The verdict on plaster in clay firing

[JRW]

Hello,

After reading many posts on the subject in this forum and others,  I remain deeply confused about the dangers of a bit of plaster being present in clay during firing. Many people take it as a given that even a speck of plaster embedded in clay will cause an “explosion” or, at the very least, cracking. They say this has to do with the different properties of the plaster, which will not shrink at the same rate as the clay that surrounds it.
 

Many other folks say that the only danger with plaster in clay is that it can lead to “lime pops” AFTER firing.  They say that the plaster calcines during firing and then absorbs water later, which causes problems, but nothing that happens chemically during the firing itself will harm the piece. 
 

Still others say it depends on the size of the piece of plaster in the clay.
 

It seems to me like there must be some basic chemistry to all of this and the contradictory information is baffling to me. Does plaster pop out or melt into the clay body? does it shrink or expand? Is there a difference between what it does at bisque temperatures versus cone 5 or 10? Can anyone make sense of this? 

[JRW]

I should add that I do a ton of slipcasting and I’m positive that some tiny amount of plaster must have made its way into some of my pieces, and yet I have had no issues with cracking or explosions... and yet I use a shared kiln, so I want to be careful not to put others’ work at risk.

[Hulk]

Good question!

I'd like to suggest test scenarios, however, not in a shared kiln; I've many tests on my own "to do" list...

[Sorcery]

I was pondering this in thinking about a popular bonsai Potter whose, it seems, signature is lime popping.

Makes me have this question as well, though I don't know that every pot breaks after some time. Maybe they do.

What I constantly wonder...

Will condensation form within voids in a vitreous pot?

If not, internal lime bits are secure.

If so....pop!

I believe every factor of "it depends" is at play.

 

Sorce

 

 

[neilestrick]

It's all about the size of the plaster pieces. If the pieces of plaster are small enough, or dust, you're not going to get pops.

[Min]

Size of the plaster pieces plus absorption of the clay is going to factor into it. In order for the lime to "pop" it needs to be a big enough nodule to create the force needed to blow out the clay covering it when it expands from moisture absorption. Bisque and low fire earthenware is going to be most likely to have issues with blow outs. If the same size bits of plaster are in a well vitrified zero absorption clay wall, with no chance for expanding due to moisture absorption, then there is less likelihood of pops. Trouble is the mere act of glazing the bisque is adding moisture to the clay so the pops can occur during the firing. As to whether there will be pops sometime in the future, regardless of if it's cone 04 or 10 or somewhere in between, I would be looking at the absorption of the clay vs the firing cone. Higher the absorption figures would equal a greater chance of lime pops if the plaster bit is large enough to expand to the point of having the strength to blow out the wall if it doesn't happen during the firing. 

Welcome to the forum.

 

[liambesaw]

I've had pops from reclaim I dried on drywall.  The chunks were small enough to not notice while wedging or throwing.  There are different types of plaster too, some based on lime, some on cement, but the most common is calcium sulfate.  The problem with calcium sulfate, is that it doesn't degrade into calcium and sulfur in the kiln, it stays calcium sulfate.  So even if you reached the melting temperature (over 2600f) in the kiln. It would still not incorporate into the clay body, and it would still expand dramatically when exposed to moisture.

I think the general consensus is to work plaster in one area with one set of tools, and do clay work separately.  Try your best not to get plaster in your clay, and that's about all you can really do.

[oldlady]

liam, your first sentence above bothers me.   are you saying that the drywall contributed to the problem?   there are truly beginning beginners on this website and your statement might make them avoid drywall entirely when it is a really beneficial product.

[liambesaw]

6 minutes ago, oldlady said:

liam, your first sentence above bothers me.   are you saying that the drywall contributed to the problem?   there are truly beginning beginners on this website and your statement might make them avoid drywall entirely when it is a really beneficial product.

Yes, it was a piece past it's prime.  I have switched to solid pieces of pottery plaster for reclaim, no chance of crumbles getting in.  

Drywall good for some things, not for reclaim.  Too weak and gets soft with that amount of moisture

[Benzine]

20 hours ago, liambesaw said:

Yes, it was a piece past it's prime.  I have switched to solid pieces of pottery plaster for reclaim, no chance of crumbles getting in.  

Drywall good for some things, not for reclaim.  Too weak and gets soft with that amount of moisture

I went with cement board for my classroom.  We will be on our fourth year, and they are holding up well (Student tested should be a separate and likely the most rigorous form of testing).

The biggest problem is getting them to consistently use them, because they are "heavy".

[PSC]

Lime does pop so plaster in clay is a hazard...so is concrete in clay too...concrete when heated goes boom...just take a torch to your driveway if you need to see for yourself. I've seen plaster blow about a 3/8” chunk out of the surface of a pot when a small 1/8” little chunk got trapped in the wall of a thrown pot.