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Exploding Pots During Glaze Firing


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Hi fellow ceramicist, I have been practicing ceramics for about 2years. I am making slab built dodecahedron pots in white grogged earthenware with amaco underglaze and clear earthenware glaze on top. I bisque fire to maturing temperature 2102 F and the pots come out perfect. I then wet spray the inside of with a water gun and swill the clear glaze inside the pot. I then underglaze and spray gun the clear glaze to the outside. Leave it to dry for five days in this cold English climate and then left it in a home made warming cupboard at 66.2F for five days and fired to the same cycle as the bisque firing but only up to the glaze temp of 1940F. The pots are exploding on opening an older electric kiln three days later. The glaze surface appears unblemished suggesting to me it is on the cooling cycle I am getting the failures.

 

kind regards,

Pete

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Exploding, during the glaze firing?  I've never heard of that happening.  

 

Why do you wet spray the wares?  Is it to control the spray application?  I don't spray glaze, so that's not familiar to me.

 

The general reason things explode in the kiln, is due to moisture, which is why it happens in the bisque firing.  Bisqueware has no moisture, so it shouldn't explode.  Glaze doesn't have much moisture in it.  I've put freshly glazed wares, in the kiln, and fired them immediately with no issues.  It is possible to have some glaze defects arise, from doing so, but it shouldn't affect the ceramic body at all.

 

If the body was saturated with water, and you tried to glaze fire it, they could theoretically explode, much like how certain kiln shelves will do the same if they get wet, and aren't dried properly.  

However, you seem to be bisque-firing to the clay's maturation point.  It shouldn't be that porous, when you go to glaze.  So I really am not sure what's going on.

 

I guess I should ask, what the wares look like.  Are they coming completely apart?  If they are just cracking violently, it could be a glaze fit issue.  The glaze is shrinking more than the body, and the glaze is it to fracture.

 

Trying posting some pictures, and others may be able to help.  I'm stumped.

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Benzine

I wet the inside of the pot so that when I swill the glaze on the inside of the pot it does not build up to thickly. I have had a number of successful pots in the past but recently a lot more failures.

Marcia Selsor

I am opening the kiln at 77F and I leave the bung in place.

Wyndham

I have had quite a number of successful pots in the past. That is a simple but great solution to test the firing by putting the bisque through a second time with the glazed pot and certainly I could try not wetting the inside of the pot first. I had thought of going back to a normal bisque at 1652 

Neilestrict

I have had them doing both but the last three firing with the turn in the weather have exploded

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That's cracking, not exploding in that photo. Exploding is hundreds of little pieces, shards in the element grooves, a total mess. It's really, really, really hard to blow up pots in an electric kiln that have already been bisque fired. They have to be fairly moist and fired really fast. The only time I have seen it happen is in raku firings where pieces weren't quite dry after glazing.

 

What's your firing schedule? I'm thinking that either the second firing is too fast either heating or cooling. Try slowing it down and see what happens.

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Hi Neilstrick

My firing cycle for bisque is

104F to212F;   374F to 428F;   248F to 1112F; Full power (probably around 266 F an hour) soak for 20mins.

My firing cycle for glaze is

the same as bisque but only going up to 1940F as the minimum temp for the clear glaze.

 

I am so glad it is only cracking not exploding - thanks for putting me right on that.

 

kind regards

Pete

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Hello, it's been awhile since anyone commented.  Have you made any changes?  Are the results the same?  Recently, I refired a large platter on the chance that the refire and re-glazing would correct some of the pin holes and voids.  Unfortunately, the platter cracked apart completely and the crack was very similar to yours.

 

Why do you bisque fire at such a high temperature?  Have you tried a bisque fire at a temperature lower than your glaze fire?  What were the results?  Would it be possible that your first fire brings the clay to near vitrification and the second glaze fire stresses the clay such that you have the cracks? 

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mrthumbs, I wonder if your platter had some hidden stress cracks, that the second firing finally brought out. Ceramic items can only take so much stress.

 

Alternately, the additional glaze could have been too much, causing the same dunting, that seems to be the original poster's issue. One of the glazes, I use often, generally gives me no issues. But one time I was a little too "liberal" with it, and it tore a large vase apart.

 

As to why he chooses to fire so high for the bisque, perhaps it is to vitrify the clay. Since low fire glaze firing isn't generally hot enough to do this, it would help seal the wares. Of course it would make them tougher to glaze.

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No, my platter did not have any stress cracks (pinged nicely before I placed into kiln); I think that my firing was a bit too fast and I don't plan to do any refires.  Agree with a higher bisque. I think we need to know why he fires that high in the first place.    

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 Are these pots slab built, I think it looks like a dunting crack. Are the edges ofthe crack sotra smooth or sharp, smooth  cracked on the way up sharp, cracked on the way down. Are these spots towards the top onf the kiln with no shelf above them? Could be uneven cooling if so.

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Hi Everyone 

I really really appreciate all the feedback I am getting and I so pleased it represents so many years of experience and training so let me try and answer your questions.

MarkC.

It is slab built with a bisected dihedral angle to the edges from a plastic template joined with deflocculated slip.

Benzine

I had problems in the beginning with the joints fracturing on firing (I now have a bespoke 15 blade knife that scores each edge with two passes giving a gap between each score mark of approximately 7/32") and read that bringing the pot to maturing temperature first would cause it to fail first and less likely on glaze as I spend sometimes one and a half days undergalzing the words on the pots I thought this a better way to go so that the glaze temp would be the minimum recommended plus I like to use red a lot, so i did not want to lose this at maturing temp for a final fire.

Wyndam

I will talk to the manufacturer about the stresses on maturing the clay first. And in practice I slice 1/4" slabs from a block and roll them on a plaster bat to 3/20" and dry them in 1/2" plasterboard (rockboard?) bats for a day before assembly. Initially I would dry them for a day in the bats after slicing them and some of them would split which I then rejoined with deflocullated water before rolling them on a cloth backing to 3/20".

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Babs

I have normally fired two pots in the kiln at one time one on a shelf but I kept the bung in place and did not try to force cool it which is why the firing cycle is 2&1/2days.

 I will have a look in detail at the sharpness of the fracture I think they are sharp smooth but you would have to explain to me the differences I should look for to tell the two types of crack apart from each other. 

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Peter,

 

I have been working with low fire, the past ten or so years.  The clay has been put through the extremes, both by me, and especially my students.  I've pulled glazed wares from the kiln, when they were still hot.  I generally glaze fire them quickly, especially at the end of the term.  I've even had the kiln opened when it was cooling at over 700 F (Not by my choice or doing).  And through all that, the only thing that caused similar results, to what you are seeing, was an issue with glaze fit (dunting). 

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My firing cycle for bisque is

104F to212F;   374F to 428F;   248F to 1112F; Full power (probably around 266 F an hour) soak for 20mins.

My firing cycle for glaze is

the same as bisque but only going up to 1940F as the minimum temp for the clear glaze.

Are you using a manual or computer controlled kiln? If manual, are you using cones to verify the temperatures your are trying to reach?

 

You might want to think about slowing down the rate of temperature increase as you approach peak -- for both bisque and glaze. Maybe work out a schedule where the last two hours your rate of increase is 108F. That will reduce some stress on the clay and glazes. Also, once you complete your soak, it is not as abrupt a decline in temperature as you start cooling.

 

Can you tell us a bit more about your claybody. You bisque to around Cone 3, and glaze to Cone 04. What is the firing range of the clay? Does it vitrify at Cone 3 or can it go higher?

 

Are you using the same glazes as when the firings were successful? Or have you mixed/bought new glazes?

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Well if the pot cracked on the way up the glaze edges to the crack will soften and smooth somewhat as the glaze matures. If the crack is on the way down, the glaze will be sharper to the touch as the glaze will have cooled and "set" so to speak. Experts will give hte science to clarify my dayspeak.

Are your shelves smooth and non catching?

Does that roundish curving crack follow the slight foot ring?

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Hi Benzine

I can see from the comment that i will need to talk to the manufacturer about the clay and the glaze fit. Thanks appreciate feedback.

Hi Bciskepottery

I use an electronic programmer with multiple programmable options on each firing cycle. I like your advice about slowing down the rate of increase nearer to final temp. I will definitely try that.

The clay is Valentines White Grogged Clay ( because it is not so prone to warping and cracking) with a firing range of 1976 F to 2156 F.

I also use Valentines smooth White Clay for the footring panel and the lips (for smoothness) with a firing range of 1940 F to 2120 F

The glaze is Leadless Clear Earthenware glaze with Bentonite suspender and fires from 1940 F to 2120 F.

I have been using the same clays and glaze for the past 18months with some success but more failures mainly due in retrospect to poor joining methods but now when it is Dunting? it is failing everywhere but along the joints.

Babs

It is never failing around the foot ring although the last failure was directly across the footring (thats the first time thats happened).

I will give close examination to the shards of my failures when i am next at the studio for the Sharpness you have explained to me thanks I now understand and without the science it makes a lot of sense to me.

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