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moh

Case Of An Enlarged Bubble?

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moh    4

Today I unloaded the kiln to find that all pieces had a case of major bubbles. Smaller bubbles in tumblers, big quarter sized ones in larger vessels.

 

First thing on my troubleshooting mind was -- air pockets?

 

No, I can rule that out because my wedging/throwing hasn't changed in years and this is the first time to encounter this issue. And this issue materialized in every piece.

 

Glaze/clay fit issue? I have fired with this combo before without problem. But the glaze I used was commerical, so could be a faulty batch maybe.

 

Anywho, have you had this problem before?

post-79579-0-44307000-1489794566_thumb.jpg

post-79579-0-99999700-1489794628_thumb.jpg

post-79579-0-44307000-1489794566_thumb.jpg

post-79579-0-99999700-1489794628_thumb.jpg

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oldlady    1,323

happened to me.  new clay.  manufacturer checked the date and gave me new boxes.

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Min    777

Did you overfire the kiln? Can also get that bloating if clay is fired past maturity, it starts to break down and off-gas causing the bloats.

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moh    4

@glazenerd, @oldlady -- How do you remedy this? The batch is from January 2017 so relatively new..

 

@min - Checked the temp on the firing, it did not go over the ^5 intended.

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curt    117

If the bloating is caused by inadequate organic burnout (which is the usual cause), alter your BISQUE firing program to include a one hour soak at 800 degrees Celsius on the way up. Also make sure your kiln has adequate airflow (all bungs out, lid just cracked,or whatever gets a bit of air in) during the firing as this is necessary for the organic burnout process.

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glazenerd    816

As a point of curiosity: wire off a slab and inspect it for color veining: it should be uniform in color. Check to see if there are white or dark brown specks.   Then follow Curt's recommendation. You did not specify; but I would assume this is stoneware.

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Make a small cone shaped test piece from your 'new' clay body and mount it like a cone in a cone pack.  include it in your next firing.  If you are over firing the test piece should show signs of deforming.   this is a good check out technique for suspect clays.
 
by 'cone shaped test piece' I mean a pinched piece of clay shaped like an large Orton cone or a large Orton stand-alone cone.
 
LT

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moh    4

@glazenerd - this is porcelain

 

I did single out one factor that was different in the this firing.

I have a 10 cubic feet kiln and loaded only 30% of capacity for this firing. Does that sound like something that could cause issue like this?

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glazenerd    816

It is rare to see bloating in porcelain because of the purity level of kaolin. However, some porcelain bodies do incorporate ball clay/s as a plasticizer. However, in this case I have reason to think that potassium was used as a flux in lieu of sodium. I have seen this kind of bloating when experimenting with porcelain bodies that incorporated mica and pearl ash (pure potassium/s) in too high of a level. (over-fluxed)

 

In your first picture: the bottom of the piece is pure white on a pure white body. As you move up the side wall, you can see a shift to a tan'ish color underneath the white glaze: a classic sign of potassium fluxes. Potassium is the off-gassing hog of the KNaO group; and it will discolor a porcelain body; if it is over fluxed.

 

Potassium flux test

 

The bottom of this test bar is pure white (vitrified), and it turns the tan color as you move up the bar: as seen in your sidewalls. As potassium off gases, most of the gas remains inside of the body: only about 3-5% is off gassed into the kiln atmosphere. When potters turn their pieces upside down and fire on the rim: they assume the gas is trapped inside- not true. Sublimation requires a complex answer, so I will leave it at that.  So I would put potassium (too much) as the primary cause at this point.

 

Firing in a semi filled kiln only effects rate of climb and rate of cool.

 

Nerd

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JBaymore    1,432

 An enlarged bubble can sometimes be caused by irrational exuberance. 

 

 

 

 

Oops......... I thought this posting thread title was about the stock market.  ;)

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JBaymore    1,432

I've seen poor mixing of porcelain bodies cause this also. A "seam" of something like feldspar not well mixed in. So a little pocket of overfired glass.... outgassing and .... bingo.

 

This is where I'd put my money.

 

best,

 

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

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Mark C.    1,797

Small chance this may be a extinct dinosaur egg growing in the clay pocket-you could keep it warm in the oven a few months until it hatches?/Just thought. Most clays come that time frame.

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Babs    385

You mentioned checked temp.

Was there an actual cone in kiln.

Bloatng occurring in pots throughout kiln?

Ware spaced evenly

What was your firing schedule?

Old friend used to say. Not the machine but the operator

Just saying

Hmm can you repeat the bloating effect? With different clay?

Bisque schedule

Curt's got the word.

John if a seam of stuff and Moth pugs and wedges would that not negate the effect?

Nerd colour on photo reflection of hand colour?

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neilestrick    1,380

In my experience, when porcelain over fires it deforms and slumps, but does not bloat. I agree with John that it's probably a poorly mixed batch.

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oldlady    1,323

moh, contact your manufacturer by phone, ask to speak to the head guy, not some receptionist or salesperson.  do not stop until you have his/her ear and tell the whole story.  ask what should be done because you bought 45 tons of that bad clay and you need help before you continue.  well, probably better to say how much you actually bought.  you should be offered a replacement.  be sure you mention the date that should be on the box.  suggest they may have a large number of returns with the same problem.

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glazenerd    816

John:  my clay futures climbed to over 2230F, but slowly, followed by a sudden fall.

 

 

I've seen poor mixing of porcelain bodies cause this also. A "seam" of something like feldspar not well mixed in

 

which is why I posted this earlier:

 

 

As a point of curiosity: wire off a slab and inspect it for color veining: it should be uniform in color. Check to see if there are white or dark brown specks.

 

Babs: in the first pic, Moh's hand is on the outside of the cup..so no.

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I was playing around last year, trying to make pickling crocks that were unglazed, but without using something costly like Polar Ice. Using my chosen porcelain (which Tom will likely not think is real porcelain), I filled the bisqued pots with a salt soloution, and left it until the pots flouresced. I dumped the soloution out, and fired to cone 9 in reduction. I wound up with blisters all over the pot, in and out, that looked very much like your pictures. So my vote is definitely for clay body being over-fluxed through some mechanism that happened in manufacture.

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glazenerd    816

 which Tom will likely not think is real porcelain),

 

Callie:

You are talking to the weird science/experiment guy, remember?

Besides, porcelain is what the clay makers say it is.........................EYEROLL!! :rolleyes: 

 

Tom

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It was Plainsman p600.

If it helps, not too many other people think it's really porcelain, either. I wasn't caring how translucent it was for a pickling crock, I just wanted zero absorption. Which I didn't get.

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