What Happens When Clay Is Significantly Overfired?

[curt]

This topic is intended to continue the discussion in another recent thread “New to clay, need help with firing temperatures”, which had digressed somewhat from the topic raised by the OP.

Bill Klieb asked what mechanisms make clay less dense and begin to bloat when severely overfired (say, 3 or more cones higher than recommended).

The reason clay bloats when severely overfired is due to the off-gassing of decomposing clay body materials and/or the re-gassing (coming out of solution) of previously dissolved gas bubbles, as temperatures increase.   Thousands of small air pockets make bubbles which start “inflating” the now glassy clay body like air in a balloon, or blowing in to very thick honey with a straw.  Chemically and microstructurally, this phenomenon was discussed somewhat in another thread in November 2015 entitled  “Bubble, bubble, toil and trouble,” started by Highwater, on page two of that thread..  

It is the same kind of off-gassing that initially results in small bloats on the clay body surface when a body is mildly overfired (see some pictures in a thread called “Pictures of bloating close up”).  Just much more exaggerated.   Anyone who has broken open some severely over-fired (ie melted mess) earthenware will notice that it looks like a honeycomb candy bar inside.  For those that have no idea what honeycomb candy is, think of heavily leavened bread.  Or a sponge.  Lots and lots of small round holes/“pores”, separated by thin glassy walls.  Of course the individual roundish pores in an overfired (very glassy) clay body are very different from the “pores” in an under-fired clay body, which are more like small twisty cave-like passages between large chunky particles.

I agree with Min and Bill that when seriously over-fired, clay density may become less dense compared to the density at the proper maturing temperature, because every cubic centimetre of clay will contain a lot of these little air bubbles suspended in a very glassy mass, and hence would weigh less than a cubic centimetre of solid glass (using weight as a proxy for density)

Clay this overfired does sound clunky when tinked due to this porous structure, and I know what you are talking about and have seen it myself.

[LeeU]

I'll have the salted maple honeycomb please. And a nice loaf of Nordic rye.   

Now, not to digress too much---but what can someone tell me about refiring a body (^5) 4-6-more times? This is not something I want to test in my L&L EZ23. I want to be very, very kind to it, such  that it may live a long and happy life. But I am wicked curious. I know the body can get a bit brittle at 1 refire, but what comes with pushing it?  Also--what are 3-6 layers of commercial mainstream non-specialty ^5 glazes (some already fired, some not) likely to do that could be a problem?  I don't care about color affects, bloating etc.--just explosions, melting, anything destructive.  This is re: scuptural, not food/drink functional. Thx. 

[Magnolia Mud Research]

LeeU asked:  what can someone tell me about refiring a body (^5) 4-6-more times?

 

Well, I don't know, but while doing an class assignment on the work of Ron Nagle, I discovered that many of his gallery pieces were fired multiple times, the one I really liked had more than 20 glaze firings.  What I remember was that his clay body was in the order of cone 5 and his repeated glaze firings were in the 06-04 range.   
 

LT

[liambesaw]

17 minutes ago, LeeU said:

I'll have the salted maple honeycomb please. And a nice loaf of Nordic rye.   

Now, not to digress too much---but what can someone tell me about refiring a body (^5) 4-6-more times? This is not something I want to test in my L&L EZ23. I want to be very, very kind to it, such  that it may live a long and happy life. But I am wicked curious. I know the body can get a bit brittle at 1 refire, but what comes with pushing it?  Also--what are 3-6 layers of commercial mainstream non-specialty ^5 glazes (some already fired, some not) likely to do that could be a problem?  I don't care about color affects, bloating etc.--just explosions, melting, anything destructive.  This is re: scuptural, not food/drink functional. Thx. 

I've fired the same thing 4 times, the glaze ended up crawling up and out of the bottom of the bowl after the third firing and got worse with the 4th.  It was a high iron body and fired in reduction with a glaze that crazed on it with a single firing, which is why I decided to refire it to see what happened.

[liambesaw]

Here's a cone 6 stoneware fired to cone 9 by accident

You can see it stuck to the shelf as well due to the iron fluxing in reduction

[Mark C.]

The more times you take the clay to or near its maturing point the weaker it gets. Refires for me only go once not two or three times.They will get the bubbles in them as Curt says pretty fast off taken to the clays maturity point again and again.

I know some sculpture folks like Nagel who fire a dozen to more times usually working down in temps. The pieces are to be looked at not used daily. 

When you see bloating on refires toss them . They are weak.

Think of them like bald tires-outlived their use.

[Pres]

Crazy things happen to overfired clay, bloating is mentioned often, and weaker body strength. However, I have seen somewhat the opposite from time to time. I was using SC 112 years ago, and fired a bisque with casseroles in it. . . about 15 with lids. I fell asleep as the bisque was late, woke  in the morning to see a white line in the 5am dark of the garage all around the garage where the light from the slight gap in the kiln lid shows. That white bight yellow white light indicates around 2200F. I figured the pots would be toast, if not the kiln. Nope, pots were definitely heavily overfired, with no bloating, however the manganese had crystalized into large 1/8 inch splots. I figured to toss them all, but my Dad had been looking for some bonsai containers. I figured why not. Gave them to him, and he tried drilling them for drainage holes. Could not do it, he said the diamond bits burned up, and all other bits would not touch the surface. I tried some myself, and could not get them to drill. Now this was back in the 80's, and diamond bits are not the quality you can get now without spending an arm and a leg. I was surprised at the strength those casseroles had. Oh yeah, I had to rap the lids off with a wooden rolling pin, as they had fused somewhat also, but not much.

 

best,

Pres

[liambesaw]

13 minutes ago, Pres said:

Crazy things happen to overfired clay, bloating is mentioned often, and weaker body strength. However, I have seen somewhat the opposite from time to time. I was using SC 112 years ago, and fired a bisque with casseroles in it. . . about 15 with lids. I fell asleep as the bisque was late, woke  in the morning to see a white line in the 5am dark of the garage all around the garage where the light from the slight gap in the kiln lid shows. That white bight yellow white light indicates around 2200F. I figured the pots would be toast, if not the kiln. Nope, pots were definitely heavily overfired, with no bloating, however the manganese had crystalized into large 1/8 inch splots. I figured to toss them all, but my Dad had been looking for some bonsai containers. I figured why not. Gave them to him, and he tried drilling them for drainage holes. Could not do it, he said the diamond bits burned up, and all other bits would not touch the surface. I tried some myself, and could not get them to drill. Now this was back in the 80's, and diamond bits are not the quality you can get now without spending an arm and a leg. I was surprised at the strength those casseroles had. Oh yeah, I had to rap the lids off with a wooden rolling pin, as they had fused somewhat also, but not much.

 

best,

Pres

But did they dong, or did they ding?

[Pres]

High pitched ting, not as loud as when normally fired.

 

best.

Pres

[liambesaw]

I like the look of a slightly overfired stoneware.  Where the bloat is still just very small occasional blisters.  Maybe I'm alone on that, but my daily mug and my favorite bowl both have that look and they don't ding or dong, they just sound like wood when you flick them.  Waiting for them to fall apart while I'm using them some day.

 

[Bill Kielb]

This is anecdotal and common accumulated knowledge for me so nothing formal, just observation. In general anything I re-fire appears to take its toll on the base ware. If it was extremely round prior it likely will deform somewhat. This make sense to me because no longer do I expect my clay to flux and melt and do its thing (let alone my base glaze which has fully developed already), its basically done. So any re-fire that appears to work is a bonus in my opinion, and I have seen folks be successful!

Now as a substitute for the I gotta repair this, I have used low fire glaze and fired down just as many China painters do.  China painters and sculptors seem to successfully fire down so I think that is a better approach with a higher likelihood of success and less stress on the ware. (I know, overstating the obvious)

With respect to using a cone 5 body with 04-06 successive decorative firings I have done this and know folks who do it with great success so @Magnolia Mud Research work with Ron seems spot on.

To that end, hopefully in another month or so I will be able to publish our tests involving the use of underglaze, overglaze defects and propose a hybrid approach using mason stains and a decent medium so brush artists can paint more freely with a know pallet and obtain more consistent fired results.

I bring this up because it all leads to firing down to reduce the influence on color and stress on the ware and involves multiple firings. (Wow, a mouthful!) 

So fire down would be my experience with the rest being risky.

And as one who works mostly with porcelain, the ding is very important to me!

Good discussion!

[Mark C.]

2 hours ago, liambesaw said:

I like the look of a slightly overfired stoneware.  Where the bloat is still just very small occasional blisters.  Maybe I'm alone on that, but my daily mug and my favorite bowl both have that look and they don't ding or dong, they just sound like wood when you flick them.  Waiting for them to fall apart while I'm using them some day.

 

I like the Stone Age look of raw clay/torn clay but bloated pimples to me are a bad omen and not attractive. I'm a ding man with porcelain-stoneware is less ding.Bloated wares for me are goners.

[glazenerd]

Porcelain reacts differently than stoneware due to flux content. 

[Mark C.]

Stoneware is is also different with multiple fires as Tom pointed out above