Bruce Girrell on mon 21 jan 02
Being one of a curious nature, I continued looking for appropriate kiln
window material even after the original thread died out.
Fused silica is about the only material up to the task, though it comes in
many forms with various additives, hydroxyl content, bubble content, color,
light transmission characteristics...
At first I thought that anything clear with a high enough melting point
would work, but I forgot about one thing - we're dealing with a glass.
Glasses have another critical temperature called the annealing point. As
glass is cooled and solidifies, it must be annealed to even out internal
stresses before it can be completely cooled. That is, as glass is cooled
from the molten state, it is held at a specific temperature for a period of
time before the remainder of the cooling to allow internal stresses to be
relieved. If the glass is heated to a temperature above the annealing point
it must be annealed again during cooling or the internal stresses build up
again.
And there is the problem. The annealing point of commercially available
fused silica products ranges from about 1000 °C to 1220 °C. So even with the
purest, most hydroxyl-free fused silica, we're barely at ^6 before hitting
the annealing point. The engineers with whom I spoke said that in high
temperature applications the observation ports are usually cooled with a
stream of lower temperature air moving past the glass to keep the window
below the annealing point.
There is also a problem with devitrification - the conversion of fused
silica into cristobalite at temperatures above 1000 °C. High purity and low
hydroxyl content minimizes this problem, though.
One sales rep (thank you Abrisa!) sent me a sample of Robax, a fused
silica/ceramic glass with a pale yellow-pink color to it. For you photo
freaks out there, it's about the color of an 81A warming filter. The sample
is 3 1/2" by 5" and he suggested that I try temperature cycling it a number
of times to see if it can handle the heat.. I will have to tear down part of
a kiln wall to install the glass so it may be a little bit until my next
update. Till then, happy potting, and don't singe too many eyebrows peering
into peep holes.
Bruce "too hot to handle" Girrell
Lynne Girrell on sun 6 jul 03
OK, so I got an index card sized sample piece of high temperature glass from
a company and installed it in the wall of our kiln. The window was installed
such that one side of the glass was always exposed to the kiln interior and
the other side was protected by a piece of IFB that could be removed for
viewing the kiln interior.
Now, for those of you who are not familiar with the work that Lynne and I
do, the max temperature that this kiln is subjected to most of the time is a
mere 700-800 ºC. Our bisque is done in a different kiln. This one was used
only for heating the pieces prior to horsehair decoration during the entire
time that the window was installed. In fact, I really did not use the window
much at all. The original intenet was to use it for observation of raku
glaze melting. The kiln had been previously used for raku firing, a point
that may be important when I discuss the results. The window was subjected
to about ten firings, though during each firing, the kiln might be taken up
to temperature and cooled down ten to twenty times.
You can see that we were really pretty gentle on our little test window. As
I dismantled our little kiln in preparation for building a new kiln, I
examined the window. I was very surprised at its condition. The window is
now warped. It's not a huge amount. It's bowed to one side in the center by
maybe 0.02 inches, but the annealing temperature of this glass indicated
that it should be able to withstand repeated firing to cone 6. I wouldn't
expect it to deform at such low temperature.
Worse, though, is that the window now has a frosted appearance. I could not
remove the frosting using a kitchen "scrubee" pad. At 40x magnification I
could not discern any particular character to the surface. I will examine it
at higher magnification soon.
There are a couple of possibilities for the frosted appearance of the glass.
One is that combustion products reacted with the glass in some manner.
Another possibility - one that I believe to be more likely - is that, since
this kiln had been used for firing raku pieces, remnants of glaze chemicals
either deposited on the glass during firing or reacted with the glass during
firing. I'm open to suggestions for other possibilities.
Regardless of the frosting mechanism, it is quite likely that similar
results would occur at high temperature as well. Quite possibly the severity
of frosting would be worse at elevated temperatures. We used very common
raku glaze formulations relying primarily on copper for color. We did not
use any copper mattes.
This is some pretty bad news for those who would want a window in a kiln.
Warping is one thing but losing the transparency of the window pretty much
renders it useless. I will see if I can determine the cause of the frosting.
There may be a way to avoid the problem or the problem may not occur with
other glass formulations. BTW, The only practical possibility for a window
for a cone 10 kiln is sapphire. Thankfully, I did not spring for a sapphire
window for this test. But then maybe sapphire is immune to the frosting
problem.
Bruce "and that's the news from Lake Wobegon" Girrell
Tom Buck on mon 7 jul 03
Bruce:
strange that the glass window should frost so soon. your description
suggests "etching" rather than "laydown" ....higher mag may reveal which.
if the glass is etched, then the culprit that tops the list is
hydrogen fluoride, HF, and most claybodies and some glaze materials
contain fluorides in small amounts, up to 1%w. sadly the pyrochemistry of
ceramic fluorides is seldom discussed on open forums like Clayart because
fluoro-glasses are vital to certain high-tech end-uses. even tech
literature and patent info tends to reveal almost zilch.
but I agree with you, if this frosting effect is in-built, then
using a glass window on any kiln will soon have a zero benefit.
til later. peace. Tom.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
John Rodgers on mon 7 jul 03
I use to think about how nice it would be to have a window in a kiln.
Then I reflected on the fact that the Space Shuttle has the "latest and
greatest" as windshields so far as glass that resists heat goes, and yet
they still have to use ablative materials to form a protective gaseous
layer to protect the windshield and front end of the Shuttle from heat
on re-entry into the atmosphere from orbit. These re-entry temperatures
would approximate or be greater than hi-fire temps in kilns. The
ablative materials are materials deliberately designed to burn off and
produce a gaseous layer to protect from higher heat.
I suspect having a truly clear glass capable of withstanding pottery
kiln operating temperatures is still out of reach. But maybe in the
future technology will produce such a thing.
As far as frosting of the glass, the glass on the windows in my
porcelain studion frosted in time with all the porcelain firings that
took place over the years. There was definitely some kind of reaction
that took place between the glass and what ever was put into the air
during firing. Doesn't speak well for being in the room while firing the
kilns.
My $0.02
John Rodgers
Birmingham, AL
Arnold Howard on mon 7 jul 03
Paragon makes a small jewelry kiln with an optional 2" square glass
window. I haven't heard complaints about a frosted surface on the glass.
Jenken also makes glass fusing kilns with glass windows.
The type of kiln window glass we use is rated to only 1700°F.
Maybe your glass would not frost if you used a downdraft vent in the
kiln.
Sincerely,
Arnold Howard
Paragon Industries, L.P.
www.paragonweb.com
From: John Rodgers
> I use to think about how nice it would be to have a window in a kiln.
> Then I reflected on the fact that the Space Shuttle has the "latest
and
> greatest" as windshields so far as glass that resists heat goes, and
yet
> they still have to use ablative materials to form a protective gaseous
> layer to protect the windshield and front end of the Shuttle from
heat
> on re-entry into the atmosphere from orbit. These re-entry
temperatures
> would approximate or be greater than hi-fire temps in kilns. The
> ablative materials are materials deliberately designed to burn off
and
> produce a gaseous layer to protect from higher heat.
>
> I suspect having a truly clear glass capable of withstanding pottery
> kiln operating temperatures is still out of reach. But maybe in the
> future technology will produce such a thing.
>
> As far as frosting of the glass, the glass on the windows in my
> porcelain studion frosted in time with all the porcelain firings that
> took place over the years. There was definitely some kind of reaction
> that took place between the glass and what ever was put into the air
> during firing. Doesn't speak well for being in the room while firing
the
> kilns.
>
> My $0.02
>
> John Rodgers
> Birmingham, AL
Les on mon 7 jul 03
I remember when John Leach was in Vancouver for a workshop, we discussed his
wood-fired kiln at some length. I may be wrong, but my memory tells me that
John has a thinck glass peephole to see into the kiln whist his pottery is
being fired.
Possibly some one that has been to John's studio would have more intimate
knowledge on this.
Les Crimp on Vancouver Island.
lcrimp@shaw.ca
----- Original Message -----
From: "John Rodgers"
To:
Sent: Monday, July 07, 2003 9:32 AM
Subject: Re: Windows in kilns - an update
> I use to think about how nice it would be to have a window in a kiln.
> Then I reflected on the fact that the Space Shuttle has the "latest and
> greatest" as windshields so far as glass that resists heat goes, and yet
> they still have to use ablative materials to form a protective gaseous
> layer to protect the windshield and front end of the Shuttle from heat
> on re-entry into the atmosphere from orbit. These re-entry temperatures
> would approximate or be greater than hi-fire temps in kilns. The
> ablative materials are materials deliberately designed to burn off and
> produce a gaseous layer to protect from higher heat.
>
> I suspect having a truly clear glass capable of withstanding pottery
> kiln operating temperatures is still out of reach. But maybe in the
> future technology will produce such a thing.
>
> As far as frosting of the glass, the glass on the windows in my
> porcelain studion frosted in time with all the porcelain firings that
> took place over the years. There was definitely some kind of reaction
> that took place between the glass and what ever was put into the air
> during firing. Doesn't speak well for being in the room while firing the
> kilns.
>
> My $0.02
>
> John Rodgers
> Birmingham, AL
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Bruce Girrell on tue 8 jul 03
> Maybe your glass would not frost if you used a downdraft vent in the
> kiln.
One of the suggestions that I got from the glass supplier was to have an air
curtain of sorts - a vent - on the hot face of the glass. I got the
impression while talking with him that this was to help keep the surface
temperature of the glass lower. Perhaps it has more than one purpose.
Bruce "cool it" Girrell
Bruce Girrell on tue 8 jul 03
> > As far as frosting of the glass, the glass on the windows in my
> > porcelain studion frosted in time with all the porcelain firings that
> > took place over the years. There was definitely some kind of reaction
> > that took place between the glass and what ever was put into the air
> > during firing.
That's interesting. We use a porcelain body as our clay.
Bruce Girrell
Eric B on tue 8 jul 03
there is a type of glass that i've seen advertised as the type they use on
some of the better wood stoves (so people can see the fire). one wood stove
store near me (in half moon bay calif) actually has a photograph depicting the
glass's resistance to thermal shock. it shows a plate of the glass with a fire
underneath it and cold water being poured on top of it, with no breakage! i
can't verify the "truth" in that advertising, but if it's legit then it sounds
like the type of stuff that you could use as a port-hole on a kiln. the owner
of the store told me that the material was invented for the space shuttle.
so, i'd suggest going to the nearest store that specializes in wood stoves and
asking about the manufacturer. if anyone who needs it is close enough that
they'd like the name and telephone of that store let me know and i'll go look it
up.
btw -- a glass peep window on a kiln sounds really cool! i would imagine one
could use it to view cones without the usual blowing and squinting (and maybe
retinal damage) that we have to go through now to view cones.
buena suerte.
eric
SpunMud
Steve Mills on wed 9 jul 03
Don't know about John's Kiln, but I had glass peep holes in the door of
my 30 cube gas reduction kiln from '77 to '84 when I had to move
workshops due to excessive rent! The glass came from the smokeless fuel
stove in my Mother's house; the door was made up of strips of the stuff
about 2 ins wide & 10 ins long. The spy windows in my Kiln were mounted
on the ends of tubular props used as spy holes. The Kiln was Fibre
lined. The props were 3 ins long with a 1.25 inch (+) hole down the
middle. and the glass lasted as long as the Kiln which was fired on
average once a month.
Steve
Bath
UK
In message , Les writes
>I remember when John Leach was in Vancouver for a workshop, we discussed =
>his
>wood-fired kiln at some length. I may be wrong, but my memory tells me t=
>hat
>John has a thinck glass peephole to see into the kiln whist his pottery i=
>s
>being fired.
>
>Possibly some one that has been to John's studio would have more intimate
>knowledge on this.
>
>Les Crimp on Vancouver Island.
>lcrimp@shaw.ca
>
>
>----- Original Message -----
>From: "John Rodgers"
>To:
>Sent: Monday, July 07, 2003 9:32 AM
>Subject: Re: Windows in kilns - an update
>
>
>> I use to think about how nice it would be to have a window in a kiln.
>> Then I reflected on the fact that the Space Shuttle has the "latest and
>> greatest" as windshields so far as glass that resists heat goes, and ye=
>t
>> they still have to use ablative materials to form a protective gaseous
>> layer to protect the windshield and front end of the Shuttle from heat
>> on re-entry into the atmosphere from orbit. These re-entry temperatures
>> would approximate or be greater than hi-fire temps in kilns. The
>> ablative materials are materials deliberately designed to burn off and
>> produce a gaseous layer to protect from higher heat.
>>
>> I suspect having a truly clear glass capable of withstanding pottery
>> kiln operating temperatures is still out of reach. But maybe in the
>> future technology will produce such a thing.
>>
>> As far as frosting of the glass, the glass on the windows in my
>> porcelain studion frosted in time with all the porcelain firings that
>> took place over the years. There was definitely some kind of reaction
>> that took place between the glass and what ever was put into the air
>> during firing. Doesn't speak well for being in the room while firing th=
>e
>> kilns.
>>
>> My $0.02
>>
>> John Rodgers
>> Birmingham, AL
>>
>>
>_________________________________________________________________________=
>___
>__
>> Send postings to clayart@lsv.ceramics.org
>>
>> You may look at the archives for the list or change your subscription
>> settings from http://www.ceramics.org/clayart/
>>
>> Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
--
Steve Mills
Bath
UK
Bonnie Staffel on thu 10 jul 03
Some time ago I mentioned that my studio windows frosted over a period of
time from firing a clay containing a lot of sulphur. My remark said it was
a reaction of the sulphur to the moisture on the glass, hence sulphuric
acid. This contention was shot down, of course, by those who understand
chemistry better than I. However, it WAS the result of the sulphur fumes in
the studio. The air would be blue during the firing. So I fired overnight
so I wouldn't be exposed to the fumes. The basic element here is that the
glass was single pane. The reaction doesn't happen with the double pane
IMHO because of less exposure to the cold. Of course, we are talking about
wintertime up here in northern Michigan when it happens. I changed clay
bodies after this occurred, but it was too late for the windows, but at
least better for my lungs. All this occurs, of course, with electric kilns
and the fumes come near the end of the firing schedule, both bisque and high
fire.
Regards, Bonnie Staffel, in beautiful northern Michigan where we may get our
first rain in a long time today.
http://pws.chartermi.net/~bstaffel/
iandol on fri 11 jul 03
Dear Bonnie Staffel,=20
Small quantities of Concentrated Sulphuric acid, up to half a gallon, =
are stored and transported in Glass Bottles called "Winchesters". I have =
had a Winchester of the stuff sitting around for several years now. So I =
doubt claims that H2SO4 etch glass.
I do not dispute your knowledge that Gases derived from Sulphate =
chemicals in clay are vented from a kiln. But some clays, (I would be =
loathe to say all clays), just some clays, contain Calcium Fluoride as =
the mineral Fluorite.
Fluorine in the composition of this mineral is also vented from the =
kiln. I understand it reacts with water vapour in the air to give =
Hydrogen Fluoride which is well known for its ability to attack glass, =
and of course, human beings.
You are wise to take care, may you live long and prosper.
Best regards,
Ivor Lewis. Redhill, South Australia =20
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