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mkregor

New Plaster Wedging Table Contaminating My Clay

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Hello,

This is my first post, so please be gentle ;)

 

I just finished making my first plaster wedging table, and I'm experiencing something odd.  No matter how much I  sand, scrape, and clean the surface, little specks of plaster are contaminating my clay as soon as it hits.  I've attached a photo.  As far as I can tell, I followed the instructions for mixing the #1 pottery plaster.  I also vibrated the mold to remove bubbles, and I've waited several weeks for it to fully dry.  Has anyone ever had this problem?  I know I could switch to another surface material, but I've always preferred how the clay moves on plaster, and I cherish it's quick-drying qualities.  Any help would be greatly appreciated!

 

Thanks, Matt

post-63717-0-40815300-1402459091_thumb.jpg

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Hi and welcome!

 

Was this a fresh bag of plaster you used and did it heat up as it was setting? Is it absorbent now that it has cured? Kinda looks like it is made from plaster that has absorbed moisture from the air and is no longer viable.

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Lots of bubbles! over agitated encorporating air/ Need to whack the plaster bucket a lot before pouring the plaster to get rid of air bubbles.These will crack open and get into your clay as you wedge. Canvas cover would be the way to go with this one.

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Yeah, I agree with the above suggestions.  Either there was something wrong with the plaster or the mixing.  While plaster, even when mixed and set correctly, can chip if not used properly, it shouldn't be coming apart like that however.  

 

I would just redo it.  Once it's redone, I would put canvas over top, as suggested.  Wet clay REALLY likes to stick to plaster, but will pull off the canvas much easier.

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Ok I agree throw out that board forget the cloth and start over. I just have experiences here with folks wanting to seem to keep everything from broken pots to pitted pots to whatever so the cloth was a pothole patch.

I only use straight plaster and am not a dusty cloth guy myself.

Mark

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Thanks to all for the replies.  I guess it's clear that I should try again.

Strangely, it was a fresh bag of plaster that I picked up straight from Laguna Clay.  It did give off heat while curing and it does absorb moisture very well.  I suppose I could have over agitated it in mixing (I used a drill-powered paddle meant for tile mortar), though I kept the speed slow.  Is it possible to over-vibrate it when releasing the bubbles?  I whacked the bucket, but also vibrated the mold with a blade-less reciprocating saw -- I figured more couldn't hurt.  I never thought to flip it over.  I'll cross my fingers and check that tomorrow!

Thanks again for your time!

Matt

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I drop the bucket a few times to get the bubbles to rise after mixing before pouring.

Then after pouring vibrate the form its poured in.

As you got it from Laguna it must be fresh so its a bubble/air issue in mixing and pouring

Mark

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It just occurred to me that the photo I attached might be mis-leading.  That is not a picture of the plaster board.  It's the underside of the clay I wedged on it.  All those specks of plaster got pulled into the clay leaving behind a pitted surface on the plaster board (seen in this attachment).  Does this clarification change anyone's opinion, or is it all just a result from the same problem?

Matt

 

post-63717-0-89449000-1402553894_thumb.jpg

post-63717-0-89449000-1402553894_thumb.jpg

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I understood your initial photo as the plaster on clay, I can't speak for anyone else.  

 

It sounds like you mixed it correctly and did everything you could.  Perhaps it wasn't fully set?  Hopefully the more experienced members will weigh in again, but I do still think you should cover your plaster with canvas, irrespective of how it was mixed, bubbles, or otherwise.  Just an extra step to protect your clay.

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I thought it was your plaster in the picture. It didn't look right, that's why I thought it could be old plaster. Since the plaster was fresh it would have to be from over use of the drill powered paddle and / or not thumping it down a couple times to pop the bubbles. I just use my hand when mixing plaster then when ready to pour I thump the bucket onto the table a couple times. When pouring i pour it over my hand into the form rather than just dumping it in, it seems to produce less bubbles that way. I'm not a fan of dusty canvas on plaster (or tables), I would make another one. 

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High Bridge,

My memory is fuzzy (I let it set up for 3 weeks), but I think there might have been some surface pooling.  Does that indicate something?

 

Min,

Thanks for your advice about hand mixing.  I've noticed that other people do that, so I'll try that with the next one.  I too am not a fan of dust -- too many allergies, and asthma. :(

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Mark,

Good question.  I did weigh both the water and the plaster, and added plaster to the water.  Part of why this is perplexing is that I followed the USG product directions to the letter.  At this stage all I can assume is that it is possible to over-vibrate the plaster once poured, or that the mixing must be done very slowly.

 

Matt

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I found all my moulds to start with would do this and become soft on the top inch or so. The rest would be good. Mixing for longer until the very last point when I noticed it feeling thick and looking very glossy helped make it a much more consistent mould.

 

Also the drill looks to have added lots of air.

 

To make I sprinkle the plaster onto the surface letting it drop to the bottom. Keep adding all of it in this way and let it soak in the water for a minute or so. Mix with your hand slowly so not to add air. Keep agitating until it thickens and goes glossy. I use cold water to slow the reaction.

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Good News!

Today I opened the mold, flipped the board over and it's working beautifully -- no plaster comes off!  I think it was similar to what you described High Bridge.  I bet if I scraped off the top inch or so I'd have exposed solid plaster.  Next time I'll definitely mix by hand.

Thanks for all the advice!

High Bridge Pottery likes this

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I use a jiffy mixer and mix for many minutes. I mix at high speed These mixers do not suck air into the stream. I use cold water

and drop buctet on cement a few time and pour very carefully to keep air out.

Always vibrate table  some being poured intill bubbles stop coming if there are any.

mark

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if you do mix by hand, it helps to put the palm of your hand in the bottom of the container, raise the fingers and just sway them back and forth.  that way you do not add air by putting your hand in and out of the freshly poured liquid. 

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oldlady,

I like this tip.  It has the benefit of being both scientific and graceful  ^_^

Just to be clear, do you do this to the plaster after it's been poured into the mold, or to mix it in the bucket, or both?

 

Matt

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