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AndrewNorrell

Bloating And Crazing Issue.

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Hoping for advice on if I should continue testing, changing the firing schedules, or just choose a different glaze clay combo. Problems here are bloating and crazing.

 

I used a clear cone 6 glaze to test on cone 6 stoneware. The 4.6 cubic foot electric kiln was mostly empty, which I think let to some of my issues. Also, this is a manual kiln with no pyrometer. My temperature estimates are based on color and cones.

 

I used Heath A2V cone 6

 

Silica 35.4

Gerstley Borate 29.2

Nepheline Syenite (Theoretical) 20

EPK 6.6

Strontium Carbonate 5.7

Wollastonite (Theoretical) 2.5

Dolomite. 0.6

 

Cone 6 Stoneware

 

Cushing's Handbook (#4 on p.38)

VC Stoneware Cone 6

OM4 25

Goldart 25

Hawthorn 25

Lizella Red 10

Talc 5

Nepheline Syenite 10

 

 

 

 

I bisqued to cone 05 ( thinking of switching to cone 06) moving slowly from 200 to 1000 F, but once I passed 1300 I put all switches on high til the cone bent like we do at the kilns at my college. Through research though, I found out moving fast just after crystolysis can trap organics in the clay and this could be the cause of the bloating during glaze firing.

 

 

 

Firing to cone 6, I shot the kiln right up at about 300F an hour... All the way to cone 6... Not doing that again....

 

I attempted to hold the cone 6 temp for 10 minutes, but like I said- no temperature readout.

 

 

 

Results of the cone, test tile, and test mug are attached. Would it be wise to try this again with a slower schedule, or scrap the combo?

post-86416-0-91805500-1496970004_thumb.jpg

post-86416-0-99025900-1496970015_thumb.jpg

post-86416-0-15507200-1496970169_thumb.jpg

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I have been firing ^6 reduction and /or oxidation since 1980. I looked at your glaze recipe and thought the silica was too high. Most of my recipes are in the 18-25% for silica. I did notice one at 32%

I would recommend cutting back on the silica a could of grams at a time. do some tests, 

 

As for the bloating, there seem to be some well define layers folded in that cross sections. Could you have wedge a contaminent

into the clay while wedging? The layers seem to uniform for bloating.

 

Just an observation from your photo.

Marcia

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Hi Andrew: welcome to the forums.

 

You have bloating; which is coming from the Lizella. You also have delamination, because the flux levels are too low for cone 6.I have not ran the clay on a calculator: but I see all kinds of problems with it. Way too much large particle fire clay; the absorption rate will be much higher because of it. Way too much Om4 as well: contrary to popular opinion; throwing in more ball clay does not make it more plastic. I assume the Lizella was thrown in for iron content: but it is an unstable clay at cone 6. Swap it out for some Red Art, which has even more iron.

A cone 6 stoneware body should have around 3.15 molar KnaO levels, suspect this recipe is in the 2.60 range. ( I have not ran it). Again with the talc: I assume someone put out a clay book that said talc should be added to everything. What talc does in a low- fire or slip recipe: is not the same in a cone 6-10: high plasticity recipe.

 

I seriously doubt you have a crazing problem: you have a highly unstable clay body that is not remotely close to vitrification, which cracked your glaze. Little surprised this recipe was put out. You can fast fire a high iron body up to 2050F from bisq, but you need to slow way down from 2050F up to 2190 / 2230F to burn off sulfides and reduce bloating.

 

You have to watch Hawthorne as well: it does have large particle feldspar materials from time to time: but those usually show up as rather large blisters in the glaze from off-gassing.

50 35m

 

You can see a white chunk of feldspar in the pic above, that show ups like the pic below.

 

Clay pinhole

 

 

Nerd

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Hi Nerd, I apologize for the typo when I copy pasted the recipe. I had already subbed Red Art for availability. Is it possible to bloat an unvitrified clay?

 

From what I understand about bloating is that fast overfiring causes bloating because gasses become trapped in already formed glass, and that it takes flux to form glass at lower temps. So I thought that the ingredients might have been unstable, forming too much gas and there being too much flux causing vitrification too early. The KNaO is 2.82 molar, I'll post the rest of the fluxes molarity and a better test Wednesday. The plan is going to be to slow down at around 2000F (I have no temp readout) and fire to just below cone 6. I don't think discussing the issues is useful without a proper test.

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The primary cause of bloating is excess off- gassing. However, there is more specific application of how that occurs. Bloating is most often ( but not exclusively) seen in high carbon/ s bodies: an example would be a dark brown stoneware, and some red bodies. As the carbons burn off ( usually sulfides from lignite coal): it can build up an impermeable barrier just below the face of the clay. That barrier hinders the release of gases from the ignition of feldspar/ s. Feldspars actually begin their melt below 04, and increase until they are spent at 2190f. So some off- gassing is occurring at bisque. Your pictures detail ( to me) that more than bloating is occurring.

 

Nerd

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I just unloaded the cone 5 test. I fired slowly after cone 04 to be extra cautious, and slowed the cool with the switches at 40% for 4 hours. 

 

Tiles came out to 2.8% porosity, high but functional.

 

There is no more bloating or crazing. There was a whole set of experimental mugs ready to go on this clay body so I threw them in. 2 plates put in crazed. So the fit isn't perfect, but the mugs are functional. So, even with all the glaring warning signs in the recipes, they probably work but aren't great. These problems are mostly due to my novice approach to a manual firing, over firing then under firing a not very stable recipe. I doubt the cone 6 firing had anywhere near even temps in the kiln, causing some of it to reach cone 7. Could the cone 5 firing have left the glaze short of fitting to the clay body making the plates craze?.... instead of asking I'll do my testing first. In the future I'll get opinions on clay/ glaze recipes before using them, fire with proper schedules, and use guide cones. 

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Thought I would update this thread with some recent observations about bloating. Bloating can occur without the usual blisters associated with it.

* This would occur primarily when using high iron/ dark body stoneware, or red bodies stoneware, or red bodied clay to higher cone fire than intended.

* This should only be considered if you have followed the usual remedial fixes in glaze calc, COE adj. and firing schedule.

image.png.91585226b8ee3ba023a3b6be58da46fc.png

You can see the craze line mid picture right, and directly  below it bloat cavities that have not ruptured the surface.

The only way you can detect it without cutting open the piece is by unexplained crazing and extreme color shifts in the glaze. ( as shown below.)

image.jpeg.77b951600bd332e41f43beffa28b92ab.jpeg

At times, there can be some "dark" unexplained blotches.

Nerd

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On 6/8/2017 at 6:02 PM, AndrewNorrell said:

Hoping for advice on if I should continue testing, changing the firing schedules, or just choose a different glaze clay combo. Problems here are bloating and crazing.

 

I used a clear cone 6 glaze to test on cone 6 stoneware. The 4.6 cubic foot electric kiln was mostly empty, which I think let to some of my issues. Also, this is a manual kiln with no pyrometer. My temperature estimates are based on color and cones.

 

I used Heath A2V cone 6

 

Silica 35.4

Gerstley Borate 29.2

Nepheline Syenite (Theoretical) 20

EPK 6.6

Strontium Carbonate 5.7

Wollastonite (Theoretical) 2.5

Dolomite. 0.6

 

Cone 6 Stoneware

 

Cushing's Handbook (#4 on p.38)

VC Stoneware Cone 6

OM4 25

Goldart 25

Hawthorn 25

Lizella Red 10

Talc 5

Nepheline Syenite 10

 

 

 

 

I bisqued to cone 05 ( thinking of switching to cone 06) moving slowly from 200 to 1000 F, but once I passed 1300 I put all switches on high til the cone bent like we do at the kilns at my college. Through research though, I found out moving fast just after crystolysis can trap organics in the clay and this could be the cause of the bloating during glaze firing.

 

 

 

Firing to cone 6, I shot the kiln right up at about 300F an hour... All the way to cone 6... Not doing that again....

 

I attempted to hold the cone 6 temp for 10 minutes, but like I said- no temperature readout.

 

 

 

Results of the cone, test tile, and test mug are attached. Would it be wise to try this again with a slower schedule, or scrap the combo?

post-86416-0-91805500-1496970004_thumb.jpg

post-86416-0-99025900-1496970015_thumb.jpg

post-86416-0-15507200-1496970169_thumb.jpg

I am curious as to how the cone is set, it looks like it is laid on something?

David

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Guest JBaymore

RE the above cone picture .... you are not using large cones in a kiln sitter, Are you?

best,

......................john

 

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