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Fuzz On Plaster


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#1 Jennifer Harnetty

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Posted 04 March 2010 - 02:06 PM

A reader sent the following question...
When reconstituting clay, I use plaster slabs to draw out the moisture for workability. After a while there is this fluffy white residue that grows like crystals on the plaster surface over time. From all my readings and investigation, this seems to be a form of Sodium Silicate. From the readings, Sodium Silicate can be added to a liquid clay as a flocculent and/or to add plasticity. What would happen if this substance is mixed into a low/medium/high fire clay body for throwing or hand building? I normally just brush the stuff off into the garbage because everyone has told me to. Would this crystal substance actually help or harm a regular clay body? I fire to cone 6 electric if that is any help.
Anyone have an answer?

Jennifer Poellot Harnetty
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Ceramic Arts Daily
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#2 Sherman

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Posted 18 March 2010 - 02:35 PM

What you are seeing is called efflorescence. Your online research has returned results on sodium silicate because it happens a lot with slip-casting molds, where the slip has been formulated with this soluble deflocculant. This can happen with plaster slabs used for reclaim for similar reasons, but at a slower rate. Sodium silicate is not typically added to throwing or handbuilding bodies because it will actually make it less plastic at a certain point, so that is likely not your problem (unless casting slip is being poured on this slab), but it is possible that other soluble salts are in your clay body as trace materials from other components. Over time, these soak into the plaster with the water coming out of your reclaim. When the water evaporates back out of the plaster, it leaves the surface into the air, but the salts cannot and therefore build up on the surface in the fuzzy crystalline matrix you are seeing. Over time, these soluble materials will break down the plaster from the inside, and it
will begin to get soft and crumbly. While the materials themselves are likely to be in such small amounts that they will not adversely affect your clay body, they will eventually affect the plaster to the point of disintegration, and pieces could start coming off into your clay. At that point, it’s time to cast a new plaster reclaim slab. I usually trash my slabs as I start to see the fuzz, because it’s just a matter of time.
Sherman Hall
Editor, Ceramics Monthly
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http://www.ceramicsmonthly.org

#3 AnnieM

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Posted 31 March 2010 - 09:05 PM

Thanks for this info. I cover my plaster with canvas to prevent plaster residue in my clay. I haven't seen any sign of salts leaching into the canvas. Is the plaster still being degraded under the canvas or does the fabric leach away the salt?
AnnieM

#4 Sherman

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Posted 02 April 2010 - 07:12 AM

Thanks for this info. I cover my plaster with canvas to prevent plaster residue in my clay. I haven't seen any sign of salts leaching into the canvas. Is the plaster still being degraded under the canvas or does the fabric leach away the salt?


AnnieM,
The canvas will wick moisture from the plaster and the evaporation of the water would take place from there, so it wouldn't hide the salt crystals. If you don't see them on the canvas, you don't have this problem.
Sherman Hall
Editor, Ceramics Monthly
Co-host, Ceramic Arts Daily
http://www.ceramicsmonthly.org




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