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Pottery Cracking In Glaze Firing


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On 12/24/2019 at 12:19 PM, neilestrick said:

Depends on the size of the kiln and how tight it's packed. My baby kiln can cool from cone 6 to  unload in 5-6 hours. My 4 cubic foot can do it in 8 hours if it's packed light. Both of those situations can be too fast for a wide flat piece.

I have not experienced it, but maybe. My comment was more on gradient though, which whatever rate the kiln is cooling most of it is cooling at that rate. To be truthful I have really never experienced cristobalite issues as well at cone six  another  cause cited frequently. I find cristobalite issues to be pretty rare actually. Now as to opening the top of the kiln at some temperature is definitely a killer. Even cooling (uniform) - is definitely a plus for all wares for sure.

I have watched folks unload at 250 degrees with no damage, but when they unloaded that hot and put the wares on a metal cart. Crackalama!  So evenness is surely important but making sure the ware can grow and shrink freely is a huge one during firing.

Interesting to graph a kiln Cooling rate. Right at the end of fire it may cool down at 800 degrees per hour or more initially, and slowly decrease until the very end to maybe  25 degrees per hour or less. What is too fast? I don’t really know actually. Cooling evenly seems important though.

Edited by Bill Kielb
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The bottom of this dish seems much thicker than the walls.

How is this form made?

Does it ping after the bisque fire?

Qhite a lot of manipulation after throwing. 

I have always found forms which rise abruptly from the flat base more troublesome. I would try to trim bottom leaving a central nub to the same thickness as wall.

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6 hours ago, Babs said:

The bottom of this dish seems much thicker than the walls.

How is this form made?

Does it ping after the bisque fire?

Qhite a lot of manipulation after throwing. 

I have always found forms which rise abruptly from the flat base more troublesome. I would try to trim bottom leaving a central nub to the same thickness as wall.

Thanks for the input!

I think the walls and bottom were pretty consistent in thickness (it was slab built), so don't think the issue lied there, but the change in directions of the form -almost 90degree angle from base to walls makes sense to me as a possible cause of trouble.  Will try "spreading" the sides next time to create a slant angle. Also put something beneath it for the firing seems to be a predominant idea. Only have to figure out the Spanish for alumina (I know grogg is "chamote", but not sure about alumina). ☺️

Also might try to throw this shape, rather than slab buid, maybe that'll help as well!

IMG_1613.jpg

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On 12/23/2019 at 1:25 PM, liambesaw said:

Can we get a close up picture of the crack, where the glaze and clay interface?

And was it split like that when you found it, or have you separated the halves for clarity

Here's the closeup! From all the comments I'm getting, seems to me it has to do with separating a bit from the bottom next time (either alumina, grog or some wadding). Also the tension as the form moves while cooling, so will check the times for that as well... And placing it on the topmost shelves rather than bottom... 

Thanks a lot!

 

IMG_1612.jpg

IMG_1613.jpg

IMG_1614.jpg

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On 12/23/2019 at 3:00 PM, Min said:

@Sopita on the Rocks!, if you run your fingertip across the crack is the glaze sharp on the edges of the crack with no smoothing over of the glaze? Also, how fast do you cool the kiln and how hot was it when you removed the peep hole plugs and cracked open the lid?

Not sure about the question of the crack/glaze, but attaching a photograph that maybe shows it better. As for the cooling, once it's reached 1240˚C I let it at that temperature for 15', and after that let it cool all night, opening once it's reached 100˚C or less (it was 90 something when I opened it, if I recall correctly).

IMG_1613.jpg.782361914f4733fb1b00c136ded7b949.jpg

IMG_1614.jpg.bbd33283c2323f5a7364b5e5a29fa8ee.jpg

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@Sopita on the Rocks!, looks like the glaze is softened / slightly rounded over along the crack on the base.  Rounded / softened glaze edge indicates the crack was there before the glaze melted, as the kiln was heating up and before the kiln reached top temperature. Sharp edge indicates the crack occurred on the cool down. It does look like the glaze on the wall has a sharp edge and that there is a glaze thickness difference between the base and the wall, the base being a fair bit thicker. 

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10 minutes ago, Min said:

@Sopita on the Rocks!, looks like the glaze is softened / slightly rounded over along the crack on the base.  Rounded / softened glaze edge indicates the crack was there before the glaze melted, as the kiln was heating up and before the kiln reached top temperature. Sharp edge indicates the crack occurred on the cool down. It does look like the glaze on the wall has a sharp edge and that there is a glaze thickness difference between the base and the wall, the base being a fair bit thicker. 

Thank you so much for these insights! Will try looking at it and paying close attention to this when I get home! I think I understand what you mean. 

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When making hanging planters with flat to the wall backs I had a bit of similar cracking happening. Throwing the flat back and attaching front instead of slab work really helped with that problem.

Check the bisque is uncracked. Flick the pot and listen for a pinging sound.

Your clay slab has been under pressure of manipulation to get from flat to that shape.

Fire on bisqued thin sausage coils of  clay placed like an asterisk.

 

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Does anyone suggest using cookies or coasters under pots?  I've been working on some footed pieces and firing them in a shared kiln where sand or grog could cause a  problem.  I don't know if this would work for something flat.  A cookie is a flat piece of the same kind of clay that will shrink at the same rate as the pot and prevent drag.  It is usually a waste piece, but could be a plate or trivet.

Cynthia

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47 minutes ago, Cline Campbell Pottery said:

Does anyone suggest using cookies or coasters under pots?  I've been working on some footed pieces and firing them in a shared kiln where sand or grog could cause a  problem.  I don't know if this would work for something flat.  A cookie is a flat piece of the same kind of clay that will shrink at the same rate as the pot and prevent drag.  It is usually a waste piece, but could be a plate or trivet.

Cynthia

Yes,

Works, but sometimes the waster piece drags on the shelf as well so something like alumina wax can still be a good practice on the foot of the ware.

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  • 2 weeks later...
6 minutes ago, Sopita on the Rocks! said:

Thanks for all the comments and help! I tried firing again this weekend and used alumina beneath a new dish I made, to great success!!! 

1.jpg

 

Nice work and nice solution! Easy peesy lemon squeezey ! Glad it worked, go make more cool stuff!

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  • 7 months later...

Help 

I’ve been working on 3 nativity scenes for Christmas, and the big barn center piece for the manger keeps cracking! I don’t fire them myself but have been going to a ceramic shop and she’s been firing them for me. The first barn had a deep crack( not all the way through) and then the next two had a fine line crack.. I don’t know what’s being done wrong but I’ve put in so much time on each piece ,the last two were already fired in the glaze and I had put the mother of pearl on them and the6 cracked in that firing.. any idea what’s going on? 

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It is usually easiest to upload images straight from your phone. Most new models will automatically ask which size of file you’d like to use. If you’ve got an older model, you can email the picture to yourself, and when you hit send the email program will ask you which size you’d like to send. 

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