Bloating

[Val]

I am experiencing bloating with Lugana cone 6 B mix. After reading this : http://www.lagunaclay.com/support/bloating-and-blackcoring.php I am wondering .........I bisqued to cone 04 on slow cycle but as I have a kiln vent with a small hole in the top and botton of the kiln I did not open the larger side vents. Could this be my problem? I glaze fired on medium speed also just using the small kiln vent holes. My cone 6 was bent perfectly so I don't think I over fired. But was medium too fast?

[Mark C.]

Laguna has in the past had some bloating issues in some bodies-they always say to bisque it hoter.I do not fire in this range so I'm no help on this body. I thin there are many here who use this body lets see what they say.I think offcenter uses this body some.

Mark

[OffCenter]

I am experiencing bloating with Lugana cone 6 B mix. After reading this : http://www.lagunacla...blackcoring.php I am wondering .........I bisqued to cone 04 on slow cycle but as I have a kiln vent with a small hole in the top and botton of the kiln I did not open the larger side vents. Could this be my problem? I glaze fired on medium speed also just using the small kiln vent holes. My cone 6 was bent perfectly so I don't think I over fired. But was medium too fast?

 

I use cone 6 B-mix a lot and have never had a bloating problem. I bisque to cone 010 as fast as my kiln will go after about 250 degrees and glaze fire to 6-7 sometimes a fast as possible with a cool down as fast as possible and sometimes (depending on the glazes) with a slow final approach to peek and then a controlled cool down. I don't use vents so don't know how they affect a firing. Something that Laguna is not going to suggest to you is that you got a bad batch of clay. I got half a ton of clay (not B-Mix) from them one time that caused me a lot of trouble. Have you used cone 6 B-mix before? Did you have the same problem?

 

Jim

[bigDave]

I am experiencing bloating with Lugana cone 6 B mix.

 

I staring at a box of B-Mix 5 as we speak, Got a big ole 5 on it. Some of the clays go one or two cones higher, wondering maybe just over fired.

 

You say 6 was all the way down? like a dead snake or just a flag at half mast ?

 

Used lots, always perfect at 5 for me.

 

Hope you figger it out asap

 

big D

[OffCenter]

I am experiencing bloating with Lugana cone 6 B mix.

 

I staring at a box of B-Mix 5 as we speak, Got a big ole 5 on it. Some of the clays go one or two cones higher, wondering maybe just over fired.

 

You say 6 was all the way down? like a dead snake or just a flag at half mast ?

 

Used lots, always perfect at 5 for me.

 

Hope you figger it out asap

 

big D

 

Granted it says "5" on the box, but since most midfire glazes are cone 6 most potters fire to B-Mix 5 to 6. I fire it to a flat 6 or soft 7. The clay has more of a bell tone to it at 6-7 than at 5.

 

Jim

[Claypple]

This is what the manager from Laguna said about bloating.

The main thing is he suggested to send him some of your clay back along with the batch #

 

"There are a couple of contributing factors to bloating. Not just

temperature. First, gasses have to be present and trapped inside the body.

Then the body has to be soft enough to allow the gasses to expand, but fused

enough not to let them escape. With many White bodies the clay/glaze

interface layer can become a barrier to gasses escaping.

 

To overcome this I generally recommend a bisque firing to at least ^04 with

a good soak at the end. If you've got a tightly packed bisque kiln I'd start

slowing the firing down about 100*f from the end and proceed at 50* and hr

and then the last 25* over the last hour. Electric Kilns are notorious for

not firing evenly top to bottom and even from the edge to the center with

the larger diameter ones. Slowing down at the end allows the heat to

equalize over the entire kiln and throughout all the ware.

 

And if you're convinced it's the clay ---send some of your clay in to me

along with the batch number and I'll check it out.

 

Best regards,

Jon Pacini

Clay Manager

Laguna Clay Co"

[Claypple]

I use cone 6 B-mix a lot and have never had a bloating problem.

Jim

 

I knew that!!!!!!

[Mark C.]

I have been around 3 types of laguna clays that had bloating issues-It was the clay batch in each issue but Laguna did not fess up right away and only in one case as I recall and it was high fire (cone 10)B-mix.

Overfiring can do it to just about any body.

Send Jon those batch #s and some of the clay if you still have some so he can run a test on it.

I do know folks who fire cone 5 clays to 6 and have no issues and some who do.So some cone 5 clays do not like to be fired higher- If Jim has been doing it you should also be able to.

Mark

[Natania]

I use primarily B-mix 5 but I don't fire it as hot. I do fire to witness cone 6 but I know that my kiln fires a little cool. I've fired it to cone 5 before too and it was vitrified. Why not go to 5 and see what happens?

[Guest JBaymore]

A PRIME cause of bloating is that in the bisque firing there is not enough OXYGEN present (and able to circulate thru the wares) in the chamber to allow all of the trace materials that require oxygen to be present for the reactions to go to completion.

 

This can happen due to poor (or no) local pickup ventialtion, or loading a kiln tighter than usual for the same oxygen flow from the vent. Particular issues can happen with the interior pieces when one object is inside another object which is inside another object... and so on (nested bowl forms, for example).

 

A clay body suddenly containing slightly more carbonaceous materials than "normal" can cause this to happen in the same kiln, with the same vent system, on the same firing schedule.

 

Then in the glaze firing these compounds can cause out-gassing that gets tapped in the vitrifing clay.... and bingo. Bloating.

 

Another issue is the potential for slight levels of reduction happening in the clay body as carbonaceous materials try to burn.... but can't. The iron compounds get reduced to the Fe) state... which is a powerful low temperature acting flux on silica... and this can cause a brittle iron fluxed glass to form near the nodules of iron. These later can become overfired and start to vaporize also as they are tiny "overrfired" pockets of melt.

 

When we all bisque fired in fuel fired kilns (I still do) ... that could easily have large volumes of oxygen flowing.... this was less of an issue than maybe it is today with most folks bisquing in electric kilns.

 

 

best,

 

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

[Val]

Thanks everyone, I will try firing to only cone 5 next time and see how it goes.

[Ginny C]

A PRIME cause of bloating is that in the bisque firing there is not enough OXYGEN present (and able to circulate thru the wares) in the chamber to allow all of the trace materials that require oxygen to be present for the reactions to go to completion.

 

This can happen due to poor (or no) local pickup ventialtion, or loading a kiln tighter than usual for the same oxygen flow from the vent. Particular issues can happen with the interior pieces when one object is inside another object which is inside another object... and so on (nested bowl forms, for example).

 

A clay body suddenly containing slightly more carbonaceous materials than "normal" can cause this to happen in the same kiln, with the same vent system, on the same firing schedule.

 

Then in the glaze firing these compounds can cause out-gassing that gets tapped in the vitrifing clay.... and bingo. Bloating.

 

Another issue is the potential for slight levels of reduction happening in the clay body as carbonaceous materials try to burn.... but can't. The iron compounds get reduced to the Fe) state... which is a powerful low temperature acting flux on silica... and this can cause a brittle iron fluxed glass to form near the nodules of iron. These later can become overfired and start to vaporize also as they are tiny "overrfired" pockets of melt.

 

When we all bisque fired in fuel fired kilns (I still do) ... that could easily have large volumes of oxygen flowing.... this was less of an issue than maybe it is today with most folks bisquing in electric kilns.

 

 

best,

 

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

 

 

So, is it helpful to leave the spy holes open, or one or two of them, during the entire bisque firing?

 

Ginny C

[Wyndham]

There is also the possibility that the clay was not fully mixed before being puged.

It might be interesting to take a pound or two slake it down to slip, mix and screen it then dry and wedge it. Turn bisque and glaze fire with another from the same bag and compare results.

I have seen several clays that have not been properly mixed show bloating as well as over fired clay that bloats

There have been times when a clay maker subs different ingredients, clays, feldspar's, etc in a batch that also might contribute to bloating.

Several possibilities,

Wyndham

[timbo_heff]

I agree with Baymore: try addressing the bisque firing:

Do a "slow" bisque and a a one or two hour hold just below the point of sintering: (say around 1550 F)

this gives all the junk a chance to burn out before the clay skins over and traps the gasses that cause bloats.

 

This schedule prevents these problems even in the most troubled clay bodies.

[neilestrick]

 

 

So, is it helpful to leave the spy holes open, or one or two of them, during the entire bisque firing?

 

Ginny C

 

 

If the kiln does not have a downdraft vent attached, leave the top spy open for the entire firing. If the kiln does have a vent, leave the spy holes plugged, since leaving them open will spoil the draft of the vent.