Those darned bubbles!

On Tue, 9 Oct 2007 13:01:03 +0200, "Bubbles_"
wrote:

Original post below.

I am just wondering about something. My wheel is aluminium, and it oxidizes,
so that I get a black stain on the back of my left hand from centering - but
I also get it in some of the clay that I later reclaim.

Is aluminium a possible cause of these bubbles? Should I stick to
new-bought clay and/or treat my wheel with something so it doesn't let off
particles?


No, this aluminum should not cause any problem at all. Note that
aluminum oxide melts at a very high temperature (stiffens glazes),
whereas whatever is causing the bubbles is something that is actually
boiling, or more likely burning, at your firing temperatures. The
combustion gasses are released as bubbles that appear in the
slow-to-heal glaze. (There are other possibilities, like release of
chemically-bound gasses from some minerals, which don't actually
constitute "burning" as such, but the result is the same.)

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!

On Tue, 9 Oct 2007 13:01:03 +0200, "Bubbles_"
wrote:

Original post below.

I am just wondering about something. My wheel is aluminium, and it oxidizes,
so that I get a black stain on the back of my left hand from centering - but
I also get it in some of the clay that I later reclaim.

Is aluminium a possible cause of these bubbles? Should I stick to
new-bought clay and/or treat my wheel with something so it doesn't let off
particles?

Marianne


Not to worry, aluminum oxide (alumina) is one of the primary
components of clay and glazes. And the amount you might get in your
clay from your aluminum wheel head is very, very small.

My vote as to what pottery "chemistry" book you should get is "Clay
and Glazes for the Potter" by Rhodes. The latest edition is by Rhodes
/ Hopper, but for what you'll want an older edition is fine.

But don't expect to be able to jump in and easily fix glaze problems.
That's an art as much as a science and pinholing can be caused by any
number of problems - underfired bisque, underfired glaze, overfired
glaze, the phase of the moon, orientation of your kiln to magnetic
north, etc., etc., etc.

deg

I just re-read this and am real confused - is that 1270 C? In which case
you are firing to ~ cone 10?

Wow that must be hard to get to in a a small hobby kiln. How many hours
does it take? Why are you firing that high? They actually have some pseudo
porcelain at cone 6 (what I fire to).

This is my typical firing scedule (the hold brings it up to cone 6)



Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr)
72(22.2)
1 100(37.8) 220(104.4) 0
1.48
2 350(176.7) 2000(1093.3 0
5.09
3 108(42.2) 2185(1196.1) 0.33
1.71
4 -500(-295.6) 1900(1037.8) 0.17
0.57
5 -125(-87.2) 1400(760) 0
4.00

13.34

"Bubbles_" wrote in message
...
EWWWW!!

I keep getting bubbles on my finished pieces. I have tried up to 40
minutes hold time at 1270 degreess, but they still show up! I am getting
rather frustrated by it all. I can "repair" them by adding glaze to the
holes (that are more like inverted craters going all the way down to the
clay) and refiring once or even twice, but I have a very small kiln at
home, and find it both a waste of space and power, while at the same time
not wanting to "lose" the pieces.

Any ideas on other things I could try? Last kiln was 6 hours to 650, then
up to 1270 and hold for 30 minutes. The cooling was over almost a day.
Policy being that over about 400, I don't even let the cool air of the
laundry room in, but after that, I take out a couple of plugs (top center
and bottom) and under 200 I start opening the lid of the kiln a little.
That shouldn't affect the bubbles, though, as I am sure they have set at
much higher temperatures.

BUT - AGH!!!

Any input and suggestions greatly appreciated!!!

Marianne

"Bob Masta" wrote in message
...

No, this aluminum should not cause any problem at all. Note that
aluminum oxide melts at a very high temperature (stiffens glazes),
whereas whatever is causing the bubbles is something that is actually
boiling, or more likely burning, at your firing temperatures. The
combustion gasses are released as bubbles that appear in the
slow-to-heal glaze. (There are other possibilities, like release of
chemically-bound gasses from some minerals, which don't actually
constitute "burning" as such, but the result is the same.)


Hi Bob

If aluminium stiffens glazes, couldn't it be that the release of it from the
clay, trying to get out through the glaze - causes the glaze to stiffen and
thus the bubbles not to melt back "in position"?

I know that may be a stupid question, but I have found that I only learn if
I ask questions about things I don't understand :-)

Marianne

"Dewitt" wrote in message
...

Not to worry, aluminum oxide (alumina) is one of the primary
components of clay and glazes. And the amount you might get in your
clay from your aluminum wheel head is very, very small.

My vote as to what pottery "chemistry" book you should get is "Clay
and Glazes for the Potter" by Rhodes. The latest edition is by Rhodes
/ Hopper, but for what you'll want an older edition is fine.

But don't expect to be able to jump in and easily fix glaze problems.
That's an art as much as a science and pinholing can be caused by any
number of problems - underfired bisque, underfired glaze, overfired
glaze, the phase of the moon, orientation of your kiln to magnetic
north, etc., etc., etc.

Hi Deg

I am beginning to wonder if I shouldn't make a blood sacrifice before each
glaze kiln now! LOL!

Thanks for the book suggestion. Added to my list :-)

Marianne

"DKat" wrote in message
...
I just re-read this and am real confused - is that 1270 C? In which case
you are firing to ~ cone 10?

I do think that my kiln-computer actually shows a bit higher temperature
than the actual, so 1270 might be 1250. I would love to figure out a way to
test the hypothosis. Must ask my teacher on Friday if she has any
suggestions. As I said earlier, I have never heard of folk here using cones.

Wow that must be hard to get to in a a small hobby kiln. How many hours
does it take? Why are you firing that high? They actually have some
pseudo porcelain at cone 6 (what I fire to).

Well - I stopped watching over the kiln when it is firing, but I would say
something close to 10-12 hours to get to top heat. But that is with my
firing schedule as well. Actually, I use 2 different ones for high glaze and
have been adjusting them a tad each time because of the problems with the
bubbles.

This is my typical firing scedule (the hold brings it up to cone 6)

I don't understand what you mean by segment, sorry!

You fire to almost 1200 - my glazes are meant for about 1220 to 1270
according to the lables.

I only have 1 hold on my computer, and that is at top temp. I vary between
20 and 30 minutes - having tried 40 once, to no avail.

Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr)
72(22.2)
1 100(37.8) 220(104.4) 0
1.48
2 350(176.7) 2000(1093.3 0 5.09
3 108(42.2) 2185(1196.1) 0.33
1.71
4 -500(-295.6) 1900(1037.8) 0.17
0.57
5 -125(-87.2) 1400(760) 0
4.00

13.34

D - thank you so much for taking the time to help me try to figure out this
problem! You are an angel, as are all the folks on here who have such
patience with a pseudo-beginner like myself!

Hugs!

Marianne

Let me see if I can rephrase into common ground. There are low fired,
mid-fired and high-fired pottery. Low fire is typically your earthenware,
raku, majolica, etc. Mid-fire can be functional stoneware, high fire is
stoneware and porcelain of the translucent variety. Mid fire is what many
people with small electric kilns are now working it. Your clay is vitrified
(if it is a true midfire clay) at midfire range. It is not at lowfire .
Typically the midfire range is cone5 to cone7 or 1183C to 1237C if you are
firing @ ~60C/hr. If I were to soak at peak temperature I would fire to a
lower temperature because the heat work would take me to the cone I want.
The cone is a clay made to 'melt' with a certain heat work. What I want to
know is when the clay and glaze is mature and this happens with heat work
rather than just a certain temperature. That is why cones are important.

Segments in my firing are simply different periods in the firing - so if you
are running your kiln manually (without a computer program) you might run
the first 6 hours with one switch on low and two switches off (assuming you
have three separate controls). That would be one segment. The next 1 hour
with 2 switches on low. That would be the second segment. The next hour
with 3 switches on low. The fourth segment might be 1 switch on medium and
2 on low. etc. So you are slowly coming up to a temperature you want. How
fast you go from one temperature to the next is the ramp. Typically you go
very slowly until all of the water in the clay has safely escaped from the
clay. From there you can go fast until you get to the point with the clay
changes physical structure. Here you want to again go at a rate where you
do not get sudden fast changes. If you have glazes that form crystals you
want to decrease the temperature slowly at the temperature where the
crystals are forming. So you would not just turn off the kiln when it
reached temperature and leave it. You would monitor it and when it reached
the temperature you wanted to slow the cooling you would have it on low for
a certain period (how you would do this manually would depend on how quickly
your kiln cooled on its own).


I know they use cones in Europe. Go to this site. The person is very nice
and I am sure would answer questions (I would think she could tell you where
and how to get cones). But there you can see what cone 6 glazes look like.
She does a lot of testing. If you can fire at a lower temperature until you
have more experience, I think you would find life easier.

http://www.alisapots.dk/

Donna

P.S. How do you know what temperature you are going to if you don't use
cones? Are you doing this simply on timing given you are not sure if you
are at 1270 or 1250? Communicating hard...


"Bubbles_" wrote in message
...

"DKat" wrote in message
...
I just re-read this and am real confused - is that 1270 C? In which case
you are firing to ~ cone 10?

I do think that my kiln-computer actually shows a bit higher temperature
than the actual, so 1270 might be 1250. I would love to figure out a way
to test the hypothosis. Must ask my teacher on Friday if she has any
suggestions. As I said earlier, I have never heard of folk here using
cones.

Wow that must be hard to get to in a a small hobby kiln. How many hours
does it take? Why are you firing that high? They actually have some
pseudo porcelain at cone 6 (what I fire to).

Well - I stopped watching over the kiln when it is firing, but I would say
something close to 10-12 hours to get to top heat. But that is with my
firing schedule as well. Actually, I use 2 different ones for high glaze
and have been adjusting them a tad each time because of the problems with
the bubbles.

This is my typical firing scedule (the hold brings it up to cone 6)

I don't understand what you mean by segment, sorry!

You fire to almost 1200 - my glazes are meant for about 1220 to 1270
according to the lables.

I only have 1 hold on my computer, and that is at top temp. I vary between
20 and 30 minutes - having tried 40 once, to no avail.

Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr)
72(22.2)
1 100(37.8) 220(104.4) 0
1.48
2 350(176.7) 2000(1093.3 0
5.09
3 108(42.2) 2185(1196.1) 0.33
1.71
4 -500(-295.6) 1900(1037.8) 0.17
0.57
5 -125(-87.2) 1400(760) 0
4.00

13.34

D - thank you so much for taking the time to help me try to figure out
this problem! You are an angel, as are all the folks on here who have such
patience with a pseudo-beginner like myself!

Hugs!

Marianne

"DKat" wrote in message
...
Let me see if I can rephrase into common ground. There are low fired,
mid-fired and high-fired pottery. Low fire is typically your earthenware,
raku, majolica, etc. Mid-fire can be functional stoneware, high fire is
stoneware and porcelain of the translucent variety. Mid fire is what many
people with small electric kilns are now working it. Your clay is
vitrified (if it is a true midfire clay) at midfire range. It is not at
lowfire . Typically the midfire range is cone5 to cone7 or 1183C to 1237C
if you are firing @ ~60C/hr. If I were to soak at peak temperature I would
fire to a lower temperature because the heat work would take me to the
cone I want. The cone is a clay made to 'melt' with a certain heat work.
What I want to know is when the clay and glaze is mature and this happens
with heat work rather than just a certain temperature. That is why cones
are important.

Hi D!

I fire to 1250 and prefer this because of the life in those glazes and that
the pottery is properly vitrified, so it can be used in the garden and I
don't have to take it inside in the winter.

The mid-fire glazes I have seen up until now have been a bit dull for my
tastes. I typically only use max 2 glazes (1 inside the bowl, one outside -
possibly 1 on the rim) on my pieces, so I really like for the glaze itself
to add life to the pot.

Segments in my firing are simply different periods in the firing - so if
you are running your kiln manually (without a computer program) you might
run the first 6 hours with one switch on low and two switches off
(assuming you have three separate controls). That would be one segment.
The next 1 hour with 2 switches on low. That would be the second segment.
The next hour with 3 switches on low. The fourth segment might be 1
switch on medium and 2 on low. etc. So you are slowly coming up to a
temperature you want. How fast you go from one temperature to the next is
the ramp. Typically you go very slowly until all of the water in the clay
has safely escaped from the clay. From there you can go fast until you
get to the point with the clay changes physical structure. Here you want
to again go at a rate where you do not get sudden fast changes. If you
have glazes that form crystals you want to decrease the temperature slowly
at the temperature where the crystals are forming. So you would not just
turn off the kiln when it reached temperature and leave it. You would
monitor it and when it reached the temperature you wanted to slow the
cooling you would have it on low for a certain period (how you would do
this manually would depend on how quickly your kiln cooled on its own).

I have a thermostat in my kiln. The controller is quite simple, and only
lets me put in 4 requirements.
time until
temperature (650 C always)
temperature to be reached
soak time

This has worked fine for glazes I use without this special "foredler"
(beautifier) on top. But the beautifier changes the glaze immensly and makes
it so lively with flares and beautiful highlighting colors (such as bright
blue in the black of the glaze).

I know they use cones in Europe. Go to this site. The person is very
nice and I am sure would answer questions (I would think she could tell
you where and how to get cones). But there you can see what cone 6 glazes
look like. She does a lot of testing. If you can fire at a lower
temperature until you have more experience, I think you would find life
easier.

http://www.alisapots.dk/

WOW! I have driven through Aabenraa and even stayed there once! Every summer
when we drive up to Norway, we pass through Aabenraa! I will definitely get
in touch with her. Her homepage is really great as well - and how generous
of her to share all about her glazes!

P.S. How do you know what temperature you are going to if you don't use
cones? Are you doing this simply on timing given you are not sure if you
are at 1270 or 1250? Communicating hard...

The thermostat in the kiln. Although this isn't 100% accurate. But if I have
been firing to 1250 and I increase to 1270, I know that even if 1250 is
really 1230 in the kiln, then I am increasing the temperature by about 20
degrees. Thus the new temp might in fact be 1250, but the important thing (I
thought) was to know what change I am making, and not necessarily the exact
temperature.

Also - the clay sinters at 1220 C - but can be fired at everything from 1000
to 1300 degrees. That means that I am well within the parameters as far as
that goes.

The glazes are for 1250 C.

Thanks for your time, D. I think that D stands for DEAR! :-)

Marianne




"Bubbles_" wrote in message
...

"DKat" wrote in message
...
I just re-read this and am real confused - is that 1270 C? In which case
you are firing to ~ cone 10?

I do think that my kiln-computer actually shows a bit higher temperature
than the actual, so 1270 might be 1250. I would love to figure out a way
to test the hypothosis. Must ask my teacher on Friday if she has any
suggestions. As I said earlier, I have never heard of folk here using
cones.

Wow that must be hard to get to in a a small hobby kiln. How many hours
does it take? Why are you firing that high? They actually have some
pseudo porcelain at cone 6 (what I fire to).

Well - I stopped watching over the kiln when it is firing, but I would
say something close to 10-12 hours to get to top heat. But that is with
my firing schedule as well. Actually, I use 2 different ones for high
glaze and have been adjusting them a tad each time because of the
problems with the bubbles.

This is my typical firing scedule (the hold brings it up to cone 6)

I don't understand what you mean by segment, sorry!

You fire to almost 1200 - my glazes are meant for about 1220 to 1270
according to the lables.

I only have 1 hold on my computer, and that is at top temp. I vary
between 20 and 30 minutes - having tried 40 once, to no avail.

Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr)
72(22.2)
1 100(37.8) 220(104.4) 0
1.48
2 350(176.7) 2000(1093.3 0
5.09
3 108(42.2) 2185(1196.1)
0.33 1.71
4 -500(-295.6) 1900(1037.8) 0.17
0.57
5 -125(-87.2) 1400(760) 0
4.00

13.34

D - thank you so much for taking the time to help me try to figure out
this problem! You are an angel, as are all the folks on here who have
such patience with a pseudo-beginner like myself!

Hugs!

Marianne

On Thu, 11 Oct 2007 02:45:36 +0200, "Bubbles_"
wrote:


"Bob Masta" wrote in message
...

No, this aluminum should not cause any problem at all. Note that
aluminum oxide melts at a very high temperature (stiffens glazes),
whereas whatever is causing the bubbles is something that is actually
boiling, or more likely burning, at your firing temperatures. The
combustion gasses are released as bubbles that appear in the
slow-to-heal glaze. (There are other possibilities, like release of
chemically-bound gasses from some minerals, which don't actually
constitute "burning" as such, but the result is the same.)


Hi Bob

If aluminium stiffens glazes, couldn't it be that the release of it from the
clay, trying to get out through the glaze - causes the glaze to stiffen and
thus the bubbles not to melt back "in position"?

As Dewitt mentioned, aluminum oxide is very common in clays and
glazes, and the amount from your oxidized wheel is trivial by
comparison. However, even if you had a lot more (say, if you
deliberately worked powdered alumina into the clay or something),
you'd find that alumina by itself wouldn't melt at kiln temperatures
without a flux. It wouldn't "release" from the clay.

Alumina that was on the surface and mixed with glaze fluxes
would retard their melting in the mix zone. My guess is that
this surface alumina would not migrate far up into the glaze
itself... I don't think it would dissolve in it very easily and raise
its overall melting (freezing) point, unless you held the kiln
at a high temperature that would keep the glaze molten long
enough to allow the alumina to mix. my guess is that if you
held it that long, assuming the glaze didn't run off the pot,
that all the bubbles would have time to work out through
the molten glaze, anyway.

But to reiterate, I don't think alumina is your problem at all.

I know that may be a stupid question, but I have found that I only learn if
I ask questions about things I don't understand :-)

Marianne


More than one instructor has said that "the only foolish question
is the one you don't ask"!

Best regards,



Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!