Help with high fired bowl bottom problem

[Chris Campbell]

All of my Southern Ice porcelain Nerikome pieces are made, dried and fired to Cone 10 Oxidation in another bowl/mold in order to prevent slumping etc. I also program a slowed cooling ... every load is fired on the same Ramp/Hold program.

Usually all is fine but every once in a while a wide bowl ( 10 - 14" is wide for me ) will puff up in the middle ... not lie flat as it should. It does not really affect the looks much but it bugs me that I don't know why it happens.

This is not consistent with any particular mold form ... I think it happens in the clay molds rather than the castable refractory ones.

I am posting images but my bowls are so ... hmmm ...'patterned' that it is really hard to get a photo to show the problem.

Does anyone know if this is a process error or a firing one? Ideas welcomed ... Please and Thank You:(

[TJR]

Chris;

Wow! Beautiful work! I want to say that it's a drying problem, but obviously, you wouldn't fire them if they were bowed in the middle. I sometimes have problems with my porcelain warping or plucking at cone 10 as well. Can you change your clay body to a more refractory porcelain? Just an idea.

Tom[TJR[

[Chris Campbell]

Changing clays is not an option! Took me too many years to learn this one.

[Guest JBaymore]

Chris,

 

 

 

I've been thinking through the mechanics/chemistry/?????? of what could cause the upward bowing action in the center bottom for you. Not a lot of obvious things. I'm leaning toward firing issues.

 

One thing that MIGHT do it...........

 

If the pieces are thin (and I think your work is), and the clay is pretty vitrified (some colorants may also make it more vitrified than the "white" body), then the potential influence of the thermal mass of the slump mold setters you use to support the forms plus the setter sitting on the underlying kiln shelf MIGHT be causing a thermal lag in the maturing of the outside of the porcelain bowl form, when compared to the interior surface of the bowl that is getting some direct radiational heat transfer and some convective transfer also.

 

So when the kiln hits the end point cone, the interior open bowls surface is slightly more matured than the clay that is on the outside bottom and is not only shaded from direct radiative thermal transfer, but also is in tight contact with a lot of mass that has to be heated up (slump mold plus kiln shelf).

 

So the inside is maybe a little bit more mature than the outside, and the shrinkage of the interior surface is therfore a slight bit higher than the outside's is. Inside smaller, outside larger, porcelain clay at a pyroplastic state..... = upward bow.

 

One "theory testing procedure" ......... make a really thick bottomed shallow bowl slump mold. See if a bowl in that mold tends to do this more than a really thin slump mold does.

 

Or try taking the existing mold and propping it up on three small stilts to get it up off the underlying shelf (to decrease the thermal lag of the center bottom area).

 

Or slow down the whole firing cycle with a good soak athe end to allow the lage to not happen.

 

I'm grabbing at straws here. Hope something there is useful.

 

best,

 

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

[Chris Campbell]

That really makes sense John based on whIch forms it happens to. The ones that are raised don't have the problem and the solution of raising them is really simple ...THANKS ... Going to try it in the next firing ... Long ways off ... but I will report back.

[Guest JBaymore]

That really makes sense John based on whIch forms it happens to. The ones that are raised don't have the problem and the solution of raising them is really simple ...THANKS ... Going to try it in the next firing ... Long ways off ... but I will report back.

 

 

I'll be very interested to hear the results, Chris. And what you just said "The ones that are raised don't have the problem....... " makes it sound more like the possibility.

 

We'll see.

 

best,

 

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

 

 

PS: And as a side note, your new work is STUNNING!

[Matt Oz]

My theory is, even though the bowl is fired in a form, there still must be some downward force on the rim when it starts to slump at high temperature., that movement is transferred by the rigid edge of the bottom to the center, bulging it upward (just a guess).

 

I think a form with steeper sides would help, or a bowl with a larger bottom diameter and smaller rim. Maybe some test bowls of uncolored porcelain will help figure it out.

 

I think your work looks great too, Skinner blends are involved right?

 

 

[yedrow]

You do amazing work Chris!

 

Not that it will contribute anything to the subject, but my first thought was some type of plastic deformation with a non or not-so plastic portion "moving" a plastic portion of the piece. In reading John's response I got a much better idea how something like that would happen, w00t.

 

Joel.

[Guest JBaymore]

My theory is, even though the bowl is fired in a form, there still must be some downward force on the rim when it starts to slump at high temperature., that movement is transferred by the rigid edge of the bottom to the center, bulging it upward (just a guess).

 

I was assuming here that the bowl is fully supported in the refractory mold in the kiln from Chris' description of how she fires her work in other postings/discussions. The bowl will shrink as the firing progresses and the clay will hopefully "slip" freely down the walls of the slump mold/saggar settling slightly more toward the bottom.

 

But if it is not fully sopported at the rim so that it can splay outward a bit, then what you say there also is a real possibliity. Good thought, Matt.

 

best,

 

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

[Chris Campbell]

What do you mean by "supported at the rim"? How would I support it at the rim?

[Matt Oz]

This is what I was thinking

 

If you take a strip of paper, crease the edges then push down on the edges, the center will bulge. Clay that becomes pyroplastic seems to behave in a similar way.

[Guest JBaymore]

Excellent illustration of the possibility, Matt. I'll buy that one.

 

But if that was the case, shouldn't ALL of the similar pieces treated that way in the saggare/suport molds exhibit the same behaviors?

 

The missing matching "symptom" so far is that she says the ones that are raised off the kiln shelves don't seem to exhibit it. Maybe that idea is a mis-diagnosis / observation on Chris' part? (Chris???).

 

Chris... do the bowl bottoms sit flat on the bottoms of the mold when they are loaded.... or will the bowls "settle" down into the mold and eventually reach the bottom surface as they are fired?

 

 

best,

 

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

[Chris Campbell]

Great illustration of what might be happening ... I went and checked on all the recently fired bowls and found the problem occurs in the wide clay molds not in the castable refractory ones and not in the clay molds with a small base. I was so hoping the clay molds would work!

UGH ... so this means looking forward to another week spent making more castable molds which is an evil, terrible, horrible, messy job. The person who told me it was not a difficult job lied ... or worked construction for a few years.

 

Here are two images ... the one on the right shows the form that fires in the clay molds with no problems. The left is the shape of the buckling prone ones.

[Lucille Oka]

Great illustration of what might be happening ... I went and checked on all the recently fired bowls and found the problem occurs in the wide clay molds not in the castable refractory ones and not in the clay molds with a small base. I was so hoping the clay molds would work!

UGH ... so this means looking forward to another week spent making more castable molds which is an evil, terrible, horrible, messy job. The person who told me it was not a difficult job lied ... or worked construction for a few years.

 

Here are two images ... the one on the right shows the form that fires in the clay molds with no problems. The left is the shape of the buckling prone ones.

 

 

 

Chris, your work is very nice, colorful and interesting. There appears to be a 'story' in the vessels but unfortunately the photography prevents the full view. Your work is definitely 'got to see in person work'.

[Guest JBaymore]

Aha Chris.... from those shots it look lke there is not as distinct a "shoulder" flange wall versus "floor" in the bowls like in the "paper crease" idea. They look wavy, variable width and angle "flange" and generally smoothly sloping all the way, more or less.

 

Can you also post pictures of the slump molds you use of the there general types? refractory castable, wide clay ones, and narrow clay ones.

 

What castable are you using for the castable ones? What clay for the clay ones? Are all the clay ones the same body?

 

And back to that question of ......... does the floor of the bowls touch down on the "floor" of the miolds when they are being stacked into the finish fire?

 

Do they have a "foot ring" of some sort?

 

best,

 

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

 

 

 

PS: and you are right.... the color zings with that Southern Ice.

[Chris Campbell]

Thanks Lucille for the lovely compliment.

 

Short version of the story is: a commentary on the meeting points of humans and nature and the battle for available space. This is the urban version ... I have a suburban one and am working on an ocean one and maybe even a sky one.

[Chris Campbell]

The castable refractory is Kast-O-Lite 26 LI Plus

 

The clay was either Zella or Phoenix from Highwater Clays ... both Cone 10 Stoneware. They haven't warped at all and have survived repeated uses ... so far.

 

The bottoms and sides meet the bottoms and sides of the molds. Shrinkage in excess of 15% though. No foot rings.

 

Here are images of the molds. The one on the left is the problem shape with the castable being thicker and about 12" across, the clay is about 15" across.

The second image is the wavy ones which fires flat every time even though I thought that shape would be the problem.

The third is a standard shape that gives no problems in castable or clay.

[Guest JBaymore]

The castable refractory is Kast-O-Lite 26 LI Plus

 

Hummmmmmmmmmmmmmmmmmm.........................

 

The Kast-o-lite series of castables is an INSULATING castable. It is a very odd choice for making any kiln furniture out of. Normally you want furniture that has a high thermal conductivity so that the furniture heats through as easily as the ware. One of the main attributes of silicon carbide is that it has a high thermal conductivity. Better than many other kliln furniture choices. So using insulating castable in this application is likely not helping things.

 

Personally I'd make them out of a dense non-insulating castable. Something like the Super Kast Set line or the Mizzou line (if you can find it) or Avon-Crete.

 

I'm back to the guess that in the insulating castable molds, the exterior bottom area of the clay wall is not firing to as high a cone as the interior well exposed surface.....given the general firing schedule. The mold is likely even "protecting" the bottom of the bowl from some of the thermal energy migrating through the underlying kiln shelf via conduction.

 

And if the clay molds are doing the same thing... I'll bet that the bottom area of those particular molds are significantly thicker than moldes that do not exhibit the issue.

 

We're still working on narrowing down on this one Chris. Remember... it could still be MULTIPLE factors that conspiore topgether to cause it.... which makes figuring it out extra porblematic.

 

best,

 

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

[Lucille Oka]

Thanks Lucille for the lovely compliment.

 

Short version of the story is: a commentary on the meeting points of humans and nature and the battle for available space. This is the urban version ... I have a suburban one and am working on an ocean one and maybe even a sky one.

 

 

 

Ceramic Narratives, wonderful.

[Chris Campbell]

Is the Super Kast Set material any easier to work with? ... she asks hopefully.

 

The clay molds are more problematic. They are uniform in width since they were slab built - not thrown.

 

Now my husband is challenging me to create an image that depends upon the center swelling slightly. The next step in the series is the Ocean so who knows what could pop up?

[Lucille Oka]

Is the Super Kast Set material any easier to work with? ... she asks hopefully.

 

The clay molds are more problematic. They are uniform in width since they were slab built - not thrown.

 

Now my husband is challenging me to create an image that depends upon the center swelling slightly. The next step in the series is the Ocean so who knows what could pop up?

 

 

 

That is a marvelous idea. If you can control the swelling or at least know when it will happen, utilize it.

[Matt Oz]

As far as the mechanical aspect, this is what I'm pondering now.

 

Where and how many support points after the clay shrinks. If there aren't any, when the clay starts to slump where are the first contact points, and how do they support the object. Just thinking out loud.

 

I mentioned the paper thing because for the past year or so I've been experimenting with clay formulas that slump into or over molds, similar to glass slumping. Doing this I noticed the clay behaved somewhat like office paper does if you tried pushing it into a mold, or wet paper depending on how glassy the clay gets.

 

 

 

 

The Ocean idea sounds interesting, don't forget the sailboat

[Chris Campbell]

Matt -

Your images really make sense to me and I am now trying to think of how to support those sides when they shrink away. I am not totally in love with the form so it might be just a matter of changing angle of the slope and making it out of the other castable product John recommended ??? It is such a terrible job ( for me anyway ) that I would like to be somewhat sure of the theory before I make a bunch of molds.

[Matt Oz]

I actually do have a few solutions for you,

 

For the mold use a clay that shrinks close to 15 % (12 or 13% may even be enough) and doesn’t slump to much, make a mold out of it but don't fire it, apply the alumina separator, place your bowl inside and fire them to maturity together. Of course this only works once.

 

Or, fire the southern ice close to maturity without slumping, then fire it again inside a close fitting pre-fired mold.

 

Last, place you bowl in a mold that is too small for it and let the bowl shrink into it during firing.

 

 

 

 

If those don't work, then its got to be thermal like John said.

[Chris Campbell]

I hear you ... plus, that explains why porcelain people do not re-use their clay molds. Some of these were on multiple use.

 

So, question two ... why are the castable molds problem free? They are not moving or shrinking at all ... maybe because they are thicker and spreading the heat better??