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Bubble Glaze Experiment With Close Ups.


Matt Oz

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A while ago I did some experiments with a cone 6 clear in a electric kiln that has lots of tiny bubbles in it when applied thicker, I made some small tiles (about 1 1/8" square) out of black clay so the bubbles would have good contrast and because they're fired horizontal I got a nice even spread of bubbles.

Some tests have additions of Mason stain.

15999776945_c28ff35279_b.jpg

The first tile has white clay inlaid in the black clay with small pieces of a glassy porcelain I have that were placed on top of the glaze prior to firing.

Tile three has 1% Mazerine stain added to the clear with a streak of white glaze on top and a ball of porcelain.

The last one is 4% Praseodymium with porcelain additions.

 

Close ups:

 

Tile one

15998774292_246794d560_o.jpg

 

Tile one closer

15998773912_321f388012_o.jpg

 

Tile three

15812070378_4fcb1a647c_o.jpg

 

The glaze is:

 

25 3134

15 Custer

20 Kaolin

20 Silica

20 Wollastonite

 

Not the most compatible of glazes but bubbly. I don't remember exactly how thick the clear was but at least 4 coats and if applied too thick the glaze becomes too cloudy.

I haven’t experimented with this enough to know if it's useful or practical to do it this way, but found it interesting.

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marvelous to look at!   can't wait for the chemists to comment.  how did you apply it?  your reference to "coats" makes me think it is brushed, correct?  how much did you enlarge the pictures?   are the bubbles pinpoint size?

Thanks old lady,

 

You are correct brushed on coats, I didn't take good notes so I'm only guessing, could have been more than 4.

 

The first photo of all the tiles is close to how it looks with the naked eye the bubbles look like tiny translucent specks floating in the glaze, close to pinpoint size and you can't tell there bubbles without a magnifier, It also has more depth to it than the photo can convey. My mediocre camera has a macro lens and says 8X zoom lens.

 

A lot of glazes have these micro-bubbles in them for instance Chun Glazes.

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Do we know what causes these bubbles?

 

It might prove helpful to be able to control this effect to one's liking.

 

I hypothesize that some parts of the glaze are becoming molten at a temperature below the temperature where the others are finished outgassing. Im thinking the frit.

 

Have you tried reformulating to eliminate the frit?

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Outgassing of the frit? I don't think there are any volatile components in the frit (compositionally, it looks something like a borosilicate glass), and as for the rest of the glaze recipe, there is just the possibility of the kaolinite dehydrating (could that be responsible for these bubbles?). What about the clay body? Presumably these tiles have been bisqued, so any volatiles there would have already been cooked out. Residual moisture from the water of the glaze suspension? Hard to imagine that sticking around long enough to still be there when the glaze starts to melt. But clearly there's got to be a source of gas somewhere, apparently from within the glaze, in order to form these bubbles.

 

In volcanic rocks, geologists call bubbles like these "vesicles" and the texture of the rock is called "vesicular." Pumice would be a good example here.

 

Matt, it would be interesting to know the firing and cooling schedules....

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Well, I can add that this very same effect happens with a clear glaze we use:

 

"SG4"

Frit #3195 - 22

 

Wollastonite - 26.6

 

Neph Sye - 4

 

EPK - 26.6

 

SiO2 - 16

 

GB - 4.8

 

I'm not sure about the pedigree of this glaze, so I can't say where it comes from or who developed it. When it's applied thinly (in-out dip without pausing) the layer is so thin the bubbles only appear where it drips/double glazes if you will. On porcelain, it's very clear, almost bluish, but with a very striking bubble effect, particulary on tile applications. The low expansion glaze

G1215U

PIONEER KAOLIN 14.0

SILICA 26.0

WOLLASTONITE 14.5

F-4 FELDSPAR 24.0

Frit 3249 20.0

Zinc Oxide 2.0

 

will also slightly bubble when applied thickly, has a greenish blush, but fits bodies well. If wollastonite is the culpuit as Tyler mentions, both of these glazes have the effect and have wollastonite. However, they both have kaolin, silica and some frit, too.

 

Oops, edit: the answer is here I think:

 

http://digitalfire.com/4sight/glossary/glossary_glaze_bubbles.html

 

offgassing from the glaze or the body. Solutions are listed here as well: not using raw kaolin, increasing frits, changing the body to a high-density porcelain.

 

Nice to know.

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Yes, I guess the Wollastonite could be a source -- but that means that the material is not mineralogically pure Wollanstonite, which wouldn't have any carbonate left. It is likely that any mined source of Wollastonite would likely have a significant amount of Calcite contamination.

 

FWIW, Wollastonite is a metamorphic mineral formed by the reaction of Calcite (CaCO3) with Quartz (SiO2), the combination of which is not uncommon (e.g. "dirty limestone" which metamorphoses to marble).

 

I guess if you didn't want bubbles, you could pre-fire your Wollastonite powder to 500+°C for an hour to drive off the CO2. (Dumping the powder in a vat of Hydrochloric Acid could be fun, too, but then I don't know what the resulting Calcium Chloride salts would do in the glaze.)

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Not that complicated, I think.  Glass forms bubbles as it fuses, which is why commercial glass is fritted before being used for anything and why ancient glass always has lots of bubbles in it. The frit's already off-gassed all it's going to, so it makes sense that the wollastonite has something to do with it.

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Yes, except that pure Wollastonite, CaSiO3, has no volatile component -- nothing to off-gas -- so it's got to be that the Wollastonite powder actually contains some other minerals, namely Calcite, which would be expected given the geology of Wollastonite.

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Because it still has the potential to off gas its oxygen and does so as new silicates form from the introduction of the CaSiO3. At higher temps the borates in high boron glazes want to separate from the silicates and you get weird bubbling then too. So my theory is that the glaze is off gassing because of the introduction of the wollastonite because without that you have something not too far off some low-mid fire clears that don't bubble.

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Outgassing of the frit? I don't think there are any volatile components in the frit    (SNIP)

 

Matt, it would be interesting to know the firing and cooling schedules....

I meant that the frit was probably the ingredient that was becoming liquid at the lowest temperature thus trapping the outgassing of other chemicals. I did not word it very clearly. Sorry.

 

It would be nice to know the firing schedules.

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Do we know what causes these bubbles?

 

It might prove helpful to be able to control this effect to one's liking.

 

I hypothesize that some parts of the glaze are becoming molten at a temperature below the temperature where the others are finished outgassing. Im thinking the frit.

 

Have you tried reformulating to eliminate the frit?

No I haven't tried reformulating, I hav'nt spent much time on clear glazes. I do have some control over the effect by varing the thickness of the glaze, if it's very thin than the bubbles aren't really noticable.

 

Here is a thread about bubbles in clear glazes from clayart that you and others might find interesting.

 

Bubbles in clear glazes @ cone 10r

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The black clay I used was a porcelain with black mason stain. These were a single fire, but I have used it on bisque ware and there are still bubbles if it is thick.

 

Firing schedule:

I have a slow warm up at the beginning but after that its just

300 degF/hr to 1980

110 degF/hr to 2180 or whatever bends cone 6

No slow cool or hold.

 

Here is one more of tile one through a magnifying glass, shows ever-smaller bubbles. Now I have to go fire up the Electron microscope,  where did I put that thing?

15826740530_4cd4780c26_o.jpg

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Thanks for sharing your very interesting findings.

 

The clayart reference you posted includes the following

The fact is, the bubbles in a glaze don't move due
to buoyancy. They are formed right where they are
found.
Don't take my word for it, though. Break any piece
of pottery with bubbles in the glaze and examine the
cross section of the glaze with a good microscope.
The magnification doesn't have to be very high--25 to
100X will do. The average distance from the center
line of the bubbles to the glaze-body interface will
be about the same whether the sample is from a part of
the pot with the glaze above the body, below the body,
or on a vertical surface. If bubbles rose in a glaze
these three samples would not look the same. In my
observation they just don't vary. The bubbles don't
move... Dave Finkelnburg

Which makes me wonder what would happen if you fired a vertical test tile.

 

BTW Dave Finkelnburg seems to have written an Alfred MSc thesis called

Bubble Evolution and Sintering in Whiteware Glazes, anybody know how to

see a copy?

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  • 3 months later...

I find it interesting that the dispersion of the bubbles is so uniform all the way across the tile. almost like whatever was making those bubbles had been well processed and made very homogenous somewhere along the way.  Were those glazes ball milled? 


 


I recently got a $25 usb microscope off of ebay and have been using it to look closely at some of my glaze surfaces.  Unfortuantely the images are bitmap so for the moment I cant upload them here, but I will when I get them converted.  They are also pretty low res (not nearly as good as the images of the tiles above!) so next time I will pay more....


 


In the meantime, the upshot is that the bubbles I have been getting are NOT uniformly distributed.  There would be about 1/100th of the number of bubbles that the tiles above show, and they are randomly scattered around.   And they are all different sizes, although they do seem to top out at some maximum size as far as I can tell by looking at lots of them.  I am guessing most people will know the kind I am referring to.  They give any transparent glaze that (undesirable) milky look.


 


I have speculated on what is causing them, but they seem to be in the flux package, which does include wollastonite and frits.  I know this because it is standard procedure on currie test tiles (which I have been using to test various formulations of some of these glazes)to have the bottom left cell be flux only, ie no clay/kaolin and no silica.  All cells still end up having bubbles, and they are in all glazes I look at.  Only the degree varies. 


 


I WAS comforted by the fact that when I put a commercial dinner plate under the new microscope IT TOO HAD BUBBLES - only there were fewer of them and they were much smaller than on my glazes.!  So I guess even the pros have this problem...


 


I would sure still like to know if anyone has had any success MINIMIZING the number and size of bubbles. 


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OK managed to get some jpegs of bubbles uploaded to the gallery.  The images are smaller than I had hoped, but if you look close you can see the shared element in all the pictures....

 

all images are magnified 200x.  The lights in the center of the darker bubble in the middle are (I think) the reflection of the 8 small LED's my USB microscope uses to illuminate whatever it is looking at.

 

First image (hopefully) shows the bubbles in a light purple transparent glaze.  Second image shows them in a clear glaze on a porcelain test tile.  third image shows the same kinds of bubbles on a commercial dinner plate.  They all have these bubbles, only degree seems to vary.  Any comments welcome, and any credible explanations even more welcome!

 

purple bowl dark spot W bubbles

119 On Si corner C reduction

commercial dinner plate clear glaze W bubble

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BTW Dave Finkelnburg seems to have written an Alfred MSc thesis called

Bubble Evolution and Sintering in Whiteware Glazes, anybody know how to

see a copy?

 

Try going to your nearest college library and asking if they can get a copy via interlibrary loan. :-)

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  • 3 weeks later...

OK managed to find these pictures in an old glaze book, Understanding Pottery Glazes by David Green.   They are englarged photos of a "transparent lead boro silicate glaze" at various stages of firing. The vertical line in the center of each picture is a piece of platinum wire which is .003 inches in diameter embedded in the glaze to give comparative scale.

 

They seem very reminiscent of the opening post pictures, so is the glaze in those pics underfired?  Or, in these pics, is it the lead? or the boron? Or, or....??

 

If anybody, has, say written a thesis on, say Bubble Evolution or something similar this is your cue!!!  :)

 

Bubbles in Glaze X at 1030 C

Bubbles in glaze X at 1070C

bubbles in glaze x at 1100C

bubbles in glaze x at 1100C after four hours of soaking

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Curt,

Thanks for sharing your microscope pictures and other info.

 

To answer your question about if I use a ball mill, the only thing I do is put the glaze through a 100 mesh sieve. I haven’t compared it to lower meshes for bubbles, so I don't know what the differences are.

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