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Sillyhav15

Removing Greenware From Ceramic Mold Questions

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Hi everybody,

I am new to pouring molds and I have been having a few issues. I am pouring low fire slip into plaster molds and some of my pieces have tiny pin holes in them. Is there any way to prevent this? I'm assuming I can patch these by maybe brushing some slip on them once they are a little firmer in the greenware stage is this right?

 

Also, Its been raining here and these things are taking forever to dry so I can unmold them. I have heard that leaving them too long in the mold can crack it, so in fear I have been pulling them out too early and they literally mush and collapse. Is there a good way to tell when you can un mold them? Can I reuse these botched pieces of un dried slip for anything?

 

Some of my pieces are large and have literally sat all night and are still soft.

 

Thanks in advance!

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I have been able to rub the pinholes out at greenware.  Depending on what your mold looks like will depend on if your piece will crack.  How many pieces is your mold?  Is it mostly enclosed or have a large open area?  I have had some luck on a 3 piece mold taking one side and the bottom off, leaving the other side horizontal with the piece still inside of it. Then putting the first side back on and flipping it and exposing the wetter side to air.  

You can reclaim the botched and dried pieces - takes time, but it can be done.

It takes practice and time to make good slipware.  Good luck

Nancy

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I will try to rub my pinholes out thanks!

 

Have either of you done a Christmas tree? I have one say 14" tall... I poured yesterday, let it sit all night then unmolded two sides this morning. Well I let that sit about 5 hours unmolded and I believe one side has collapsed. Grrrr guess it needed to sit longer.

Please let me know if you have experience with these. Oh and how long did you let it sit before pouring the slip out?? I think mine sat to long making the top solid lol not sure if this one is going to pan out.....

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I've made vases about 18" and 14" tall, not cone shaped though.  

Sometimes as you pour the excess slip out it will 'glug' causing the soft slip to pull away from inside the mold.  Be very careful and slow as you do the pouring.  

Experience will show you how long to let the slip stay in the mold before you decant it.  I found that the drier my mold, the less time I had to let it set up.  

Perhaps one of your mold pieces has something on it that resists water, that can slow the movement of moisture into the plaster and cause thin/weak spots.

Nancy

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You can run a small heater or dehumidifier near your molds, which will help them dry. We let larger pieces sit until the wall-the solid layer next to the mold-is about 1/4" thick before pouring off the excess.

Pinholes can be caused by dirty molds. Older molds often leave pinholes, as the plaster has started to degrade with use. If you are using old molds, you may have to recast a master and recast the mold in new plaster. Most of the time a damp sponge or chamois will take care of them. If the top of the piece is plugged with semi-solid clay, you can poke it with a dowel or small sharp knife, then pour out the excess slip still inside the mold. Pieces with small gates (pour holes) often need this, especially if the rest of the piece is broader, like a tree mold will be.

The many variables of slip viscosity, temperature, mold dryness, air humidity, etc.,etc. mean you need lots of practice to gauge your cast pieces readiness to be pulled. Be prepared to experiment until you have a good idea of how the slip will act. You can reclaim your clay.

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