Louis Katz on sun 24 apr 05
Yes. All clay is porous to some extent. Even fired stoneware. Re-fires
that have been exposed to water can have pores full of water. Because
the clay is harder it holds the pressure longer and when it goes it
does so more violently and puts stuff everywhere. It the refires neve
get wet it is not a problem.
It can be particularly difficult to come to grips with the reality
here. Reglazed re-fires have to be dried (fired) maybe even more
carefully than greenware if you want to never blow one up. Smaller
pores, some nearly closed.
We always used to put them at the top of the kiln, and then only
hestantly. Haven't blown one up in years though.
Louis
On Apr 24, 2005, at 9:14 PM, Randy McCall wrote:
> I just lost 30 pieces in a glaze firing because one piece blew up into
> a millon pieces. I a trying to figure out what happened. I use
> Desert Bluff from Highwater, but had refired a piece of Standard 112.
> The Standard 112 had I think been refired once or twice before. My
> question is this can refiring pieces cause them to shatter like this.
> I am pretty sure it was the Standard that exploded as I can see the
> specs in the pieces.
>
> Has this happened to anyone else before? I am really discouraged and
> want to prevent this from happening again. I had a lot of work and
> expense in this firing.
>
> _______________________________________________________________________
> _______
> 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.
>
>
Louis Katz
http://www.tamucc.edu/~lkatz
Earl Brunner on sun 24 apr 05
I once had some mugs that fired poorly and were laying around outside the
kiln shed in the snow awhile, I had some empty spots in the kiln and
figured, "what the heck......." One of the worst mistakes ever. They had
soaked up too much moisture and high fired clay is too dense to let it out
fast. Tiny, tiny shards ALL OVER the kiln inside the other pots.
The only disasters worse where time I turned on the burners on my propane
fired kiln BEFORE I lit the paper taper to stick under the kiln to light it.
Blew the stacked brick door right off the kiln on top of me; thank goodness
for soft brick :)
Then there was the time my friend cut insulating fire brick into shelf
supports for his first firing in his new kiln because his shelf supports
were back ordered. He didn't leave them hefty enough and they crumbled and
collapsed the whole stack under the heat and weight.
But yeah, if the high fired piece has enough moisture in it and you heat it
too fast it will happen.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Randy McCall
Sent: Sunday, April 24, 2005 7:15 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Firing Disaster?
I just lost 30 pieces in a glaze firing because one piece blew up into a
millon pieces. I a trying to figure out what happened. I use Desert Bluff
from Highwater, but had refired a piece of Standard 112. The Standard 112
had I think been refired once or twice before. My question is this can
refiring pieces cause them to shatter like this. I am pretty sure it was
the Standard that exploded as I can see the specs in the pieces.
Has this happened to anyone else before? I am really discouraged and want
to prevent this from happening again. I had a lot of work and expense in
this firing.
Randy McCall on sun 24 apr 05
I just lost 30 pieces in a glaze firing because one piece blew up into a =
millon pieces. I a trying to figure out what happened. I use Desert =
Bluff from Highwater, but had refired a piece of Standard 112. The =
Standard 112 had I think been refired once or twice before. My question =
is this can refiring pieces cause them to shatter like this. I am =
pretty sure it was the Standard that exploded as I can see the specs in =
the pieces.
Has this happened to anyone else before? I am really discouraged and =
want to prevent this from happening again. I had a lot of work and =
expense in this firing.
Snail Scott on mon 25 apr 05
At 10:14 PM 4/24/2005 -0400, you wrote:
>I just lost 30 pieces in a glaze firing because one piece blew up into a
millon pieces. I a trying to figure out what happened. I use Desert Bluff
from Highwater, but had refired a piece of Standard 112. The Standard 112
had I think been refired once or twice before...
When you fired it before, to what temperature did you
fire? If it was fired close to maturity, or if it had
a vitreous (glaze) or semi-vitreous (underglaze)
coating, this can cause potential problems. Even work
that's only been fired to bisque temps can be trouble
if it has a coating that's started to sinter, even if
it's nowhere close to maturity.
A piece that's been fired close to its peak but which
still has some absorption can take on moisture after
firing, especially if it's re-glazed. The moisture
which escapes to readily from bisqued clay has a much
tougher time getting out of a tighter, more vitrified
body.
Even worse can be re-firing a piece that's been bisqued
with full surface coverage of underglazes or engobes.
They can sinter and begiin to vitrify at bisque temps,
which is one reason people fire them that way. However,
if the piece is glazed before re-firing, the moisture
from the glaze can get into the absorbent clay underneath,
and have a very tough time getting back out through the
semi-vitrified underglaze. (The same idea applies to glaze
over unvitrified clay.) It's like a cracked headlight -
moisture can get in more easily than it can escape.
I've seen several previously-fired pieces (both bisque
and higher-temp) pieces blow up due to moisture trapped
under a glaze or underglaze layer. Re-glazed pieces
(or pieces glazed over an all-over bisqued coating of
underglaze) need LOTS of force-drying, for a long time.
-Snail
Eva Gallagher on mon 25 apr 05
Hi Randy - The only time that pots have exploded for me in a glaze firing
has been if they have been refired. Then mostly it's those pots that have
been sitting around for a while and have had water put in them. If the glaze
is a bit crazed and the body not fully vitrified water gets into the body
and explodes when heated up in the kiln. (Not sure why it does not do that
in the microwave - unless in the microwave the water heats up evenly from
the centre of the wall?). So I'm always nervous about refiring old pots -
maybe a slow heating up in the kitchen oven overnight might be a safeguard.
Eva Gallagher
Deep River Potters' Guild
----- Original Message -----
From: "Randy McCall"
To:
Sent: Sunday, April 24, 2005 10:14 PM
Subject: Firing Disaster?
I just lost 30 pieces in a glaze firing because one piece blew up into a
millon pieces. I a trying to figure out what happened. I use Desert Bluff
from Highwater, but had refired a piece of Standard 112. The Standard 112
had I think been refired once or twice before.
______________________________________________________________________________
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.
Vince Pitelka on mon 25 apr 05
> I am very surprised at what I am reading. I have re-fired and Multi-fired
> pots many,many times and I have NEVER had any problems.
Mark or Cindy -
Nothing to be surprised about. No one said that you couldn't refire already
glaze-fired wares. Pots don't "explode" in refiring unless they have
entrapped moisture. In general, I feel that it's usually better to just
make more pots rather than spending time refiring those that didn't turn out
well, but I know that there are always exceptions.
High-fired pots have little thermal-shock resistance in comparison to
bisque-fired pots, and therefore when refiring, it is best to place any
previously high-fired pieces in the center of the set, where they will be
heated more gradually. If exposed to sudden heat, a bisque-fired pot on
it's way up to glaze firing will be unharmed, while an already high-fired
pot would likely crack. But it won't explode unless it contains moisture.
As explained in other posts, when previously high-fired pots truly explode
during re-firing, it is because of impacted moisture locked in the vitrified
clay. I can think of no other rational explanation.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Mayssan Shora Farra on mon 25 apr 05
On Sun, 24 Apr 2005 22:14:43 -0400, Randy McCall wrote:
but had refired a piece of Standard 112. The Standard 112 had I think
been refired once or twice before. My question is this can refiring pieces
cause them to shatter like this. has this happened to anyone else before?
.
Hello Randy:
It did to me. didn't hurt anything as it was a flat platter on a shelf by
itself and it broke in a cleant line in 2 pieces that were 2" apart when I
picked them up. But it was a refiring and on furthere inspection, the crack
must have been in there after the first firing and when I redipped, it just
helped it separate that far.
It was Standards 112 which is sometimes finicky but beautiful and I don't
refire it anymore.
Best regards,
Mayssan Shora Farra, in slowly climbing back into spring weather again
Charelston WV
http://www.clayvillepottery.com
John Rodgers on mon 25 apr 05
Randy,
In my on sad experience, I have learned the hard way to heat the ware
over night on very low with electric kilns - or candle overnight with
gas kilns - in order to heat dry the ware before ramping up the
temperature. Ever since I began to do this, I have had no explosions.
Prior to my getting the message about this, I blew up stuff all the
time. Entrapped moisture turning to steam is the culprit, and you simply
must drive that moisture out before ramping to temperature.
I perceive that the entrained moisture begins to be forced out through
the pores of the piece as it heats. At room temperature most of the
moisture simply works it way out of the clay through the pores. However,
when the temperature is ramped up the water molecules are pushed out at
a rate faster than the pores are capable of handling and the result is a
pressure vessel effect. The pressure of the water in the clay begins to
rise as the temperature increases. The physical confinement of the water
as the temperature rises results in the water beginning to "Super Heat",
or go above the boiling point of water, yet remain in liquid state
between the interstices of the clay. When the temperature of the liquid
water reaches some critical point above boiling, the water suddenly
turns to steam. Somewhere in the clay, under the increased pressure,
there will be a small failure in the clay structure, and the sudden but
tiny pressure reduction results in the superheated water turning to
steam. This is called a steam explosion, this sudden release of
pressure. When this happens, the pot shatters.
I have had pieces drying for weeks and weeks in the summer heat here in
the south only to have them explode into a millions pieces when bisque
fired. After a lot of deductive reasoning I came to the conclusion that
the piece still had moisture equal to the moisture in the air - which is
quite high in the south in the summertime - and that amount of moisture
was sufficient to explode the piece, even when the pot felt completely
dry. So, I began to heat my work overnight on low, before ramping the
temperature next day. I have not had an exploded pot since.
Load your kiln, prop the door or lid, candle or set to low overnight,
and ramp heat the next morning.
Works for me.
Regards,
John Rodgers
Chelsea, AL
Randy McCall wrote:
>I just lost 30 pieces in a glaze firing because one piece blew up into a millon pieces. I a trying to figure out what happened. I use Desert Bluff from Highwater, but had refired a piece of Standard 112. The Standard 112 had I think been refired once or twice before. My question is this can refiring pieces cause them to shatter like this. I am pretty sure it was the Standard that exploded as I can see the specs in the pieces.
>
>Has this happened to anyone else before? I am really discouraged and want to prevent this from happening again. I had a lot of work and expense in this firing.
>
>______________________________________________________________________________
>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.
>
>
>
>
Vince Pitelka on mon 25 apr 05
Randy wrote:
"I just lost 30 pieces in a glaze firing because one piece blew up into a
millon pieces. I a trying to figure out what happened. I use Desert Bluff
from Highwater, but had refired a piece of Standard 112. The Standard 112
had I think been refired once or twice before. My question is this can
refiring pieces cause them to shatter like this. I am pretty sure it was
the Standard that exploded as I can see the specs in the pieces."
Randy -
I can only imagine what you experienced upon opening that kiln. I have been
there, and I know what came out of my mouth, but more importantly, I know
what went through my head. It is a terrible thing to open a kiln and
discover your hard work ruined.
This happens most often when the piece in question has absorbed moisture.
The most spectacular (in the worst possible way) ware explosions I have ever
seen are of the kind that Earl Brunner described - where the ware have
absorbed moisture from being left outside or being used in the kitchen. I
knew a guy once who had a beloved teapot that he used every day, but he
wasn't QUITE satisfied with the glaze, and decided to stick it back in the
kiln. He just didn't know better. Well, as in the scenario that Earl
described, this teapot exploded into about a thousand shards, which
liberally dispersed themselves on top of and inside all the pots in the
kiln, ruining almost everything.
We have a two-chamber bourry-box wood kiln with a primary firebox off the
end of the kiln and a secondary firebox between the two chambers. When I
came to the Craft Center almost 11 years ago there was a 24"-tall
blue-glazed wood-fired pot sitting on top of the primary firebox. The
charging port is at the end, so this pot was not in the way, and it had
become a permanent fixture. It was there for years. However, in one
particularily horizontal rainstorm, the rain came in under the roof and a
fair amount accumulated inside this vase, and sat there, soaking into the
clay. Several weeks later we fired the kiln. We got the first chamber up
to temperature and sealed up the primary firebox, and had just moved to the
secondary firebox halfway down the front of the kiln, when that pot
exploded. The pieces ricocheting off the metal roof and adjacent building
walls sounded like bullets. I hate to think of what might have happened if
we had still been gathered around the primary firebox when that pot let go.
Good luck on your next firing. Be cautious with re-fires.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Charles Moore on mon 25 apr 05
Amen to Mayssan's email.
"it was a refiring and on furthere inspection, the crack must have been in
there after the first firing and when I redipped, it just helped it separate
that far."
I have found that some clays tend to bloat or blister when refired to ^6. I
do not have an explanation, except to say that the clay that most frequently
does bloat is probably really a ^5 clay, where it does beautifully
Charles
Sacramento
Mark Pitney on mon 25 apr 05
Hi all,
I am very surprised at what I am reading. I have re-fired and Multi-fired
pots many,many times and I have NEVER had any problems. I always spray the glaze
on for a re-glaze as I have found I get a much better adhesion but I have
gone from the spray booth to the kiln and never had a problem. I am also
wondering about multi firing? Basically the difference is re-glazing wasn't planned
where as multi-glazing is planned. Based on what I am reading it would mean
that multi-firing could or should give these same blow ups?
Mark and Cindy
The Mudpit
228 Manhattan Ave.
Brooklyn,NY. 11206
_www.mudpitnyc.com_ (http://www.mudpitnyc.com/)
718-218-9424
mudpitnyc@aol.com
Veena Raghavan on tue 26 apr 05
Quite sometime ago, I asked everyone on Clayart about refiring a casserole at
cone 6. It had been fired at that temp, but the glaze had not come out well.
It was for my daughter, and she asked if I could reglaze it. It had been
washed.
I was advised to heat it up slowly in the oven a few times before glazing and
refiring. I did this and it was refired successfully.
I was very grateful for the advice I got from several people on Clayart. No
explosion, just a nicely refired casserole. I hope this is of some help.
Veena
VeenaRaghavan@cs.com
Jennifer Boyer on tue 26 apr 05
I have had stoneware platters split in half on refiring. I tend to
agree with Vince that for myself it makes more sense to make more pots
rather than refire. But I don't have any glaze combos that rely on
multiple firings to get the right colors.
I can get a good price for seconds in my studio sales, so I keep my
kiln space full of new pots....
I tend to use fine grained clay with small grog, and I've always had
the gut feeling that a refired pot would be more susceptible to
thermal shock once it was sold to a customer that used it in the
oven, of doe hot beverages.
Am I being overly paranoid?
Jennifer
On 4/25/05, Vince Pitelka wrote:
> > I am very surprised at what I am reading. I have re-fired and Multi-fir=
ed
> > pots many,many times and I have NEVER had any problems.
>=20
> Mark or Cindy -
> Nothing to be surprised about. No one said that you couldn't refire alre=
ady
> glaze-fired wares. Pots don't "explode" in refiring unless they have
> entrapped moisture. In general, I feel that it's usually better to just
> make more pots rather than spending time refiring those that didn't turn =
out
> well, but I know that there are always exceptions.
--=20
***********************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT 05602
Snail Scott on thu 28 apr 05
At 02:23 PM 4/25/2005 EDT, you wrote:
>Hi all,
>I am very surprised at what I am reading. I have re-fired and Multi-fired
>pots many,many times and I have NEVER had any problems...Based on what I
am reading it would mean
>that multi-firing could or should give these same blow ups?
A lot comes down to how much moisture gets absorbed
into the clay body UNDER the previously-fired glaze
or underglaze. (Crazing cracks or gaps in coverage
are potential entry points, as is any semi-vitrified
coating.) Spraying (as you say) tends to put less
water onto the pot than dipping, and doesn't tend to
hit the exposed clay of the foot, either, so it's
probably the best way to reglaze if it's a problematic
piece. If it's vitrified clay underneath, though, I
wouldn't worry much; the greater hazard is from under-
vitrified clay. (Special caution here to any folks
who think that their clay is vitrified, but who fire
a "^5-10" clay body only to the lower end of that range!
Most earthenware folks know their clay is absorbent,
but some mid-range folks don't realize how absorbent
an underfired stoneware body can be. If it says ^5-10,
it's NOT gonna be vitrified at ^6. Functional, maybe,
but NOT vitrified.)
-Snail
Jennifer Boyer on sun 1 may 05
Hi Ron,
So are you saying that if I get a pot through a refire successfully it
will be as oven proof as a pot that's been through one glaze firing?
I'm using a gine grog white stoneware from Sheffield called 20231G.
Jennifer
On 4/30/05, Ron Roy wrote:
> Hi Jennifer,
>=20
> What you are describing is different from exploding pots.
>=20
> Yours happended becuase of thermal shock - sides of platter heat up faste=
r
> than the center - if there is cristobalite in your clay (what kind you
> using) that would explain it. If your inside glaze had a too low CTE that
> would do it.
>=20
> If you refired really slow it probably will not be a problem - it's uneve=
n
> heating that is the problem I think.
>=20
> RR
>=20
> >I have had stoneware platters split in half on refiring. I tend to
> >agree with Vince that for myself it makes more sense to make more pots
> >rather than refire. But I don't have any glaze combos that rely on
> >multiple firings to get the right colors.
> >
> >I can get a good price for seconds in my studio sales, so I keep my
> >kiln space full of new pots....
> >
> >I tend to use fine grained clay with small grog, and I've always had
> >the gut feeling that a refired pot would be more susceptible to
> >thermal shock once it was sold to a customer that used it in the
> >oven, of doe hot beverages.
> >
> >Am I being overly paranoid?
> >
> >Jennifer
>=20
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>=20
>=20
--=20
***********************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT 05602
| |
|
