles that usually do form.

G
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  #10  
Old 03-22-2009, 02:58 PM
Andrew Werby Andrew Werby is offline
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Re: help! pinholes in my plaster castings

Yes, it's a lot better to avoid getting bubbles in the first place than to patch them afterwards. People who do this a lot use either pressure or vacuum chambers to get rid of bubbles when the plaster is liquid. Surfactants either mixed into the plaster or applied to the surface of the mold can help too. Jewelers use stuff called "debubblizer" if they aren't going to vacuum their molds; if that's in the plan they use "vacufilm" which doesn't foam up in the bell-jar.

But if it's too late for all that, use shellac to seal the surface of the plaster before attempting to patch it. Use a dilute coat first, cut heavily with denatured alcohol. Then follow up with a full-strength coat. After that, you can use spackle to fill the holes without having the plaster suck out all the moisture in it, which prevents it from setting properly. After that, use a shellac-based primer like BIN to give you a uniform white surface.

Andrew Werby
www.unitedartworks.com
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  #11  
Old 03-24-2009, 07:22 AM
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Mold Man Mike Mold Man Mike is offline
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Re: help! pinholes in my plaster castings

Personally, I like to wet the surface of the mold out (breaking surface tension) with the plaster (or concrete) using my hands before I pour the rest of the plaster in. Then I use a mallet or hammer and tap the table my plaster-filled mold is sitting on to vibrate bubbles out. I dont cast plaster in silicone much. Silicone is pretty expensive. Typically I use urethane molds and use a home made solution of Dawn liquid dish soap diluted with water at a 1Dawn:20water ratio and use a spray bottle to mist the mold surface before I pour plaster. I hope this helps.
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