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Using plaster in the studio/contaminated clay?

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Hello I hope someone might be able to help with this:

I've been using a plaster bat to reclaim clay and when I flip the clay over there are some tiny specs of plaster stuck to the clay. I scrape these off and dispose of them. But I am really worried now that my clay might be contaminated with plaster. I was trimming a pot earlier, and came across a small hard white lump maybe 3x3mm. I poked it out and disposed of this and finished the pot. Is it possible this could be anything else or is it likely to be plaster?

I never saw any pieces of plaster this big stuck to the clay after reclaiming on the bat. I'm unsure what to do about my recent batch of pots, whether to fire them or not, and what to do about the reclaim. And going forward if it's worth removing plaster from the studio, or if it's likely that my plaster bat just didn't set right (it was a few years old potters plaster)?

Many thanks! 

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If the pieces are very small then they probably won't be an issue. Larger pieces can cause pop-outs after the piece is fired, even weeks after the firing. If you're concerned about your plaster bat, then I'd remake it or simply put a sheet over it before putting the clay on it. A good bat shouldn't have any bits coming off. Make sure you use the proper ratio of water to plaster when making it so it sets properly. If using #1 pottery plaster (recommended), use 7 parts water to 10 parts plaster by weight.

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Thanks for your reply. Could old plaster be the cause of this? I borrowed it from a potter friend but she said it was about 5 years old. I'm also worried about all my reclaimed clay, is there a way to remove the plaster from the reclaim? If i slake it down and use a fine mesh sieve? 


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1 hour ago, indianlotus said:

Could old plaster be the cause of this? I borrowed it from a potter friend but she said it was about 5 years old

Yes! #1 Pottery Plaster should be used within 6 months of the manufacturer date stamped on the full bags.



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I'll amend Min's comment ever so slightly: 

Pre-covid, my local plaster suppliers, were able to deliver plasters that were fairly fresh and within the 6 month time frame Min references, 

Post covid, that has changed. Sometimes I've purchased plaster that was 4-6 months old. 

No ideal but I'll suggest a new timeframe of 6-10 months. (from the date on the bag))

Old plaster, having gone through a humid period (seasonal changes in Minneapolis for instance) will set quicker than fresh plaster and MAY be softer. Plaster that is several years old is best used for making solid plaster objects which won't interact with clay.

A possible solution would be to drape the plaster surface with a thin cotton sheet. The plaster will still absorb the water but the clay doesn't interact with the plaster surface directly.

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Thanks! Yes the plaster was atleast 5 years old, and despite leaving it for weeks in the summer to cure, it's still very soft and i can scratch bits off with a fingernail easily. I found some chips on the edges of the bat so i'm sure it was plaster that got into the clay.

I have about 4 bags of reclaimed clay that could be contaminated and I don't want to use this to make anything important in case of pop outs. I've heard it's unlikely that they would cause explosions in the kiln but rather just cracks and chips after the piece has been fired, sometimes after months even? 

I've also heard that if the plaster specs are small enough it won't cause a problem, and thought about putting the slaked clay through an 80 mesh sieve before reclaiming? Would using an immersion blender also work in the same way? 

I'll put the soft plaster bat inside of a pillowcase until I have chance to make a better one. I'm thinking i'll just use the reclaimed clay as an excuse to make myself something, and for test tiles and  bisque moulds.

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If your plaster is still as soft as you describe after several weeks of curing, that’s a good indication it’s past its best before date. Or the plaster was measured by the “peaks” method instead of weighing the water. 

Plaster becomes a problem in a fired object if there’s a large enough piece inside the wall of the item. If conditions are right, the plaster can absorb water from the environment (think of porous earthenware possibly absorbing water in the washing) and swell inside the piece. Then you get something called lime pops, and a piece will chip off the side of the pot. You are correct that if the plaster bits are too small to create enough force to break the piece, you’ll likely never notice them. If it’s evenly dispersed in the clay body, it’s essentially a little extra calcium. 

Sieving with an 80 mesh isn’t a bad idea, as long as you aren’t removing any desired grog or temper. If you slurry mix your reclaim and you can’t see any lumps, it’s similarly unlikely to cause any issues. If you don’t want to reclaim 5 bags worth, you could use it for something like cob planters or other unfired projects. 


Plaster contamination bits that haven’t dried fully may release steam in the bisque, possibly causing the dreaded explosion. It’s not a lot different than if the rest of the piece was too wet and the early stages of the firing were too fast. 


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 Suggested fixes in order of least likely to cause problems to most:

1. discard the clay (not ideal, as there’s 5 bags you suspect)

2) do a project that doesn’t involve firing the clay, or use it for glaze cookies/wasters/cone packs/waste clay for mould making.  (again, a lot of clay to do this with, but you don’t have to do it with all.)

3) Slurry mix and sieve your reclaim to remove suspected plaster lumps (lots of work, time consuming, but 0 risk of changing clay chemistry, involves owning a suitable sieve)

4) slurry mix your reclaim until there are NO lumps anywhere in it (time consuming, lots of work, low risk of changing clay chemistry, involves having a heavy duty drill and attachment)


If there’s plaster lumps you either want to remove them entirely, or basically dilute trace plaster with the rest of the clay. Given the amount of clay you mentioned and that reclaim slip is usually pretty thick, using a stick blender will burn the motor out. If you want to try blending the slurry you’d have to use a drill with enough torque (usually the corded kind) and a  grout or mortar mixing attachment to get every single lump out. Or sieve it. It doesn’t have to be an 80 mesh glaze sieve, a fine enough kitchen strainer will work. 

If you think you've got dry pieces with hidden plaster bits, you’re better off discarding them and making  new. I do not recommend attempting to get around this with slow bisque at all. Especially if you are in any kind of group setting and you don’t own the kiln. 


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it appears you really want to save the clay.   what size bag are you using?  the amount of work it will take to "save" the clay is probably not worth the enormous effort you seem to be anticipating.   just toss it and then you do not have to think about it another minute.  clay is cheap, stress is expensive. 

btw, it is not necessary to use plaster in your studio.  there are lots of ways to reclaim clay.

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