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bubbles in clear glazes @ cone 10r

updated thu 9 aug 07

 

Ben Shelton on tue 3 jul 07


I have been making some tests of 4 different base glazes with red iron oxide
additions to experiment with celedons. I have noticed that some of the bases
tend to form a haze of tiny bubbles suspended in the glaze. These glazes
also seem to not have a completely smooth surface which gives the appearance
of a reduction in the gloss of the glaze.

I really like the color I get from the following and it also seems to have
the most bubbles.
40 P feldspar
30 silica
20 whiting
10 epk
+1 iron

Fired to ^10R on white clay it makes a nice light turquoise celedon. Breat
color with no muddines.

Now, about those bubbles.

I'd like to do some experiments to affect the smoothness of the surface and
the clarity of the glaze but I really don't know what causes the bubbling so
I don't know where to start.

Thanks all,

Ben

Lynne and Bruce Girrell on wed 4 jul 07


Ben Shelton wrote:
>I'd like to do some experiments to affect the smoothness of the surface and
>the clarity of the glaze but I really don't know what causes the bubbling

Dave Finkelnburg presented some evidence at this year's NCECA that indicated
that bubbles - at least in the glazes he studied - were formed in place by
air/gases trapped between particles of the glaze when it was applied. If
glaze melting initiated at the clay/glaze interface and proceeded outward,
the glaze would have few, if any, bubbles. But if the melt started on the
outside, the bubbles would be trapped and could not migrate out. While
bubbles may expand, they do not move, as the glaze is too viscous.
(did I get that right, Dave?)

While I don't have any specific suggestions for you, perhaps you might find
something in Dave's tantalizing tidbit* of information to help you.

Bruce Girrell

*Dave had only 20 minutes to present the results of what was clearly a _lot_
of work.
I, for one, would love to see a much more detailed account of Dave's study.

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Paul Herman on wed 4 jul 07


Ben,

I have used that same glaze for years, as the base for my celadon. It
does get little bubbles, which give the celadon a nice soft, semi
opaque quality. So, for me the bubbles are a good thing.

I think they are the result of the decomposition of the whiting,
which gives off gasses as it is fired. If you want to get rid of the
bubbles, you could try reformulating the glaze using wollastonite,
which contains calcium and silica, but no carbonate to make bubbles.
I suspect eliminating the bubbles will also change the color somewhat.

Good luck,

Paul Herman

Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com


On Jul 3, 2007, at 9:02 PM, Ben Shelton wrote:

> I have been making some tests of 4 different base glazes with red
> iron oxide
> additions to experiment with celedons. I have noticed that some of
> the bases
> tend to form a haze of tiny bubbles suspended in the glaze. These
> glazes
> also seem to not have a completely smooth surface which gives the
> appearance
> of a reduction in the gloss of the glaze.
>
> I really like the color I get from the following and it also seems
> to have
> the most bubbles.
> 40 P feldspar
> 30 silica
> 20 whiting
> 10 epk
> +1 iron
>
> Fired to ^10R on white clay it makes a nice light turquoise
> celedon. Breat
> color with no muddines.
>
> Now, about those bubbles.
>
> I'd like to do some experiments to affect the smoothness of the
> surface and
> the clarity of the glaze but I really don't know what causes the
> bubbling so
> I don't know where to start.
>
> Thanks all,
>
> Ben
>

Craig Martell on wed 4 jul 07


Ben was asking:
>I'd like to do some experiments to affect the smoothness of the surface and
>the clarity of the glaze but I really don't know what causes the bubbling so
>I don't know where to start.

Hello Ben:

The bubbles are caused by the high loss on ignition of whiting. The LOI
value of whiting is about 43.9%. Whiting is calcium carbonate
CaCO3. After firing it is just CaO and you are losing the CO2 (carbon
dioxide) on ignition. This makes bubbles. You can eliminate the whiting
and use wollastonite (calcium metasilicate) which has a zero LOI. This
isn't a direct substitution and it works best if you calculate the
formula. You will need to factor in a little more wollastonite for the CaO
and remove some silica because the wollastonite is providing some silica
while the whiting is not.

regards, Craig Martell Hopewell, Oregon

Fredrick Paget on wed 4 jul 07


Perhaps the answer lies in the old story bout the basket completely
full of water melons that had room for a bunch of other fruit and
vegetables of progressively diminishing size down to peas and sesame
seeds that pack into the spaces between the larger ones. could you
have a gradation of sizes of ingredients down to ball clay which is
tiny so as to fill up all the spaces leaving o room for air?
Fred



>Ben Shelton wrote:
>>I'd like to do some experiments to affect the smoothness of the surface and
>>the clarity of the glaze but I really don't know what causes the bubbling
>
>Dave Finkelnburg presented some evidence at this year's NCECA that indicated
>that bubbles - at least in the glazes he studied - were formed in place by
>air/gases trapped between particles of the glaze when it was applied. If
>glaze melting initiated at the clay/glaze interface and proceeded outward,
>the glaze would have few, if any, bubbles. But if the melt started on the
>outside, the bubbles would be trapped and could not migrate out. While
>bubbles may expand, they do not move, as the glaze is too viscous.
>(did I get that right, Dave?)
>
>While I don't have any specific suggestions for you, perhaps you might find
>something in Dave's tantalizing tidbit* of information to help you.
>
>Bruce Girrell


--
Twin Dragon Studio
Mill Valley, CA, USA

Ivor and Olive Lewis on thu 5 jul 07


Dear Ben Shelton,=20

This situation has been aired on Clayart over the years and I suppose =
the answers that are proffered tend to evolve as people do more and more =
research.

I have not read Dave Finkelnburg's research but he is a dedicated =
researcher so he will have graphic evidence to support his thesis. It is =
known that as particles reshape during the sintering and fusion stages =
of glaze maturation that small pockets of gas move and collect at the =
corners of the polyhedra which form. These remain in situ during fusion. =
If the glaze has a high viscosity they may not move to cause pinholing =
The question then would become, can you see these with your naked eye =
given the screening size of the raw materials ? At a fraction of the =
size of the original particles I would have doubts.

The outgassing or Whiting has a logical feel to it. But does this fit =
with the facts. It is Carbon dioxide which is trapped. Decomposition =
commences at about 825 deg C and should be complete before reaching 1000 =
deg C. In your recipe there is no frit that would melt and seal your =
glaze coating. Hence the gas would vent into the kiln chamber. The =
second thing to consider would be the volume of gas you are observing. =
If it is Carbon dioxide then one Mol of Whiting (about 100 grams) will =
discharge one Mol of Carbon Dioxide (about 44 grams) at ambient =
temperature and atmospheric pressure. This would occupy a volume of 22.4 =
litres, a fair amount of gas (Sam volume as about 6 gallons of =
gasoline).

Reformulating the glaze to eliminate the whiting also seems to have a =
good feel about it. Some of the group may recall I reported on a series =
of glaze samples where the source of Calcium oxide was Wollastonite and =
most of them had gas inclusions, small bubbles less than 1mm dia.

In the end I attributed this effect to the presence of iron =
contamination in my raw materials. The argument here is that Red Iron =
oxide, Fe2O3, dissociates evolving Oxygen when heated above about 1300 =
deg C. When there is a good proportion of RIO in the recipe it is this =
reaction which provides the energy to stir the glaze, with smaller =
bubbles coalescing to form large bubble that burst to give the Oil Spot =
Tenmoku effect.

So, as Iron oxide is necessary to your quest, you may have to live with =
the effect.

Best regards,

Ivor Lewis.
Redhill,
South Australia.

Ben Shelton on sun 29 jul 07


Well I have completed a small experiment in trying to determine the source
of the bubbles I have been seeing in the following glaze

Leach clear (4321)

custer feldspar 40
silica 30
whiting 20
epk 10


add 1% iron for celedon
8~10% iron for temmoku

In both the celedon and clear versions of this glaze I noticed a haze of
tiny bubbles suspended in the fired glaze. Many possible sources for these
bubbles have been suggested. One that I could easily test was that the
whiting might be the source as some of it converted to gasses at higher
temps. Another suggestion was that the bubbles are the result of air spaces
left in the dried glaze coating. I dont really have a way to eliminate these
spaces but I did wish to see just how thinly I could apply this glaze and
have it look like a glaze.

The first thing I tried was replacing the whiting with wollastonite. This is
not a 1 to 1 substitution so I strted up my Glaze Master software and came
up with this.

Leach clear w/wollastonite

custer feldspar 43.5
silica 19.6
wollastonite 27.2
epk 9.8


This glaze looked exactly like the original when both were dipped onto test
tiles for 3 seconds at a specific gravity of 1.45 which is about what you
get with most glazes if you add an equal weight of water to the dry batch
amount. ex: 100 grams dry gets 100 grams water. It happens to be a good
starting point for most glazes and is easily thinned for testing.

So, the whiting is not to blame for this glaze.

For the next test I thinned the base glaze little by little and dipped test
tiles noting the specific gravity as I went. Here I had a surprise. The
bubbles went away as the glaze coating got thinner. I had tiles that ranged
from 1.45 down to 1.28 and at 1.28 there were no noticable bubbles. The
glaze coating was thick enough to cover the clay with a glossy surface and
was fairly clear. This is great for me because I was looking for a smoother
and clearer glaze as a foundation for dipping other glazes over with wax
resist. I am not sure if the glaze coating was thin enough not to trap any
air to begin with or if it was thin enough at temp to allow the bubbles to
escape but the bubbles were gone.

If anyone needs a good graduated cylinder to measure specific gravity try US
plastics. they have them in 100 and 1000 ml sizes at very reasonable prices
(Cheap, they are plastic after all).

That's all for now,
thanks for all the suggestions,
Ben

Dave Finkelnburg on mon 30 jul 07


John,
Thanks for your note. I don't dispute your
observations at all. What you describe is well
documented. As a glaze melts, bubbles which form at
or so near the surface that the surface film over the
bubbles are thin, the bubbles burst, and that's what
you see. Oilspots are a special case in which you
purposely produce such large bubbles that they all
burst at the surface, thus "boiling" the glaze and
producing those beautiful surface effects.
The fact that bubbles form near enough to the glaze
surface to burst there in no way says bubbles rise in
a glaze. However, you and a whole lot of other
potters and folks in science and industry for a long
time have leaped to that conclusion.
Bubbles certainly rise in a glass of beer or soda.
However, those bubbles are large compared to what we
usually find in a glaze, and the beer or soda is very
"thin," or low in viscosity compared to a glaze.
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.
That's my story, John, and I'm stickin' to it! :-)
I hope this finds you well,
Dave Finkelnburg, being serenaded by the
cricket chorus again tonight...

--- John Britt wrote:
> Hate to argue with Stokes Law but I have watched oil
> spot and other glazes
> bubble quite vigorously. Perhaps I missed some
> critical detail since I
> just joined this discussion.
> I have also watched tons of bubbling in Raku kilns.



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Ivor and Olive Lewis on mon 30 jul 07


Dear Ben Shelton,=20

Perhaps my reply to your original post was lost recently when Clayart =
had a glitch.

The solution to your dilemma of your haze of bubbles is to eliminate the =
Fe2O3 from your recipes. This means not adding it as a colourant and =
selecting ingredients that have minimal Iron contamination.

Fe2O3, red iron oxide, decomposes in the cone 10 range, releasing =
oxygen, even without the intervention of a reducing agent.

Success in the creation of Oil Spot Tenmoku glazes depends on preventing =
reduction of the glaze and firing to Cone 10 or higher. The violence of =
the reaction creates large bubbles which burst and provide the =
decorative patterning. This is well documented in an article in Ceramics =
Monthly from a couple of years ago.

If Iron oxide is essential then a longer dwell time may be needed to =
allow bubbles, be they carbon dioxide, carbon monoxide or oxygen, to =
migrate to the surface, burst and heal

Best regards,

Ivor Lewis.
Redhill,
South Australia.

Ben Shelton on mon 30 jul 07


Do you think that there is enough iron in this glaze to have this effect. I
looked up the latest analysis of the materials I used and there is only
0.127% iron in this glaze. The only way to lessen the iron would be to use
Grolleg Kaolin and G-200 feldspar.

I tried the grolleg kaolin substitution but didn't really notice a lot of
difference.

Do you think 0.127% iron has that much of an effect?

Interesting thoughts,


Ben

Dave Finkelnburg on mon 30 jul 07


Dear Ivor, Ben,
You are correct, as always, about iron oxide
decomposing at high temperature. At the risk of
opening an extremely large can of worms, though, I
have to disagree strongly with your advice to Ben
Shelton. Bubbles which form close enough to the
surface of a glaze to burst can indeed heal. However,
bubbles which are not that close do not "migrate to
the surface." A glaze which will not flow off a
vertical surface is too viscous, it is molten for too
short a time(a few hours at most), and the bubbles are
too small (less than 250 microns diameter) for
significant (more than a few microns) movement of a
bubble to take place. Do the Stokes Law calculation
and you will arrive at this conclusion, too.
From Ben Shelton's tests it is clear what happened
was he applied successively thinner coats of glaze
until most of the bubbles were able to burst at the
glaze surface and heal over and the glaze became
bubble free that way.
By the way, IF the kiln is fuel-fired then it may
be possible to reduce the iron in the glaze and remove
the problematic oxygen that way. This is provided the
reduction occurs before the glaze melts and the kiln
is held in reduction until the glaze melts.
Good potting,
Dave Finkelnburg

Date: Mon, 30 Jul 2007 16:30:27 +0930
From: Ivor and Olive Lewis
Dear Ben Shelton,
The solution to...your haze of bubbles is to eliminate
the Fe2O3 from your recipes. This means not adding it
as a colourant and selecting ingredients that have
minimal Iron contamination.
Fe2O3, red iron oxide, decomposes in the cone 10
range, releasing oxygen. If Iron oxide is essential
then a longer dwell time may be needed to allow
bubbles, be they carbon dioxide, carbon monoxide or
oxygen, to migrate to the surface, burst and heal.



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Dave Finkelnburg on tue 31 jul 07


Dr. Bastarache,
Bubbles in a melted glaze stay put because the
glaze is too viscous, too "thick," like molasses in
January! If the glaze were fluid enough to permit a
lot of bubble movement, the glaze would all run down
onto the floor of the kiln, or onto the shelves, or
into the bottom of the pots. The bubbles do move in a
glaze due to buoyancy, but the movement is so slight
that it would be very difficult to measure, on the
order of a micron.
Good potting,
Dave

From: "Edouard Bastarache Inc."

David,
why is it they do not move?

" 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."






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Dave Finkelnburg on tue 31 jul 07


Linda,
I once attempted to make a list of all the possible
sources of bubbles in a glaze. It got to be a long
list! :-(
Yes, organics might burn out(C + O2 = CO2) and the
C02 could become trapped as a bubble in the body.
What would drive the bubble into and/or through the
glaze? The only thing I can think of would be if
there were so much CO2 evolving that the body couldn't
hold it all. Trapped gases, most likely due to pore
spaces in the body, do expand and we see that in
poorly mixed clay or overfired clay as bloats,
blisters of the body. Virtually all clay bodies, by
the way, have voids or pores in them after firing.
I've had mixed results refiring glazes with serious
bubble problems. As often as not the blisters have
gotten worse, not better. :-(
By the way, as you know paper burns at 451-degrees
F. Remember Fahrenheit 451? :-) So I would think
most organics would be burned out by the time a glaze
melted unless the kiln was seriously short of oxygen
until well into the firing. In other words, a clean
firing should prevent this from happening.
I hope this answers are least some of your
question.
Dave Finkelnburg

From: Linda - Pacifica
Is it not true then that bubbles can arise from
unburned organics in
the clay body? I thought I'd seen that proposed as
another potential
source of bubbles in glazes. If that is to be true,
it seems to me
that the bubbles must migrate through the glaze, but
I'm prepared to
be proved wrong.
Is time lapse photography out of the question?

Also, it's said by some that the bubbles can be
eliminated in some
cases with a re-firing. Given what you've said, I'd
guess that the
surface bubbles might heal ok but that others would
remain embedded
in the glaze. Or indeed new bubbles might be formed
as old bubbles
heal.



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Linda - Pacifica on tue 31 jul 07


John,

Is it not true then that bubbles can arise from unburned organics in
the clay body? I thought I'd seen that proposed as another potential
source of bubbles in glazes. If that is to be true, it seems to me
that the bubbles must migrate through the glaze, but I'm prepared to
be proved wrong.
Is time lapse photography out of the question?

Also, it's said by some that the bubbles can be eliminated in some
cases with a re-firing. Given what you've said, I'd guess that the
surface bubbles might heal ok but that others would remain embedded
in the glaze. Or indeed new bubbles might be formed as old bubbles
heal.

Just wanna know the facts.

Thanks for all the valuable information,
Linda
On Jul 30, 2007, at 10:11 PM, Dave Finkelnburg wrote:

> John,
> Thanks for your note. I don't dispute your
> observations at all. What you describe is well
> documented. As a glaze melts, bubbles which form at
> or so near the surface that the surface film over the
> bubbles are thin, the bubbles burst, and that's what
> you see. Oilspots are a special case in which you
> purposely produce such large bubbles that they all
> burst at the surface, thus "boiling" the glaze and
> producing those beautiful surface effects.
> The fact that bubbles form near enough to the glaze
> surface to burst there in no way says bubbles rise in
> a glaze. However, you and a whole lot of other
> potters and folks in science and industry for a long
> time have leaped to that conclusion.
> Bubbles certainly rise in a glass of beer or soda.
> However, those bubbles are large compared to what we
> usually find in a glaze, and the beer or soda is very
> "thin," or low in viscosity compared to a glaze.
> 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.
> That's my story, John, and I'm stickin' to it! :-)
> I hope this finds you well,
> Dave Finkelnburg, being serenaded by the
> cricket chorus again tonight...
>
> --- John Britt wrote:
>> Hate to argue with Stokes Law but I have watched oil
>> spot and other glazes
>> bubble quite vigorously. Perhaps I missed some
>> critical detail since I
>> just joined this discussion.
>> I have also watched tons of bubbling in Raku kilns.
>
>
>
>
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Edouard Bastarache Inc. on tue 31 jul 07


David,

why is it they do not move?



" 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."



Later,




Edouard Bastarache
Le Franšais Volant
The Flying Frenchman

Sorel-Tracy
Quebec
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John Britt on tue 31 jul 07


Dave,

Ok, now I understand. My mistake.

I understand that you are talking about bubbles rising due to buoyancy. I
must have missed that part. I guess, from what you are saying, what I was
referring to was bubbles that are created and then become so large that
they "burst" at the surface, either from oxygen being produced, chemical
water being released or silicon carbide, etc.

Just from a practical point of view, I would not think that bubbles in
glazes rise due to buoyancy otherwise you would see them at the rims of
pots (vertical travel) rather than "rising" to the surface of the glaze
which would mean horizontal travel.

Anyway, I will write again about your CM article.

Thanks for filling me in.

Thanks,

John Britt
www.johnbrittpottery.com

John Britt on tue 31 jul 07


Linda,

I am not sure about it, but maybe Dave will chime in.

I am not sure what he would think about the term "migrate". He seems to
specifically be talking about "buoyancy".

I have never cut a cross section of a pot and looked at it with a
microscope but I have seen photos.

I will wait to see what Dave says.

Thanks,

John Britt
www.johnbrittpottery.com

Ivor and Olive Lewis on tue 31 jul 07


Dear Ben Shelton and Dave Finkelnburg,=20

To calculate the amount of Oxygen released assume the reaction is Fe2O3 =
to 2FeO+1/2O2.

I get this to give 1/2(22.4) litres of molecular oxygen per mol of =
Fe2O3, that is 11.2 litres. So 100g Fe2O3 will discharge 6.25 litres of =
gas, 1 gram... 0.0625 litres. which is 62.5 cubic centimetres. So I =
would expect your glaze with 0.127 gms of Iron oxide to liberate about 8 =
cubic centimetres of gas. Correct that to the absolute temperature of =
the reaction, 1300 deg C ~ 1573 K and it is a significant volume. Check =
my figures, I could be wrong.

With regard to Dave's comments, I am sure I would get the same results =
if I were to do the Stokes law calculation. As a matter of principle =
Dave, can this route be used to define the motion of a spherical void in =
horizontal direction ? I recall working with glass spheres in Glycerine =
at Uni during our Science foundation course. Motion due to gravity !

I have had samples where the diameter of the bubbles is up to the 1000 =
micron range.

Best regards,

Ivor

Edouard Bastarache Inc. on wed 1 aug 07


David,

thanks !!!


Later,



Edouard Bastarache
Le Franšais Volant
The Flying Frenchman

Sorel-Tracy
Quebec
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Ivor and Olive Lewis on thu 2 aug 07


Dear Edouard Bastarache,

Could it be that we are ignoring the Diffusion and Permittivity =
Processes. These qualities of many gases in Silicate melts have been =
studied and measured.=20

Best regards,

Ivor

Edouard Bastarache Inc. on thu 2 aug 07


Hello Ivor,

thanks.

Could we read more from you about these processes.


Later,


Edouard Bastarache
Le Franšais Volant
The Flying Frenchman

Sorel-Tracy
Quebec
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Chris trabka on wed 8 aug 07


I have a wonderful bright shiny pale green celedon that if not fired to
maturity will have a frosty look that is caused by innumerable "bubbles"
entrapped within the glaze (my last firing had 5 pieces with the "bubbles"
and 30 or so pieces that had the correct surface characteristics - the
glaze is used on both vertical and horizontal surfaces). On refiring to the
correct temperature the "bubbles" are no longer present and the glaze is
clear and glossy. The base glaze is a ^11 glaze to which I add 1% yellow
iron oxide (synthetic). I start reduction at ^015.

My suspicion is that the gasses in the "bubbles" are absorbed within the
glaze matrix when the glaze is brought to the correct temperature.

Chris