Chris Schafale on wed 11 jan 06
OK, I'm taking bets on this one....
I normally make my goblet or chalice stems open at the bottom, but this
time, the customer specifically requested that they be closed. So, I threw
the stems with a bottom, attached as usual, and left them to dry a bit. I
came back later and poked a pinhole in each stem, and was interested to
realize that I could hear and feel the trapped air whistling out the hole
since it had been under pressure as the stem dried and shrank.
I let the chalices dry thoroughly, and fired with a slow preheat to be sure
all moisture had time to exit. I lost only one out of 15 chalices, and the
bottom blew out so neatly that I can probably salvage the piece for another
order just by sanding a little around the base.
My question now is, when I glaze these pieces, and the little pinhole gets
covered with glaze, will they explode in the glaze firing? My husband
thinks air will escape during heating, before the glaze has melted, but
then the hole will be plugged during cooling, so I will have vacuum in the
stem which could cause a nifty implosion if the piece ever gets
dropped. What do you think?
Chris
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA (near Raleigh)
www.lightonecandle.com
Galleries of Pottery Forms by Clayart Potters: www.potteryforms.org
Email: chris at lightonecandle dot com
Fredrick Paget on sun 15 jan 06
I sort of doubt they will hold a vacuum for very long in as much as
the clay is not completely vitrified. Even if it does it won't be a
very good vacuum and since the volume is pretty small it won't make
much of a pop if it is dropped on a hard surface.
I have fired a number of small egg size ornaments that were hollow
and have a pin hole in them which is plugged by the glaze when it
melts and then cool as a sealed chamber. None has exploded in firing
and I have not dropped one yet so can not give definite answer about
the "pop". One was even the size of a baseball and fired in the Tozan
noborigama. On that one the pinhole stayed open so if that happens
there is the danger of getting water inside.
Bev Tailor of the Tozan Society has been making dozens of small
discus shaped floating ornaments that are hollow. There is a pinhole
and she has no glaze on them as they are made of white and colored
clay. I think she puts epoxy in the pinholes before they go to the
customers to float in their fishponds.
--
From Fred Paget,
Marin County, CA, USA
fredrick@well.com
http://homepage.mac.com/fredrick/FileSharing1.html
Charter Member Potters Council
Working on those verse containing decals for the gift traveling mugs.
I have decided to print all the replies I got and you can take your
pick in the Clayart room (free).
Chris Schafale on sun 15 jan 06
It was pointed out that my computer's calendar was off by a few days, so
this message may have gotten shoved to the bottom of the email pile. In
hopes of getting more responses, here's a repost:
_______
OK, I'm taking bets on this one....
I normally make my goblet or chalice stems open at the bottom, but this
time, the customer specifically requested that they be closed. So, I threw
the stems with a bottom, attached as usual, and left them to dry a bit. I
came back later and poked a pinhole in each stem, and was interested to
realize that I could hear and feel the trapped air whistling out the hole
since it had been under pressure as the stem dried and shrank.
I let the chalices dry thoroughly, and fired with a slow preheat to be sure
all moisture had time to exit. I lost only one out of 15 chalices, and the
bottom blew out so neatly that I can probably salvage the piece for another
order just by sanding a little around the base.
My question now is, when I glaze these pieces, and the little pinhole gets
covered with glaze, will they explode in the glaze firing? My husband
thinks air will escape during heating, before the glaze has melted, but
then the hole will be plugged during cooling, so I will have vacuum in the
stem which could cause a nifty implosion if the piece ever gets
dropped. What do you think?
Chris
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA (near Raleigh)
www.lightonecandle.com
Galleries of Pottery Forms by Clayart Potters: www.potteryforms.org
Email: chris at lightonecandle dot com
Carole Fox on mon 16 jan 06
On Sun, 15 Jan 2006 21:42:00 -0500, Chris Schafale
wrote:
>
>My question now is, when I glaze these pieces, and the little pinhole gets
>covered with glaze, will they explode in the glaze firing? My husband
>thinks air will escape during heating, before the glaze has melted, but
>then the hole will be plugged during cooling, so I will have vacuum in the
>stem which could cause a nifty implosion if the piece ever gets
>dropped. What do you think?
>
>Chris
>
Chris-
I don't know where you put your pinhole, but if it's in the bottom, why
not just stick a toothpick in it to prevent it from filling up with glaze,
and fire with it open? That's what I do when I make a certain type of
hollow handle. I put the pinhole on the underside of the handle where
it's not noticeable.
I won't speculate whether your pieces will explode in the glaze firing,
but once several years ago I did have a piece with a hollow section that
made it through the bisque firing but blew out a chunk in the glaze firing.
Carole Fox
Dayton, OH
Mayssan Shora Farra on mon 16 jan 06
On Sun, 15 Jan 2006 21:42:00 -0500, Chris Schafale
wrote:
>the stems with a bottom, attached as usual, and left them to dry a bit. I
>came back later and poked a pinhole in each stem, and was interested to
>realize that I could hear and feel the trapped air whistling out the hole
>since it had been under pressure as the stem dried and shrank.
>
>My question now is, when I glaze these pieces, and the little pinhole gets
>covered with glaze, will they explode in the glaze firing? My husband
>thinks air will escape during heating, before the glaze has melted, but
>then the hole will be plugged during cooling, so I will have vacuum in the
>stem which could cause a nifty implosion if the piece ever gets
>dropped. What do you think?
>Chris
>
>Light One Candle Pottery
>Fuquay-Varina, North Carolina, USA (near Raleigh)
>www.lightonecandle.com
>Galleries of Pottery Forms by Clayart Potters: www.potteryforms.org
Dear Chris:
Do not worry; nothing will implode or explode. the one that broke happened
probably from moisture.
I made a few enclosed forms with the pinhole that hisses and after that I
glazed over the pinhole and no problems.
According to Pete Pinnel and I have to believe him and agree that trapped
air does not explode things as it heats and then contracts with the clay.
as it does not change much in volume, while water, as it heats up it
changes to steam which is way bigger in volume thus causing an explosion.
He wrote a nice article about that in his colemn in Clay Times a couple of
issues ago.
I hope this helps,
Mayssan in again back to spring Charleston WV USA
Ron Roy on mon 16 jan 06
I can't stay quiet any longer - if water can get into the handle and
someone heats up something in that goblet in a microwave oven - they may
burn their hand when they take it out.
I would try firing them without any hole - they will not blow up - water
can pass through bisqued ware easily - and through unfired glaze - best to
let them dry well before firing I suppose.
The only thing that can happen - if the space is enclosed and the clay gets
soft - like in some porcelain - the trapped heated air will expand the clay
somewhat - it may even split the clay - I have seen that - but it will not
be hard to do the testing.
RR
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Brad Sondahl on mon 16 jan 06
The answer is positively not. What's more, even though if you make
holes purposefully (such as for a colander) they will fill with glaze,
for some reason it's very hard to fill a pinhole with glaze, both when
you dip the pots, and in the firing. I kind of suspect that if it leads
into an air pocket, the air pocket will suck the glaze in as it cools,
but that's only a guess.
Brad Sondahl
--
For my comics, pottery how-to videos, original art, music, pottery, and literature, visit my homepage
http://sondahl.com
Warren Heintz on mon 16 jan 06
Excuse my piggy-backing on your reply,but I've deleted the original posting. As small as it is,couldn't the pin hole have liquid wax applied prior the glazing, filling the hole using a fine brush and a bit on the other surface?
Brad Sondahl wrote: The answer is positively not. What's more, even though if you make
holes purposefully (such as for a colander) they will fill with glaze,
for some reason it's very hard to fill a pinhole with glaze, both when
you dip the pots, and in the firing. I kind of suspect that if it leads
into an air pocket, the air pocket will suck the glaze in as it cools,
but that's only a guess.
Brad Sondahl
--
For my comics, pottery how-to videos, original art, music, pottery, and literature, visit my homepage
http://sondahl.com
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Vince Pitelka on mon 16 jan 06
The worst thing would be to have a pinhole still there in the finished
product. As I mentioned in an earlier message, expansion and contraction
from use and washing will suck moisture into the hole, and over time alien
cultures will grow in there. Then, when the goblet heats up again, some
horrible slimy dark substance will come seeping out of the hole. I've seen
this happen and it is disgusting.
As was recently discussed on Clayart, the simple solution is to do as
someone suggested - leave a pinhole in the clay through the bisque fire, and
then glaze it shut in the glaze firing. To make sure it glazes shut, use
your fingertip or a little spatula to force some glaze into the hole, and
then glaze in a normal fashion. As the kiln heats up, the pressure can
still escape through the porous, non-fired glaze in the hole. Once it is
fired, the vacuum inside is no consideration at all. It is possible that
the vacuum will neutralize via the very slight porosity in the clay, but
whether or not it does is of no consequence.
- 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/
Carole Fox on tue 17 jan 06
On Mon, 16 Jan 2006 13:45:09 -0500, Ron Roy wrote:
>I would try firing them without any hole - they will not blow up - water
>can pass through bisqued ware easily - and through unfired glaze - best to
>let them dry well before firing I suppose.
Ron - I don't doubt your logic, but I wonder if there are some conditions
that might still cause a problem. As I had previously posted, I
absolutely did have a piece that blew out a segment of a small hollow
section during the glaze firing. The fragments that blew out were
collectively about the size of a quarter. I was told by an instructor at
the time that I should not have let the pinhole fill up with glaze.
I am now curious to know what might have caused that blowout it if wasn't
the result of closing the hole. This was years ago, but ever since this
incident, I have been diligently keeping small holes open through the
glazing process in closed hollow sections, apparently unnecessarily. What
other factors could cause a chunk to explode off the pot? I'd love to
know.
Carole Fox
Dayton, OH
Digest Ack on tue 17 jan 06
Chris,
I make goblets and chalices in two parts--the cup, and the stem. I join them using a paper clay slip I make myself. The stem is closed at the bottom, and has a decorative rim at the top to partially disguise the join with the cup.
When I stamp and sign the bottom of the stem, I poke a needle tool through one of the lines in my signature. You have to look *really* closely to see it. Of course, I do not glaze the bottoms of these pieces, so the hole stays open and free to allow the air to enter and leave. No pressure--either positive or negative.
Et voila!
Haven't lost a goblet yet.
Good luck,
Tig Dupre
in slippery, slidey, nasty Port Orchard, Washington
John Hesselberth on tue 17 jan 06
On Jan 17, 2006, at 9:45 AM, Carole Fox wrote:
> What
> other factors could cause a chunk to explode off the pot? I'd love to
> know.
Hi Carole,
Water. Greenware pots that are not dry or glazed pots that are not
dry. It is very difficult to get these enclosed spaces thoroughly
dry. But I will say emphatically that it is water, not trapped air
that causes essentially all kiln "explosions" of pots. Even when the
outside of an enclosed space feels dry the inside may still be damp.
The only other thing I can think of that might cause it is if you
have a significant amount of glaze inside that enclosed space. Then
not only would water potentially be a problem, but things like
carbonates letting loose of their CO2 might generate enough gas
quickly enough to break a pot. Trapped air, by itself, does not cause
this kind of breakage.
Pete Pinnell addressed this question head-on in his Clay Times column
in the Nov/Dec 2005 issue.
Regards,
John
John Jensen on tue 17 jan 06
Carole;
I'm sure many others have more to offer than I...I rarely do have
explosions; but I do have a small observation to offer. Explosions from =
air
pockets tend to leave a few large chunks whereas explosions that leave =
lots
of small shards are likely to be from moisture and too rapid firing.
Best wishes.
John Jensen, Mudbug Pottery
John Jensen@mudbugpottery.com
http://www.toadhouse.com http://www.mudbugpottery.com
http://www.mudbugblues.com
.
>=20
> I am now curious to know what might have caused that blowout it if =
wasn't
> the result of closing the hole. This was years ago, but ever since =
this
> incident, I have been diligently keeping small holes open through the
> glazing process in closed hollow sections, apparently unnecessarily. =
What
> other factors could cause a chunk to explode off the pot? I'd love to
> know.
>=20
> Carole Fox
> Dayton, OH
Ron Roy on tue 17 jan 06
Hi Carole,
I don't know what happened - could it be related to air in the clay and
firing too fast? Steam in an air pocket in freshly glazed ware could
possibly do it if the beginning of the firing was fast.
When you see air in clay - especially in thrown clay it tends to be flat
voids rather than bubbles - which could account for the shape of the
blowout.
RR.
>>I would try firing them without any hole - they will not blow up - water
>>can pass through bisqued ware easily - and through unfired glaze - best to
>>let them dry well before firing I suppose.
>
>Ron - I don't doubt your logic, but I wonder if there are some conditions
>that might still cause a problem. As I had previously posted, I
>absolutely did have a piece that blew out a segment of a small hollow
>section during the glaze firing. The fragments that blew out were
>collectively about the size of a quarter. I was told by an instructor at
>the time that I should not have let the pinhole fill up with glaze.
>
>I am now curious to know what might have caused that blowout it if wasn't
>the result of closing the hole. This was years ago, but ever since this
>incident, I have been diligently keeping small holes open through the
>glazing process in closed hollow sections, apparently unnecessarily. What
>other factors could cause a chunk to explode off the pot? I'd love to
>know.
>
>Carole Fox
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Gay Judson on wed 18 jan 06
Chris, Last night I recalled that once my husband's favorite bowl was
suddenly leaking cereal milk--much to our amazement. It was the pin
hole I had made to check the depth of the base. Because we were fond
of the bowl I tried sealing the tiny hole with the ceramic epoxy made
by East Valley Supply. I had bought the expoxy to mend the handle on
a teapot--which has held very well. As did the patch in the bottom of
our bowl. You might leave pin hole in the bottom of the chalice and
then apply the expoxy after glaze firing. The epoxy can be colored to
match the glaze.
Gay Judson in San Antonio, TX
PS the epoxy can be seen/ordered at:=20
www.EVSupply.com
I have no connection with the company--just pleased with their
product.
> -----Original Message-----
> From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Chris
> Schafale
> Sent: Wednesday, January 11, 2006 7:53 AM
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: closed goblet stems -- will they explode in glaze fire?
>=20
> OK, I'm taking bets on this one....
>=20
> I normally make my goblet or chalice stems open at the bottom, but
> this
> time, the customer specifically requested that they be closed. So,
> I threw
> the stems with a bottom, attached as usual, and left them to dry a
> bit. I
> came back later and poked a pinhole in each stem, and was
> interested to
> realize that I could hear and feel the trapped air whistling out the
> hole
> since it had been under pressure as the stem dried and shrank.
>=20
> I let the chalices dry thoroughly, and fired with a slow preheat to
> be sure
> all moisture had time to exit. I lost only one out of 15 chalices,
> and the
> bottom blew out so neatly that I can probably salvage the piece for
> another
> order just by sanding a little around the base.
>=20
> My question now is, when I glaze these pieces, and the little
> pinhole gets
> covered with glaze, will they explode in the glaze firing? My
> husband
> thinks air will escape during heating, before the glaze has melted,
> but
> then the hole will be plugged during cooling, so I will have vacuum
> in the
> stem which could cause a nifty implosion if the piece ever gets
> dropped. What do you think?
>=20
> Chris
>=20
> Light One Candle Pottery
> Fuquay-Varina, North Carolina, USA (near Raleigh)
> www.lightonecandle.com
> Galleries of Pottery Forms by Clayart Potters: www.potteryforms.org
> Email: chris at lightonecandle dot com
>=20
> ____________________________________________________________________
> __________
> Send postings to clayart@lsv.ceramics.org
>=20
> You may look at the archives for the list or change your
> subscription
> settings from http://www.ceramics.org/clayart/
>=20
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
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