Lajos Kamocsay on fri 30 nov 01
Hello,
Steven Branfman in his Raku book writes:
"Let me debunk another pottery misconception: pots do not crack and =
explode because of air bubbles. Cracking usually can be traced to clay =
that was still wet when fired." (page 51)
I'm a beginning potter, havn't had things exploding in the kiln, and I =
have yet to "debug" my wedging, so I'm pretty sure I have some fired =
pots with small air bubbles in them.
So is he right that it's not the air bubbles?
Thanks,
Lajos
CINDI ANDERSON on fri 30 nov 01
Yes, he is right! (I can't believe it but I even saw a recent column in Popular Ceramics talk about air bubbles causing explosions, but it is not true.) If something explodes it is
because of moisture. Someone on ClayArt conjectured that maybe a big air pocket would collect moisture as the clay dried and be the last thing to dry out, but it is still moisture.
If you let the clay dry long enough, the only problem with an air bubble is that it can cause your pot to be off center or otherwise throw funny, and a big one left in a wall could
weaken a wall.
Cindi
Fremont, CA
Hank Murrow on fri 30 nov 01
>Hello,
>
>
>Steven Branfman in his Raku book writes:
>
>"Let me debunk another pottery misconception: pots do not crack and
>explode because of air bubbles. Cracking usually can be traced to clay
>that was still wet when fired." (page 51)
>
>I'm a beginning potter, havn't had things exploding in the kiln, and I
>have yet to "debug" my wedging, so I'm pretty sure I have some fired pots
>with small air bubbles in them.
>
>So is he right that it's not the air bubbles?
>
>Thanks,
>Lajos
"air bubbles' as a major evil, is a myth perpetrated by teachers who are
misinformed or impatient in their firings.
Cheers, Hank
Snail Scott on fri 30 nov 01
At 10:22 AM 11/30/01 -0800, Lajos wrote:
>Steven Branfman in his Raku book writes:
>>"Let me debunk another pottery misconception: pots do not crack and
explode because of air bubbles. Cracking usually can be traced to clay that
was still wet when fired." (page 51)>
>So is he right that it's not the air bubbles?
Yes, he's right. The problem with air bubbles isn't the air.
It's that any moisture in the clay will turn to steam when
it gets hot enough, and steam which finds its way into these
air pockets can build up a lot more localized force than
steam spread evenly through the pores of solid clay. Water
vapor will expand to many times its original volume when it
becomes steam. Air also expands when heated, but not nearly
as much, so the pressure of trapped air alone is seldom
sufficient to crack the clay. If the clay is truly dry when
it reaches the boiling temperature of water, small air bubbles
will have no real effect.
How big is a 'small' bubble? It depends on your clay body.
A groggy, coarse clay is more permeable to both air and
steam, and even bubbles a 1/2 inch across may cause no
trouble at all. In a tight, fine-grained clay like porcelain,
such a large bubble can cause trouble, though smaller ones
may still be OK. If you fire slowly enough, even totally
enclosed hollow forms can be fired without an air hole.
How slowly? Again, that depends mainly on the clay body.
-Snail
vince pitelka on fri 7 dec 01
> Let's think about this for a minute. When you bisque, the clay NEVER
> loses porosity. There is nothing to trap the air until the clay
> starts to vitrify and seals over...fairly late in the firing.
> Bloating comes when other materials (like carbonates) off-gas after
> the surface of the clay is completely sealed...which means melted.
Tom -
Actually, when you bisque the clay gains porosity. In the greenware stage
it has very little porosity, especially if still slightly damp, and it
cannot release steam pressure very quickly. The steam gathers in thick
sections or air pockets with sufficient pressure to blow the piece apart.
With thick sections, the piece ususally blows into a zillion tiny shards.
With air pockets, a large section usually blows off.
My best example of this occurs when my intro students do slumped slab masks.
I show them how to add clay for brow ridges, chin, eyes, etc, by scoring and
applying slurry and then smearing on large dabs of clay to build up the
necessary form and thickness. And inevitably there is a student who will
score, apply slurry, and then try to press on one large mass of clay,
inevitably trapping a network of large air bubbles beneath this piece. And
inevitably, this add-on pops back off in the bisque firing. It is usually a
clean break. This can occur because of a single good-sized trapped air
bubble, or from a network of tiny trapped bubbles in the score lines when
insufficient slurry is used. To avoid this, score any leather-hard surfaces
and use plenty of slurry for attachment to leather hard, and smear small
wads of clay in place, building up the desired thickness.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Ron Roy on fri 7 dec 01
Just a bit of additional info on exploding pots - it is not from the air
bubbles - pots explode in a first firing because they are not dry enough -
when the atmospheric moisture and/or water of plasticity is still in the
clay - dry your ware out properly before firing - not cool to the cheek -
or heat below 200F/100C till the water is out.
If you are firing thicker clay this can take a lot longer than you can
imagine. Hold a mirror or some shiny metal at the top spy (with the bottom
spy open) - if you see water condensing on the cool metal or glass there is
still water in the kiln - wait and try again later. Best to do this when
your kiln is well heated - start testing for water about 150F/65C.
The reason some people think it's the air bubbles is because - when the
clay can't take the pressure anymore it comes apart at the weakest point
first - air pockets are weak points - this leads to the false assumption
that the air pocket was responsible - just not true.
Could it be that it only comes apart where there is an air pocket - yes and
it does happen but that is because the clay is weakest there - the cause is
still firing to fast when there is too much water present.
RR
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Cindy Strnad on fri 7 dec 01
Ron wrote:
__________________________________________________
___
The reason some people think it's the air bubbles
is because - when the
clay can't take the pressure anymore it comes
apart at the weakest point
first - air pockets are weak points - this leads
to the false assumption
that the air pocket was responsible - just not
true.
Could it be that it only comes apart where there
is an air pocket - yes and
it does happen but that is because the clay is
weakest there - the cause is
still firing to fast when there is too much water
present.
RR
__________________________________________________
____
Okay, I'll buy that. But a pinhole into the air
pocket is still cheap and good insurance.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Tom Wirt/Betsy Price on fri 7 dec 01
>>>Subject: Re: air bubbles?
| Ron wrote:
| __________________________________________________
| ___
| The reason some people think it's the air bubbles
| is because - when the
| clay can't take the pressure anymore it comes
| apart at the weakest point
| first - air pockets are weak points -
|
| RR
| __________________________________________________
| ____
|
| Okay, I'll buy that. But a pinhole into the air
| pocket is still cheap and good insurance.
|
Let's think about this for a minute. When you bisque, the clay NEVER
loses porosity. There is nothing to trap the air until the clay
starts to vitrify and seals over...fairly late in the firing.
Bloating comes when other materials (like carbonates) off-gas after
the surface of the clay is completely sealed...which means melted.
Tom Wirt
Tom Wirt/Betsy Price on sat 8 dec 01
In the greenware stage
| it has very little porosity, especially if still slightly damp, and
it
| cannot release steam pressure very quickly.
Vince... I thought we were talking about dry clay and air. Not water
and wet clay. Your comment makes no sense. Dry clay is porous to
air. If there is moisture present, the clay will be slightly expanded
and somewhat waterproof...but the air can still move out. The water
molecule is larger than air (gas) molecules.
I'm not sure I can agree with you that the reason a thick applied
section pops off is because of air. Differential expansion or
contraction, pressure when the combined water comes off, the clay
particles not being well enough "welded" at the joint. If the
particles aren't well enough joined to "weld" the joint, expansion in
the heating will be enough to crack the joint. If you've got some
proof that it's the air, I'd like to see it. The slurry/slip itself
is a big cause of bad joints....causing expansion of the clay right at
the joint. As that dries and contracts, a crack line is formed at the
joint because of the shrinkage. Cut way back on the amount of slip
and go to just a dab of vinegar or other flocculant. The press the
pieces together and move the clay back and forth till it "grabs" Then
do whatever luting you want. Just enough wetness to help the
particles blend together.
been there, done that
Tom Wirt
And inevitably there is a student who will
| score, apply slurry, and then try to press on one large mass of
clay,
| inevitably trapping a network of large air bubbles beneath this
piece. And
| inevitably, this add-on pops back off in the bisque firing.
vince pitelka on sun 9 dec 01
> Dry clay is porous to
> air. If there is moisture present, the clay will be slightly expanded
> and somewhat waterproof...but the air can still move out. The water
> molecule is larger than air (gas) molecules.
Tom -
I guess the point is that dry clay is not nearly porous enough to allow
steam pressure to escape.
> I'm not sure I can agree with you that the reason a thick applied
> section pops off is because of air.
I never said that it was because of air pressure. I said that the steam
gathers in air pockets or wherever else it can. The steam pressure can of
course come from atmospheric humidity in the clay, or from dissociation of
chemically combined water. Any air space acts as gathering places for steam
pressure, causing parts to pop off. If the air pockets are alligned along a
seam, such as a joint assembled with inadequate slurry, the applied part
will pop off neat as can be.
> The slurry/slip itself
> is a big cause of bad joints....causing expansion of the clay right at
> the joint. As that dries and contracts, a crack line is formed at the
> joint because of the shrinkage. Cut way back on the amount of slip
> and go to just a dab of vinegar or other flocculant.
This might work for you, but it is contrary to my experience. I have always
had best luck with thorough scoring, application of plenty of slurry, and
then enough pressure in assembly to squeeze out all the excess slurry, which
is of course removed. So there is no problem at all with shrinkage of the
slurry. It really does not take much pressure at all, as long as the slurry
is the right consistency. These joints are pretty much foolproof. I
assemble stiff-slab forms this way with no internal reinforcing, and fire
them in ^10 reduction, salt, and wood with no problems.
For applying plastic clay to stiffer clay, such as handles applied to a mug,
I apply slurry as above, and then work the handle against the surface until
it grabs, just as you describe, and wipe off all the excess slurry with a
quick swipe of the sponge. Again, I have no problems with shrinkage of
slurry, because there really is none left to shrink.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Ron Roy on thu 13 dec 01
Hi Cindy,
I agree - but that assumes we know where the air is - I thought we were
talking about air pockets from improper wedging or joining.
The idea of scoring by the way - is to get the clay wet and sticky enough
to join well - all those marks should be wiped out in such a way as to not
leave any air pockets - the more air the weaker the join.
I never score or even roughen the surface anymore - and I join porcelain -
simply because the join is more sound that way with a minimum of shrinkage
- rub in a little vinegar - twice - make sure both surfaces are rounded so
no air is trapped - and press on.
It does help a lot if the pieces to be joined are the same clay and the
same firmness but I have seen porcelain and stoneware used with no problem.
If the join is well made it is amazing what you can get away with -
especially if slow dried.
RR
> The reason some people think it's the air bubbles
>is because - when the
>clay can't take the pressure anymore it comes
>apart at the weakest point
>first - air pockets are weak points - this leads
>to the false assumption
>that the air pocket was responsible - just not
>true.
>
>Could it be that it only comes apart where there
>is an air pocket - yes and
>it does happen but that is because the clay is
>weakest there - the cause is
>still firing to fast when there is too much water
>present.
>
>RR
>__________________________________________________
>____
>
>Okay, I'll buy that. But a pinhole into the air
>pocket is still cheap and good insurance.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Craig Clark on thu 13 dec 01
OK. Now I'm confused!!!! I was taught, and have been inadverdently
passing on bad info if not correct, that it is best to "slip and score."
This is not an attempt to argue the point or methods in any fashion. I'm
simply looking for the more effective way of joining pieces of clay.
Could you explain slowly, please.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Ron Roy"
To:
Sent: Wednesday, December 12, 2001 10:26 PM
Subject: Re: air bubbles?
> Hi Cindy,
>
> I agree - but that assumes we know where the air is - I thought we were
> talking about air pockets from improper wedging or joining.
>
> The idea of scoring by the way - is to get the clay wet and sticky enough
> to join well - all those marks should be wiped out in such a way as to not
> leave any air pockets - the more air the weaker the join.
>
> I never score or even roughen the surface anymore - and I join porcelain -
> simply because the join is more sound that way with a minimum of shrinkage
> - rub in a little vinegar - twice - make sure both surfaces are rounded so
> no air is trapped - and press on.
>
> It does help a lot if the pieces to be joined are the same clay and the
> same firmness but I have seen porcelain and stoneware used with no
problem.
> If the join is well made it is amazing what you can get away with -
> especially if slow dried.
>
> RR
>
>
> > The reason some people think it's the air bubbles
> >is because - when the
> >clay can't take the pressure anymore it comes
> >apart at the weakest point
> >first - air pockets are weak points - this leads
> >to the false assumption
> >that the air pocket was responsible - just not
> >true.
> >
> >Could it be that it only comes apart where there
> >is an air pocket - yes and
> >it does happen but that is because the clay is
> >weakest there - the cause is
> >still firing to fast when there is too much water
> >present.
> >
> >RR
> >__________________________________________________
> >____
> >
> >Okay, I'll buy that. But a pinhole into the air
> >pocket is still cheap and good insurance.
>
> Ron Roy
> RR# 4
> 15084 Little Lake Rd..
> Brighton,
> Ontario, Canada
> KOK 1H0
> Residence 613-475-9544
> Studio 613-475-3715
> Fax 613-475-3513
>
>
____________________________________________________________________________
__
> 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.
>
Ron Roy on fri 14 dec 01
I can think of three situations that we call bloating.
Clay seals over - continues to melt enough to produce gases which cannot
escape and the first sign would be small bubbles or blisters - called blebs
- appear all over the surface of the ware - if the melting continues the
blisters get bigger.
Clay seals over and larger blisters form in certain areas - blisters when
broken sometimes have a fibrous look inside - almost like squash insides -
due to reduced iron from unclean bisque firing - the blisters are usually
in thicker sections when the combustibles did not get oxidized properly in
the bisque - a problem if iron is present in the clay to start with.
Clay seals over and there is an air pocket from improper wedging or
throwing - some clays soften enough that the trapped air - still expanding
- produces a bulge.
I don't think steam "gathers" in air pockets by the way - If you took
pressure reading inside the clay and the pocket they would be the same - I
do believe the first explosion will be at the point where the clay is
weakest - that will be the air pocket.
If there are no air pockets you get the over all explosion because there is
no escape point. Some times you get both but the air pocket would go first.
The reason it is felt that air produces explosions in bisque came from the
observation - some times - of the cavity of air left when you see the
remains of it where the explosion took place.
Sounds like the subject for some sort of experiment to me.
RR
> Let's think about this for a minute. When you bisque, the clay NEVER
> loses porosity. There is nothing to trap the air until the clay
> starts to vitrify and seals over...fairly late in the firing.
> Bloating comes when other materials (like carbonates) off-gas after
> the surface of the clay is completely sealed...which means melted.
> The steam gathers in thick
>sections or air pockets with sufficient pressure to blow the piece apart.
>With thick sections, the piece ususally blows into a zillion tiny shards.
>With air pockets, a large section usually blows off.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
vince pitelka on fri 14 dec 01
> I don't think steam "gathers" in air pockets by the way - If you took
> pressure reading inside the clay and the pocket they would be the same - I
> do believe the first explosion will be at the point where the clay is
> weakest - that will be the air pocket.
> If there are no air pockets you get the over all explosion because there
is
> no escape point. Some times you get both but the air pocket would go
first.
Ron -
Perhaps I am wrong to say that the steam "gathers" in any closed pocket in
the clay, but my experience proves to me that steam does apply much greater
pressure in such spaces, causing large pieces to pop off or apart. When
there are no pockets and a thick and/or damp piece is fired too quickly, we
get that terrible situation where nothing is left but a pile of tiny shards.
Where there are closed air pockets, even in a much slower bisque-firing, we
get large sections popping apart.
I have a slide of Japanese Jomon mask from circa 5000 BC. One eye is
missing, and you can see the air pockets which caused it to pop off.
Potters have been making the same mistakes for many millennia. I love to
show this slide to my students before we do slumped-slab masks, but as I
mentioned in an earlier post, even with my warnings some of them invariably
build up the eyes or noses with large clumps of clay, trapping air bubbles
beneath, and these applied sections pop off neat as can be.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Ron Roy on mon 17 dec 01
Hi Craig,
I was taught the same way - by someone who had just graduated from Alfred -
part of the problem - never made functional pots and had no studio
experience - just passing on what he had been told.
As I have said - I work with porcelain some of the time and found I could
not get reliable joins using score and slip. Many times the join failed
during drying and even more often the slip around the join cracked enough
to be able to see it through transparent glaze. Besides - porcelain does
not like too much rewetting
I could have tried other ways - like defloccing the slip but that is a lot
of work - and so is scoring and slipping.
I knew that adding vinegar to clay makes it more plastic - and old clay is
better at bending and sticking than new. Bacterial action produces acid -
like vinegar - which is the product of bacterial action - so when someone
in a workshop suggested it - I tried it.
Some swear by other mixtures - makes no difference if it works.
Anyway I have not had ANY failures since I started with vinegar.
Best thing is for you to try it and see - if it works it's gonna cut down
your joining time.
The worst part about scoring and slip - it is very difficult to get the air
out completely and that makes a joint weaker.
Cider vinegar is supposed to be more acid - so I use it - I don't believe
it but I like the smell.
Paint on vinegar where you intend to join - round surfaces of clay to be
joined so no air is trapped when you add the parts - paint vinegar on the
parts - more vinegar on the pot - rub till clay is sticky - not wet anymore
- push on parts - make sure edges of joints are sealed well.
> OK. Now I'm confused!!!! I was taught, and have been inadverdently
>passing on bad info if not correct, that it is best to "slip and score."
> This is not an attempt to argue the point or methods in any fashion. I'm
>simply looking for the more effective way of joining pieces of clay.
>Could you explain slowly, please.
>Craig Dunn Clark
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Bunny Lemak on tue 20 mar 07
I just opened the kiln and everything I bisqued to cone 04 turned out
fine, with the exception of one piece.
It is a bowl, made with B-mix ^6 clay, that was slabed then humped over a
mold, on the bottom side there is a sliver of clay that popped up during
firing. I assume it was because of a air bubble in the clay that went
undetected, until now.
Is there any way of fixing this?
Or is the bowl a total waste? Any help would be greatly appreciated.
Thanks!
Bunny
Donna Kat on wed 21 mar 07
On Tue, 20 Mar 2007 20:36:16 -0500, Bunny Lemak wrote:
>I just opened the kiln and everything I bisqued to cone 04 turned out
>fine, with the exception of one piece.
>
>It is a bowl, made with B-mix ^6 clay, that was slabed then humped over a
>mold, on the bottom side there is a sliver of clay that popped up during
>firing. I assume it was because of a air bubble in the clay that went
>undetected, until now.
>
>Is there any way of fixing this?
>
>Or is the bowl a total waste? Any help would be greatly appreciated.
>
>Thanks!
>
>Bunny
>
It is not quite clear what you have. How large is the 'sliver'? You can
certainly grind it down which should be relatively easy with a bisqued
piece. It depends on what you mean as a 'total waste'. It can certainly
be used as is as a flower pot that you can just stick outdoors in on the
ground, which won't care that it has a rough bottom. This is one of those
things that only you can really decide. Donna
Nancy on wed 21 mar 07
Bunny
Since the piece isn't perfect...what else can you do with it? I have a
few pieces that something "strange" happened. One very nice big V Bowl
cracked vertically for about 2" from the rim down. Ruined...yep I
thought..then I sold it as a centerpiece bowl with water and floating
candles and the slot....a simple flower....
If it truly is a "bubble" I would sand it so it was smooth and the
bubble is gone. Yes it will be thinner in that one spot, but you could
test glaze it....I use bad bowls to test glaze combinations that might
be runny..they'll just pool in the bowl. Maybe not usable, but you can
have a sample of what the glazes do.
Nancy
Donna Kat wrote:
> On Tue, 20 Mar 2007 20:36:16 -0500, Bunny Lemak wrote:
>
>
>> I just opened the kiln and everything I bisqued to cone 04 turned out
>> fine, with the exception of one piece.
>>
>> It is a bowl, made with B-mix ^6 clay, that was slabed then humped over a
>> mold, on the bottom side there is a sliver of clay that popped up during
>> firing. I assume it was because of a air bubble in the clay that went
>> undetected, until now.
>>
>> Is there any way of fixing this?
>>
>> Or is the bowl a total waste? Any help would be greatly appreciated.
>>
>> Thanks!
>>
>> Bunny
>>
>>
>
> It is not quite clear what you have. How large is the 'sliver'? You can
> certainly grind it down which should be relatively easy with a bisqued
> piece. It depends on what you mean as a 'total waste'. It can certainly
> be used as is as a flower pot that you can just stick outdoors in on the
> ground, which won't care that it has a rough bottom. This is one of those
> things that only you can really decide. Donna
>
> ______________________________________________________________________________
> 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.
>
>
Bunny Lemak on fri 23 mar 07
This "bubble" or "sliver" is about a 2" x 2" section, the top part is
opened but still connected at the bottom.
I have in the past tried to take these off only to find that it gets
deeper as you peel or pop it off. So it does take a big chunk out. I
don't know if air bubbles do this or not since it has only happened now 2
times to me.
Someone suggested that I could fix it with paper clay, so that is what I
started to do. If it doesn't fix, then it will be a glaze test piece.
Someone else suggested it might be a "lime pop-off" and come to the list
for help. Since I've never heard of this, could someone with experience
out there please tell me if this is what happened?
Thanks-
Bunny
Mary Leither on fri 23 mar 07
Could be a bubble but there might also have been a little dampness in the
center. I've noticed that in student work B-Mix in hand building will dry
quickly on the surface but remain damp on the interior. Is your piece
functional or not. Sometimes the most expedient course of action is to
start over than to waste a lot of time trying to mend structurely unsound
pots and there are just so many test pieces that are needed. However, the
"PC" line of epoxy resins carry an amazing line of repair mediums in varying
colors (PC7 -black, PC11 -white, PC crete- grey (or plumbers putty), PC-
wood which is stainable) which will fill and become virtually unnoticeable
if properly applied and touched up with acrylic paint or softened with
mineral spirits. Marine putty dries white and is paintable. The glues aren't
cheap but can be mixed or kneaded in small amounts. When having to repair
student classwork, I also keep those boxes of leftover crayons on hand to
rub into repairs of subtle cracks of warm non functional pieces. Sometimes
you have to cheat handsomely. Paperclay is not a good choice for these type
of repairs. Mary
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of Bunny Lemak
Sent: Friday, March 23, 2007 1:18 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: [CLAYART] Air bubbles?
This "bubble" or "sliver" is about a 2" x 2" section, the top part is
opened but still connected at the bottom.
I have in the past tried to take these off only to find that it gets
deeper as you peel or pop it off. So it does take a big chunk out. I
don't know if air bubbles do this or not since it has only happened now 2
times to me.
Someone suggested that I could fix it with paper clay, so that is what I
started to do. If it doesn't fix, then it will be a glaze test piece.
Someone else suggested it might be a "lime pop-off" and come to the list
for help. Since I've never heard of this, could someone with experience
out there please tell me if this is what happened?
Thanks-
Bunny
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7:44 AM
Gayle Bair on fri 23 mar 07
Bunny,
I never had a sliver of clay pop up from an air bubble
it was always more like a bloat.
My best advice.... trying to "fix" it is a waste of time...
But.... you could get lots of use out of it as a slump mold.
best regards,
Gayle Bair
Tucson AZ
-----Original Message-----
From: Bunny Lemak
It is a bowl, made with B-mix ^6 clay, that was slabed then humped over a
mold, on the bottom side there is a sliver of clay that popped up during
firing. I assume it was because of a air bubble in the clay that went
undetected, until now.
Is there any way of fixing this?
snip>
Russel Fouts on sun 25 mar 07
>> Someone else suggested it might be a "lime pop-off" and come to the
list for help. Since I've never heard of this, could someone with
experience out there please tell me if this is what happened? <<
If you remove the sliver and find a white or gray 'mass' at the bottom
of the hole it's a lime pop-out and there isn't much you can do about it.
What happens is that you get a piece of lime in the clay then when you
fire it, it gets calcined, removing all the water. Unfortunately what
you have now is a bit of substance (I forget the work for the property)
that will absorb moisture from the atmosphere and swell, creating
pressure from within the body and causing the pop-out.
I suppose you could dig out as much of the lime as you can, it should be
fairly soft and then fill it with paperclay or epoxy.
Russel
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