Bloating - Is it the Clay, the Glaze, or Operator Error?

[TypicalGirl]

Well, I opened the kiln today - the second firing of my new kiln - and there was much sadness.

The issue is bloating. It seems to be confined to one clay body, and certain atmospheric conditions within the kiln, but I'd like some thoughts/input before I ditch the rest of this clay and jump in to something like porcelain.

 

I should say I'm n a very tight budget.

I use this clay body because it is locally produced. anything besides it is a 2.5 hour trip each way, and I usually can't afford to buy more than 100 pounds or so at a time. But, this doesn't seem to be working for me either so...

 

Anyway, I could use some insight here.

Sorry, its a FaceBook link, but its supposed to be viewable by the public.

 

 

https://www.facebook.com/media/set/?set=a.2612443156221.144548.1409288159&type=1&l=2a77433192

 

Thanks Gurus!

[Benhim]

Bloating is caused by over firing. You can probably figure out the optimal firing schedule for the cheap clay. If you're firing this to cone 11 as your firing schedule shows, I'd say that's the culprit. If it were me I'd probably try to do a test firing at cone 9 or so and see how the clay comes out. The clay looks bloated and over fired in your Facebook pictures.

[Guest JBaymore]

Bloating is typically casued by the formation of gases within the clay body as reactions are happening there that create those gases. These reactions are USUALLY the result of reactions that have not been acomplished during either the early stages of single firing, or in the bisque firing of twice fired wares.

 

These gases accumulate in micro-fissures inside the clay body walls. As the clay gets hotter these trapped pockets of gases try to expand (remember gas pressure laws?). As the clay body becomes pyroplastic as the temperature increases and the body fluxes start some real activity, the tiny trapped pockets tend to merge together into larger pockets. The combination of pyroplastic clay and increasing temperature (and hence increasing gas pressure) cause the eventual deformation of the body and the little "bubbles" that form.

 

In some cases these pockets are caused by inadequate mixing of components of the clay body itself. If there are concentrations (nodules) of a material that supplies a flux, then the clay body in that little micro-clime around that pocket of "fluxy stuff" will get overfired at the maturing temperature of the body. So the body starts glassifying in that small little area and then evolving gases as it "fines out". It gets more or less that same reaction out of the body as the gases exert pressure on the surrounding pyroplastic walls.

 

One item that can act as a flux on the body is FeO. If there are nodules containig concentrations of Fe₂O₃, AND these materials get inadvertantly REDUCED (due to inadequate oxygen EARLY in a bisque firing), then you can get the overfluxed body scenario I described above. FeO is a powerful flux on silica at pretty low temperatuires. So the body starts fusing from this brittle and unstable iron-fluxed glass very early. By maturing time... it is WAY overfluxed. And once it starts forming a melt, the presence of oxygen in the kiln chanber later on in the firing does little because it is gas imper meable to the large O₂ molecules.

 

The most usual culprit in bloating is the combustion of organics (carbonaceous matter) that has not gone to completion, giving off CO₂. Also a possibility is the generation of CO₂ from the decomposition of things like carbonates that might not have had adequate firing time due to lack of heat penetration (thermal lag) into a tightly stacked bisque load.

 

In a bisque firing, excess oxygen beyond that needed for combustion (if it is a fuel fired kiln) needs to be present. If it is a bisque in an electeric kiln, enough oxygen circulation also needs to be provided, which is one of the main reasons for a good downdraft local pickup vent system.

 

There is also a similar looking reaction caused by overfiring. It is a variation on the "inadequate mixing" description above. As mentioned above it is easy to test this one.... drop the end point firing cone. And if the effects are only happening in certain LOCATIONS in the kiln.... put some cone packes EVERYWHERE in that kiln to see the evenness at the end point (this says NOTHING about the evenness at any OTHER given point in a firing).

 

I'd first be looking at how the bisque firing was handled. First recommendation.... longer firing cycle, and checking on getting more air in there. Just becasue a particularl cycle works OK for one clay body does not mean it is adequate for another clay body.

 

If you can eliminate the firing situation causes, then the problem points to the mixing of the clay body itself. And remember that cheap clay is only "cheap clay" if you are getting consistent saleable results from you EXPENSIVE labor.

 

 

EDIT: Just looked at the Facebook pictures. I'd bet about 90% that it is overfiring of THAT particular body. The smaller size of the "bloats" is a bit of a giveaway. I'm guessing that clay body is really a cone 6-8 body to see that amount of crap at cone 11. What does the manufacturer spec that clay at? If it is not that.... it seems to be a body mixing problem with some particular raw materials remaining in small "lumps" and causing the "overfired patches" I mentioned above.

 

Also, that is a VERY empty firing of that kiln. You should be able to get WAY more ware in there...and that will even out the firing a lot also.

 

And on to another subject.......

 

In the Facebook postings I noticed you mention that your pyrometer was saying about 2200F even though cone 11 was going down. I am betting that you asre using a Type K thermocouple. And that thermocouple is obviously not protected from the kiln gases in a protection tube. Type K thermocouples are NOT really rated for use at Orton cone 9....let alone cone 11. If you look at the industrial usage guide and ratings, their electrical outputs go very non-linear above about 2300 F..... so the accuracy really starts to go bye-bye. Additionally, as the metal corrodes from the kiln atmosphere the internal resistance of the thermocouple changes, and that can cause the meter to read incorrectly also. If you are usinig a cheap analog (dial face) meter .... that makes the situation even worse.

 

For industry, the thermocouple of choice in the cone 9-11 range would involve platinum and a platnium alloy. They are quite expensive items, and that is why most potters use Type K thermocouples des[pite the innacuracy and short lifespan.

 

 

best,

 

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

[Benhim]

Bloating is typically casued by the formation of gases within the clay body as reactions are happening there that create those gases. These reactions are USUALLY the result of reactions that have not been acomplished during either the early stages of single firing, or in the bisque firing of twice fired wares.

 

These gases accumulate in micro-fissures inside the clay body walls. As the clay gets hotter these trapped pockets of gases try to expand (remember gas pressure laws?). As the clay body becomes pyroplastic as the temperature increases and the body fluxes start some real activity, the tiny trapped pockets tend to merge together into larger pockets. The combination of pyroplastic clay and increasing temperature (and hence increasing gas pressure) cause the eventual deformation of the body and the little "bubbles" that form.

 

In some cases these pockets are caused by inadequate mixing of components of the clay body itself. If there are concentrations (nodules) of a material that supplies a flux, then the clay body in that little micro-clime around that pocket of "fluxy stuff" will get overfired at the maturing temperature of the body. So the body starts glassifying in that small little area and then evolving gases as it "fines out". It gets more or less that same reaction out of the body as the gases exert pressure on the surrounding pyroplastic walls.

 

One item that can act as a flux on the body is FeO. If there are nodules containig concentrations of Fe₂O₃, AND these materials get inadvertantly REDUCED (due to inadequate oxygen EARLY in a bisque firing), then you can get the overfluxed body scenario I described above. FeO is a powerful flux on silica at pretty low temperatuires. So the body starts fusing from this brittle and unstable iron-fluxed glass very early. By maturing time... it is WAY overfluxed. And once it starts forming a melt, the presence of oxygen in the kiln chanber later on in the firing does little because it is gas imper meable to the large O₂ molecules.

 

The most usual culprit in bloating is the combustion of organics (carbonaceous matter) that has not gone to completion, giving off CO₂. Also a possibility is the generation of CO₂ from the decomposition of things like carbonates that might not have had adequate firing time due to lack of heat penetration (thermal lag) into a tightly stacked bisque load.

 

In a bisque firing, excess oxygen beyond that needed for combustion (if it is a fuel fired kiln) needs to be present. If it is a bisque in an electeric kiln, enough oxygen circulation also needs to be provided, which is one of the main reasons for a good downdraft local pickup vent system.

 

There is also a similar looking reaction caused by overfiring. It is a variation on the "inadequate mixing" description above. As mentioned above it is easy to test this one.... drop the end point firing cone. And if the effects are only happening in certain LOCATIONS in the kiln.... put some cone packes EVERYWHERE in that kiln to see the evenness at the end point (this says NOTHING about the evenness at any OTHER given point in a firing).

 

I'd first be looking at how the bisque firing was handled. First recommendation.... longer firing cycle, and checking on getting more air in there. Just becasue a particularl cycle works OK for one clay body does not mean it is adequate for another clay body.

 

If you can eliminate the firing situation causes, then the problem points to the mixing of the clay body itself. And remember that cheap clay is only "cheap clay" if you are getting consistent saleable results from you EXPENSIVE labor.

 

 

EDIT: Just looked at the Facebook pictures. I'd bet about 90% that it is overfiring of THAT particular body. The smaller size of the "bloats" is a bit of a giveaway. I'm guessing that clay body is really a cone 6-8 body to see that amount of crap at cone 11. What does the manufacturer spec that clay at? If it is not that.... it seems to be a body mixing problem with some particular raw materials remaining in small "lumps" and causing the "overfired patches" I mentioned above.

 

Also, that is a VERY empty firing of that kiln. You should be able to get WAY more ware in there...and that will even out the firing a lot also.

 

And on to another subject.......

 

In the Facebook postings I noticed you mention that your pyrometer was saying about 2200F even though cone 11 was going down. I am betting that you asre using a Type K thermocouple. And that thermocouple is obviously not protected from the kiln gases in a protection tube. Type K thermocouples are NOT really rated for use at Orton cone 9....let alone cone 11. If you look at the industrial usage guide and ratings, their electrical outputs go very non-linear above about 2300 F..... so the accuracy really starts to go bye-bye. Additionally, as the metal corrodes from the kiln atmosphere the internal resistance of the thermocouple changes, and that can cause the meter to read incorrectly also. If you are usinig a cheap analog (dial face) meter .... that makes the situation even worse.

 

For industry, the thermocouple of choice in the cone 9-11 range would involve platinum and a platnium alloy. They are quite expensive items, and that is why most potters use Type K thermocouples des[pite the innacuracy and short lifespan.

 

 

best,

 

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

 

 

So are you thinking the atmosphere is not clean enough in the bisque trapping pockets of gas in the ware? I'd considered this also, but had figured that the gas would escape or be bonded during the body reduction phase of the firing. You can see the clay even split in the pictures making visible cracks in some of the bloated spots. I was wondering if possibly this clay had some salts in it that don't become volatile until later in the firing. If this is the case the body being over fired would cause a gas at the top temperature that could not escape through the already vitrified walls of the pot. Carbondale Red is one of the clays that comes to mind, the salts it contains cause it to change color after cone 3, but at this temperature the clay is not vitrified so the gasses should be able to escape or be bonded to the clay body.

[buckeye]

Sorry to see you having trouble, hopefully it will be corrected soon because its never fun to open a kiln and see problems like that