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Plaster, clay and kilns

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Hi, I know that plaster in clay can cause some quite exciting things to happen during firing yet we use plaster batts on the wheel and to reclaim clay.

My question is, how do you stop the plaster from ruining the clay. All my plaster batts, even after a few good washes all leave a white layer if you rub your hand over them when dry, so this must get into the clay. Is it just chunks of plaster that are the problem and not a bit of dust?

Also, what ratio plaster to water does everyone use? I've just been eyeballing it till I get what I think is the right consistency.



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Learn to mic #1 pottery plaster by weight-both with the plaster and the water


You can search US Gypsum for the right way to mix and the proportions .

When mixed right the bats will be hard and not rub off. I throw many many tons a year  on plaster bats with zero plaster issues in clay.

First quit washing them as washing plaster is not something one does not need to do .

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Every type of plaster requires a different amount of water for it to set up properly. If your plaster is powdery, then it's not right. #1 pottery plaster, which works great for reclaim bats, is mixed 10 parts plaster to 7 parts water, by weight. Don't try to eyeball it.

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I forgot to say as Neil did use only 1# pottery Plaster.

I weigh the water in a bucket and then add the plaster to water and mix with jiffy mixer for 2-4 minutes with a drill.I drop the bucket on a firm surface from  few inches above to force any air bubbles to surface-(use cold  not warm water) and slowly pour my bat molds. Let sit and scrap tops  with a straight edge after it sets some (but still soft) to keep them flat.After hoping them out of tins I cut the sharp edges off the top with a straight edge and sponge them smith at that sharp point I just cut off.This makes for all soft edges and that way none get into clay as they are all smooth. Made hundreds that way over my life. I never wash them and if you do let them dry very well. If you use different clays say white one and a stoneware  one make enough bats to keep them separate .

Edited by Mark C.

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