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Ginny C

Blisters Worse After Re-Firing

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Lovely vase came out with popped blisters all over it. (Electric kiln, B-Mix cone 5-6 clay,  bisqued to 04.  Then glaze fired to cone 6.)

 

I ground them down some with a dremel, covered each with 2 dabs of clear glaze and re-fired. Now the blisters look like the wart I had as a child, made of lots of little columns crammed up together.  Really ugly, although one visitor said it was interesting, and I should just title it "Vase—Warts and All"  

 

 Maybe it needs to go to cone 7?   Should I give it one more try, re-firing it again??   If so, should the whole pot get a new coat of glaze, rather than just over the warts?

 

Other pieces from the same clay but different glazes came out fine.  The glaze was somewhat thick on this vase. Might that have caused the blisters?

 

 

 

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I can't think of a time I ever had good results when refiring blisters. 

 

A couple kiln loads ago a bowl was sitting on a star stilt and the star stilt melted and bent and the bowl tilted into the bottom of the kiln shelf. I ground it off, applied a tad more glaze, and you'd never even notice it happened. Same with a few times I've gotten a chunk of minerals in the bottom of a bowl. Ground them off, applied more glaze, fine. 

 

Otherwise I find that it's just not worth it. If the piece is really nice otherwise I use it for a pencil cup or a planter but I'm kind of topped off on those right now. 

Marcia Selsor likes this

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The clay is doing so serious off gassing: it has nothing to do with the glaze:

 

1.stoneware bodies are most usually potash/based because many stoneware clays have naturally occurring levels of potassium to begin with, Secondly because potassium is the flux of choice in stoneware formulation. Potassium is a gas hound at cone 6, yes I have tested that assertion.

 

2. Blending at the factory was faulty, leaving large areas of poorly blended materials which contain  a large amount of potash in one section.

 

Clay pinhole

 

Enough gas is escaping in the above picture to literally rupture the glaze surface.

 

Fix: the only possible remedy is to re-fire to 2175-80F with a minimum of an hour hold to allow for off gassing. Your glaze may not tolerate that kind of hold time, and if the potash content is extremely high; it may not be enough to cure off gassing. However, it is the only probable way to attempt a fix.

 

Nerd

 

Gold Art clay is commonly used in stoneware, and it has nearly 2% naturally occuring potash. If they mined through an area with much higher levels: it could easily explain your problem. I would advise you do a simple form with the same glaze: and see if this problem occurs again. if it does: take the clay back.

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I'm going to venture that if you try and refire with an hour long hold, that glaze is going to run right off that pot. It's already showing drips trails forming, and they'll just keep falling with the addition of more heat work. You will spend less time and energy making another one. It may be worthwhile to make a test to confirm that the problem lies with the clay, so you can notify the supplier and the manufacturer. If it's a batch problem, they'll want to know.

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Thanks, all! I do think the hammer is called for now, especially since I cut my finger on a big one on the inside as I was moving it for the photo.

 

But it is the same Laguna B-Mix clay I've used for years, and other pieces from that batch of clay have  been fine.

 

I made several items from a different clay (a brown Laguna with speckles) which were bisqued in the same load as the green vase, in a shared studio in MI.  These pieces I brought home to glaze and fire, and one of them blistered and two mugs I've fired so far did not blister at all.  I have two more pieces, one quite large, from the same brown clay bisqued in MI and now waiting to be glaze fired in a friend's larger kiln, and I'm wondering whether I should re-bisque them first!  ?  If the problem is that that load was not bisqued long enough, would it be wise to re-bisque them?

 

(These pieces are unique, with inlaid agate clay that I'd hate to mess up! It's a clear glaze. This doesn't show the blisters, but there are a few there.

 

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Two potential factors -- the glaze (not the clay) and your firing schedule. In reverse order. You can get those types of blisters when the kiln temperature drops fast -- like reaching peak and then turning off. The glaze is bubbling on the pot surface; the drop in temperature "freezes" the glaze bubbles as craters. If you are not already doing so, try adding a 5 or 10 minute hold at peak temperature to give the glaze more time to melt. You might also have a glaze that does not like to fire to your top temperature, or it may have been contaminated (not unusual for glaze buckets in a community studio setting).

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Ginny:

Interesting bits of information:

 

I ground them down some with a dremel, covered each with 2 dabs of clear glaze and re-fired. Now the blisters look like the wart

(These pieces are unique, with inlaid agate clay that I'd hate to mess up! It's a clear glaze. This doesn't show the blisters, but there are a few there.

 

Any idea of the composition of the "inlaid agate clay"?  I ask because of some recent updating I am doing on my glaze calculator material list. Two companies sell agate particle sizes of their commonly used clays. They both have between 5-6% iron: which your clear glazed sample indicates the presence of iron by the color of the agate. However, they both also have 2+ plus percent of potassium, along with some sodium and calcium. Was the agate red or buff in color? (Both clay agates are 20-50 mesh size: which also seems to fit what I am seeing.)

 

You ground down the original blisters, and they reformed in a much larger/broader formation-correct? Very unusual to see even larger/and heavily populated blisters in the same location: especially after you ground them off. Interesting set of circumstances,

 

Nerd

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I used the term agate clay because I thought that referred to colored clays inlaid into a slab...Maybe that's not the right term.

 I colored some of my white B-Mix clay with iron oxide powder my son sent to me from France. Using different amounts in slip that I dried on plaster and wedged up I ended up with different shades. Layered slices of the colored clays with plain B-Mix, and then twisted a slice and rolled it out flat and thin.  Rolled it into a slab of brown speckled Laguna clay.

 

But the blisters on this piece are on both that strip plus even more on the other part of the dish. 

 

I think my idea of re-bisquing the other 2 pieces made the same way but not yet glaze fired is what I'm going to do.

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Agree with Neil Estrick. Looks like over firing to me. If it is gas in the clay,  it will leave pinholes........which can get corrected by soaking your bisque. If it is over firing, re firing will make it worse, unless you fire it lower than the previous fire. Remember the glaze already melted the first time. The second time the glaze will melt faster because it is already mixed and mingled with the other ingredients. 

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Agree Babs:  but he said "unstable glaze- not enough alumina and/or silica." Alumina and silica are refractory materials. Depending on the ratio of flux/ refractory materials, the melting point will rise and fall.......  I took a short cut in my explanation. hope it is now clearer what I meant. 

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Agree Babs:  but he said "unstable glaze- not enough alumina and/or silica." Alumina and silica are refractory materials. Depending on the ratio of flux/ refractory materials, the melting point will rise and fall.......  I took a short cut in my explanation. hope it is now clearer what I meant. 

 

Yep. If it was fired cooler it might be fine, but at that temp it needs more silica or alumina to stabilize it. If we had the recipe we could compare it to limit formulas, but without a recipe you could run tests adding equal parts alumina and silica in 2% each increments. At some point, if that really is the problem, the craters will stop. Equal parts clay and silica will generally keep the si:al ratio the same as the original, or at least very close.

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I ground them down some with a dremel, covered each with 2 dabs of clear glaze and re-fired.

I understand why the silica/alumina levels are thought to be the problem given the condition of the pot. However, in case it was overlooked: it has already been fired twice. Ginny said she ground down the original blisters: and they came back even more severe in the same locations: which is the reason I think it is the clay. You would think in two firings, any high levels of flux would be burned out of the glaze. The colorant runs and bare spots are the result of two firings, IMO. Severe blisters formed even where the glaze had thinned out from two firings: another reason I think it is the clay. Firing schedule could play a role, as would glaze thickness: but I would not expect to see this after two firings.

 

Nerd

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I ground them down some with a dremel, covered each with 2 dabs of clear glaze and re-fired.

I understand why the silica/alumina levels are thought to be the problem given the condition of the pot. However, in case it was overlooked: it has already been fired twice. Ginny said she ground down the original blisters: and they came back even more severe in the same locations: which is the reason I think it is the clay. You would think in two firings, any high levels of flux would be burned out of the glaze. The colorant runs and bare spots are the result of two firings, IMO. Severe blisters formed even where the glaze had thinned out from two firings: another reason I think it is the clay. Firing schedule could play a role, as would glaze thickness: but I would not expect to see this after two firings.

 

Nerd

 

 

Here's my thinking on this:

I've seen this many times in over-fluxed glazes after refiring. They get worst the second time around, and usually smaller bubbles. In my experience, the type of bubbles seen here- craters of all sizes, often with sharp edges or bubbles that will kind of pop or break if you rub them- are the result of an unstable glaze. Too much flux, not enough silica or alumina. You can see in the photo how fluid the glaze is, and how the bubbles are appearing in the thick runny areas. The bubbles/craters are forming at the high end of the firing.

 

The other type of common pinhole is a small pinpoint with a rounded soft edge. It forms from the clay off-gassing or some chunk of impurity burning out of the clay, and has a smooth edge because the gassing happens earlier in the firing and the glaze starts to heal over at the high end of the firing but doesn't quite make it. It is relatively consistent in size, and will usually heal over in refiring (touching up with a dab of glaze helps), because the clay is done gassing by then.

 

I just recently had a batch of glaze that usually uses frit 3134, and I was all out so I reformulated it to use Gerstley isntead. I use Gillespie Borate, which tends to be a little stronger the the current Gerstley, and I forgot to lower it a bit to compensate. My students had a number of pots with glaze issues just like the OP's pot, on 3 different clay bodies. In my experience, 95% of the time the problem with the glaze is with the glaze itself, not the clay. And with all the frits used at cone 6 it's really easy to make an unstable glaze. I learned that the hard way when I switched from cone 10 to cone 6- it was shocking to see how much EPK and flint I could put into a recipe.

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On this topic: I have some odd clay ad glazes in the studio that we use to make empty bowls with. The clay is a mix of porcelain and some stoneware. A while back I used two glazes: a midnight blue and a rusty brown (both mixed from scratch some time back) on a batch of bowls. The midnight blue bowls turned out fine. The rusty brown ones bubbled, but the bubbles was in the clay itself ( that is how it appear at least and nothing burst). I came to the conclusion that the iron in the glaze interacted with the open structure of the porcelain clay, while the cobalt in the midnight blue stayed more refractory.

 

To make a exact conclusion, I will have to look up the recipes, but maybe you have some different idea as to why this happened. All the possibilities of shelves and pots packed too tight and uneven firing is canceled out. 

This was the first time that I saw a glaze interact in this way with porcelain. 

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