Ron Wright on sat 21 feb 98
I am reposting this question at the request of Ron Roy, who was leaving
town when it was originally posted.
I have since tested 2 other glazes that are nice and clear when thin but
have billions and billions of tiny bubbles (my apologies to Carl Sagen)
where they are applied thick. The glazes just look slightly opaque or
whitish but under a microscope you can see the bubbles. Ron Roys version
is the bubbliest of all, but is great with oxides. Tried firing to cone
8 and soaking, still have bubbles. I'm trying to get a thick clear for a
decorating technique I'm working on.
I have just tested a batch of Post's PMSP-4 clear glaze from the
>GlazeBase at cone 6. It came out cloudy, and when I whipped out my new
>field microscope (another great idea from the group) I found that it was
>full of tiny bubbles. The comments on this glaze states that it is
>transparent with no bubbles. How did I screw this up?
>
>Post's PSMP-4
>cone 6-7-8
>clear gloss
>
>Ferro Frit 3134 40.13
>Wollastonite 15.73
>EPK 18.06
>Flint 19.66
Ron Roy's Version (improved expansion)
AACC#2 clear cone 6
PSMP-4 base cone 6
flint 31
Frit 3134 21
nepheline syn. 30
nepheline syn. 19
gerstley borate 21
wollastonite 20
EPK 10
flint 24
whiting 8
EPK 16
Thanks
Ron Wright
Shiresham Pottery - Chicago
http://www.concentric.net/~wrright
Ron Roy on mon 23 feb 98
Hi Ron - Ron here,
Perhaps it is time we had the "how do you fire" conversation again. The
problem may be in the glaze or the bisque schedule - or both or maybe even
neither.
It does seem to be a major problem with cone 6 clears and I am beginning to
think they will need a type of firing that will avoid bubbles.
Parmelee says - the overriding factor in the control of bubbles is through
the glost firing technique. For cone 6 lead borosilicate glazes, the bubble
count varies directly with the time between 1750F (954C) and finish.
What we need here is for someone to organize or do the experiments needed
to determine how slow is slow - this is the kind of info that that should
come with the glaze in the first place - or am I being unrealistic - again?
>----------------------------Original message----------------------------
>I am reposting this question at the request of Ron Roy, who was leaving
>town when it was originally posted.
>I have since tested 2 other glazes that are nice and clear when thin but
>have billions and billions of tiny bubbles (my apologies to Carl Sagen)
>where they are applied thick. The glazes just look slightly opaque or
>whitish but under a microscope you can see the bubbles. Ron Roys version
>is the bubbliest of all, but is great with oxides. Tried firing to cone
>8 and soaking, still have bubbles. I'm trying to get a thick clear for a
>decorating technique I'm working on.
>
>I have just tested a batch of Post's PMSP-4 clear glaze from the
>>GlazeBase at cone 6. It came out cloudy, and when I whipped out my new
>>field microscope (another great idea from the group) I found that it was
>>full of tiny bubbles. The comments on this glaze states that it is
>>transparent with no bubbles. How did I screw this up?
>>
>>Post's PSMP-4
>>cone 6-7-8
>>clear gloss
>>
>>Ferro Frit 3134 40.13
>>Wollastonite 15.73
>>EPK 18.06
>>Flint 19.66
>
>
>Ron Roy's Version (improved expansion)
>AACC#2 clear cone 6
>PSMP-4 base cone 6
>flint 31
>Frit 3134 21
>nepheline syn. 30
>nepheline syn. 19
>gerstley borate 21
>wollastonite 20
>EPK 10
>flint 24
>whiting 8
>EPK 16
>
>Thanks
>Ron Wright
>Shiresham Pottery - Chicago
>http://www.concentric.net/~wrright
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
Ron Wright on tue 24 feb 98
I have received a lot of private mail on this subject and I think the
whole point of this thread is, what causes the bubbles in the first
place. Is it the glaze materials, the clay, the oxides or a combination
of all of these. I am sure that in most cases that we could come up with
a firing schedule to eliminate the bubbles, but inguiring minds want to
know what produces them. I for one would like to be able to calculate
the bubble factor in my glazes to know when a slower firing or a longer
soak would be necessary. If we know the cause, we can control it. Maybe
even have a glaze that's super bubbly. Foamware? Froth glazes. We need a
prescription from the glaze doctor.
Thanks
Ron Wright - Chicago
Shiresham Pottery
http://www.concentric.net/~wrright
Gavin Stairs on wed 25 feb 98
At 07:49 AM 2/24/98 EST, you wrote:
>...I for one would like to be able to calculate
>the bubble factor in my glazes to know when a slower firing or a longer
>soak would be necessary. If we know the cause, we can control it. Maybe
>even have a glaze that's super bubbly. Foamware? Froth glazes. We need a
>prescription from the glaze doctor.
This one's easy. Just look at the loi of your recipe. All that material
is what turns into gases, principally CO, CO2 and SOx. Also look at the
chemical formulae of the materials. Carbonates produce CO2. Sulfates
produce SO3. Excess carbon and organics in clays produce CO, CO2 mostly,
but may also produce other gasses. And many materials have combined water,
which makes steam.
So the recipe for a bubbly glaze is: fast fire a glaze with lots of
carbonates, maybe a lot of calcium carbonate (whiting), and excess alumina,
perhaps some titania or zirconia to stiffen it up, and go light on the
fluxes. Should do the trick. But don't think this is a food safe
prescription: it isn't. I don't want any bubbles in my liner glaze, or on
the lip of my mug, please. Bubbles at the surface make a good scouring pad.
Gavin
Ron Roy on fri 27 feb 98
I have been reading up on this - I think Gavin has shed some light on this
problem. It seems - so I read - that some gases and water get sealed in the
glaze during firing. This must be more of a problem with boron glazes as
they seal over earlier than other types. I notice all three glazes being
discussed have lots of clay in them - this must be part of the problem
anyway.
Frit 3134 has no alumina in it - using another type with some alumina will
also lower the amount of raw clay needed. Frit 3195 looks like it would be
useful. I can make adjustments if you want - just pick a glaze with bubbles
and send it along.
I think another useful experiment would be to replace some of the raw clay
with calcined - being careful to try and keep the glaze the same.
I suggest calcining different amounts of the raw clay in one of the
bubbling glazes to see if it makes a difference. Perhaps the simplest way
would be to weigh out all the clay for a test batch and calcine parts of it
before mixing it.
Because we would not calcine at a high temperature there will be some
rehydration eventually so this would not be a permanent solution but it
would show if there is an effect on bubble production.
The other - more permanent solution would be to use calcined kaolin - like
Glomax - and substitute for raw kaolin on a molecular level - at different
percentages.
The other factor mentioned by Taylor & Bull - the higher the viscosity and
surface tension of the glaze the less the bubbles can grow. Substituting
higher viscosity fluxes for the alkalies might have some effect.
This means substituting CaO, MgO, SrO for KNaO will help with the problem
as I understand it.
I don't have time to do this these days - wish I had. If anyone needs some
calculation help to do it I am here.
>----------------------------Original message----------------------------
>I have received a lot of private mail on this subject and I think the
>whole point of this thread is, what causes the bubbles in the first
>place. Is it the glaze materials, the clay, the oxides or a combination
>of all of these. I am sure that in most cases that we could come up with
>a firing schedule to eliminate the bubbles, but inguiring minds want to
>know what produces them. I for one would like to be able to calculate
>the bubble factor in my glazes to know when a slower firing or a longer
>soak would be necessary. If we know the cause, we can control it. Maybe
>even have a glaze that's super bubbly. Foamware? Froth glazes. We need a
>prescription from the glaze doctor.
>
>Thanks
>Ron Wright - Chicago
>Shiresham Pottery
>http://www.concentric.net/~wrright
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
pedresel@3-cities.com on mon 2 mar 98
I've been thinking about this bubble thing for a while -- long before this
thread started but I'm a long way from understanding it. Here are some
random thoughts and questions on the subject.
If the bubbles are caused by too low or slow a bisque, would that mean that
you would tend to see them more at ^6 than ^10? I seem to remember some ^10
glaze with lots of trapped bubbles -- even on a white stoneware. It seems
to me that the impurities would burn out before a ^10 glaze melted if they
tend to get trapped at ^6. What would it be that wouldn't decompose to a
higher temperature?
If the bubbles are from materials in the glaze I have the same question.
Something like whiting or dolomite should decompose long before a ^10 glaze
melts.
Any air in the pores of a bisque pot will have to escape as the pot turns
into a dense ceramic (is there a verb for that?). If the glaze starts to
melt before that happens then the air may be trapped. Refiring might only
make it worse. Is that far-fetched?
If the glaze is viscous enough the bubbles may be more than content to stick
to the surface of the pot under the melted glaze -- like bubbles in a glass
of soda pop. There's not a really strong driving force for them to pop up
to the surface and it may be really hard to change anything in the firing to
get rid of them once they form.
Some glazes with bubble are just super IMO. You can get a fantastic
translucence as long as the surface stays smooth.
This sounds like a question for Studio Potters Technology Institute. I
think there are some tests that could be done without expensive chemical
analyses.
-- Evan
pedresel@3-cities.com
Performing thought experiments while his bisque cools and his beer ferments
in cloudy & warm W. Richland WA
John Hesselberth on tue 3 mar 98
I too have been puzzling about the bubble question. I guess it brings
out the chemical engineer that I was in a previous life. Evan's comment
(below) that if bubbles do form in the melted glaze they would not likely
release is an excellent addition to this stream of ideas. I'll put
forward a hypothesis I haven't heard yet.
I'd like to hypothesize that the bubbles aren't gas bubbles at all. They
are vacuum bubbles. Hamer, on page 363 points out that NaO, K2O, ZnO and
BaO become volatile at temperatures of 1100 C or so. Visualize a high
sodium (or K or Zn or Ba) glaze that melts and is very viscous at, say
1150. A small particle of something-or-other seeds a bubble of NaO. Evan
makes the point that this bubble isn't going to immediately release--it
will have to get to a certain size or the glaze viscosity will have to
drop to a certain point before it releases. Just about the time that
bubble grows big enough to be seen, the cone falls and the kiln is shut
down. If conditions were right I could see the glaze thickening to the
point where the bubble couldn't collapse when the vapor pressure of the
sodium oxide dropped to the point where you might expect it to collapse.
Of course, the sodium oxide would become a solid bit of dust when it
cooled enough leaving behind a vacuum bubble. If this type of phenomena
is indeed happening, it would be extremely difficult to predict and
control; although one might eventually learn not to use high quantities
of these volatile materials in glazes maturing in a certain critical
temperature range. Comments?
>
>If the glaze is viscous enough the bubbles may be more than content to stick
>to the surface of the pot under the melted glaze -- like bubbles in a glass
>of soda pop. There's not a really strong driving force for them to pop up
>to the surface and it may be really hard to change anything in the firing to
>get rid of them once they form.
>
..
>
>-- Evan
> pedresel@3-cities.com
>
>Performing thought experiments while his bisque cools and his beer ferments
>in cloudy & warm W. Richland WA
John Hesselberth
Frog Pond Pottery
Pocopson, PA USA
EMail: john@frogpondpottery.com
visit my web site at http://www.frogpondpottery.com
Ron Wright on tue 3 mar 98
With the size of these bubbles I've been getting I think that they are
not leaving the glazes with a long soak because of the viscosity of the
glaze. My reasoning is that since there are no pits, none of the tiny
bubbles are making it to the surface because of surface tension. The
bubbles are too uniform and evenly dispersed to be outgasing from the
clay, I think. Does anyone know about viscosity of molten glass. Is this
something else to worry about, expansion, unity formulas and now
viscosity of molten glaze. Maybe open the kiln at peak temp and give
the pots a good whack, to loosen bubbles? Hmmmm, slapku firing? Or maybe
just give the kiln a good kick every hour to loosen the bubbles.
BTW - If you live near Chicago, Charles Jahn has opened a ceramics
supply next to his studio in Evanston. Nice people and no more long
drives for clay. Phone no. 1-847-332-2089
Ron Wright - Chicago
Shiresham Pottery
http://www.concentruc.net/~wrright
pedresel@3-cities.com wrote:
>
> ----------------------------Original message----------------------------
> I've been thinking about this bubble thing for a while -- long before this
>
> -- Evan
> pedresel@3-cities.com
>
> Performing thought experiments while his bisque cools and his beer ferments
> in cloudy & warm W. Richland WA
| |
|
