Carole Fox on mon 5 aug 02
I know it's late to jump on the thread about the lack of a ringing when =
thumping on raku but I've been away and only now just caught up with a =
weeks worth of clayart. I have always thought that one of the reasons =
for using a clay that will not be vitrified at raku firing temperature =
is that the clay will be more accepting of the carbon...to get a good =
black. Please correct me if I am mistaken.
I've been trying to make some large horsehair raku pots but am getting =
much cracking. Is there a limit to the size of a pot that will =
successfully take this treatment? I have been putting small pieces of =
kiln shelf in with the firing to place the pots on upon removal but this =
does not do the trick. It is starting to get a little depressing.
Carole Fox- in Elkton, MD where it's hot and dry- not great for firing, =
but at least you don't have to mow the lawn too often!
cfox@dca.net
Bruce Girrell on mon 5 aug 02
Carole Fox wrote:
> I've been trying to make some large horsehair raku pots but am
> getting much cracking. Is there a limit to the size of a pot
> that will successfully take this treatment? I have been putting
> small pieces of kiln shelf in with the firing to place the pots
> on upon removal but this does not do the trick. It is starting to
> get a little depressing.
Oh, boy, can I empathize with this. Our first attempts at horsehair
decoration were met with a frustratingly large percentage of cracks. Just to
make things more difficult for ourselves, we chose to use a porcelain clay
because of its whiteness and smooth texture. Often we lost more than half of
our pieces.
People talk about the thermal shock inherent in the raku process, but raku
is nothing compared to horsehair decoration. Raku pots get thrown in a nice
cozy reduction bin surrounded by burning material, while horsehair pots sit
shivering out in the open air, over 1000 degrees hotter than their
surroundings.
Here are some suggestions based on what we have done to address this
problem:
1) Lower the bisque temperature to ^010 to ^08 (we were doing our bisque at
about ^06). Originally, we did this to help preserve the burnished finish on
the pots, but I think it helps with the cracking as well. BTW, despite the
low bisque temp, all of our pots ring when thumped unless there is a crack
somewhere.
2) Add thermal shock resistant material to the clay body. Specifically, we
have added pyrophyllite and talc. We tried wollastonite (thinking that the
needle-like crystals would act like rebar), but it did not help much. We
have had a lot of difficulty with the additives, as they decrease the
(already low) plasticity of the clay. I suggest that you try other methods
before messing with additives.
3) Throw and trim thin. The outside of the pot cools faster than the inside
and, as a result, the outside of the pot goes into tension. The thicker the
piece is, the greater the temperature difference between the inner and outer
walls, hence more cracking. The thickness of our pot walls is usually about
1/8 inch.
4) Put up a wind block to minimize breezes. Variations in surface
temperature due to one part of the pot cooling faster than another can lead
to cracking.
One unfortunate aspect of horsehair decoration is that the temperature range
at which the decoration is done straddles the quartz inversion temperature.
So you have a pot cooling at a tremendous rate that also has to undergo the
beta to alpha quartz transition. If you have the opportunity to create your
own clay body, I suggest that you use as little free quartz (flint, e.g.) as
possible. I have not had the opportunity to do any extensive testing of clay
bodies formulated specifically for minimizing thermal shock due to quartz
inversion.
I hope that helps. Just another note - we pull our pieces to a banding wheel
that has a piece of refractory ceramic fiber board on it for insulation. The
banding wheel allows us to turn the piece easily while applying the hair and
the insulation minimizes thermal shock. Even with everything that we do, it
is still a two person process. I handle the hot pieces while Lynne does most
of the decoration. We find that we have just a little over a minute in which
to complete the decoration. With our current methods we rarely suffer any
cracking. We lose perhaps one pot in 20.
Bruce "sometimes compared to a different part of a horse" Girrell
Marcia Selsor on mon 5 aug 02
Hi Carole,
I will be doing a horsehair workshop in NJ in Nov. For horsehair pots, I bisque raku clay to ^09 with a terra sig burnished finish. I take them out of the kiln at 12-1300 F. Not nearly as hot as a raku firing.
My larger horse hair pots are 12-14 inches.
If you need more info..contact me. I will be having a show at Hood in Oct-Nov. I will be in MD. area for 5 weeks. I could come by and help you through some firings.
Marcia
Carole Fox wrote:
> I know it's late to jump on the thread about the lack of a ringing when thumping on raku but I've been away and only now just caught up with a weeks worth of clayart. I have always thought that one of the reasons for using a clay that will not be vitrified at raku firing temperature is that the clay will be more accepting of the carbon...to get a good black. Please correct me if I am mistaken.
>
> I've been trying to make some large horsehair raku pots but am getting much cracking. Is there a limit to the size of a pot that will successfully take this treatment? I have been putting small pieces of kiln shelf in with the firing to place the pots on upon removal but this does not do the trick. It is starting to get a little depressing.
>
> Carole Fox- in Elkton, MD where it's hot and dry- not great for firing, but at least you don't have to mow the lawn too often!
> cfox@dca.net
>
> ______________________________________________________________________________
> 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.
Cantello Studios on tue 6 aug 02
This is an easy one, after I lost 50% to 60% of my Horse Hair production
over a six mounts period.
I started running two kilns, a raku kiln to get the pots to temp and a small
up draft running
around 500 F to decorate in. This kiln also works like a annealing oven,
which keeps the pots from cracking.
It's quit a dance at times but when your loss rate drops down to almost 0
It's a dance I'm loving.
Another thing I found is the pots must be washed after the firing process is
over and
Before a wax past or what ever is put on. If not this protective coating
will soon come off. Why??
The carbon from the hair will act as a release. This is something I learned
in Glass casting.
Happy to Help. Chris Cantello - www.cantellostudios.com
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Bruce Girrell
Sent: Monday, August 05, 2002 10:29 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: raku thumping and horsehair question
Carole Fox wrote:
> I've been trying to make some large horsehair raku pots but am
> getting much cracking. Is there a limit to the size of a pot
> that will successfully take this treatment? I have been putting
> small pieces of kiln shelf in with the firing to place the pots
> on upon removal but this does not do the trick. It is starting to
> get a little depressing.
Oh, boy, can I empathize with this. Our first attempts at horsehair
decoration were met with a frustratingly large percentage of cracks. Just to
make things more difficult for ourselves, we chose to use a porcelain clay
because of its whiteness and smooth texture. Often we lost more than half of
our pieces.
People talk about the thermal shock inherent in the raku process, but raku
is nothing compared to horsehair decoration. Raku pots get thrown in a nice
cozy reduction bin surrounded by burning material, while horsehair pots sit
shivering out in the open air, over 1000 degrees hotter than their
surroundings.
Here are some suggestions based on what we have done to address this
problem:
1) Lower the bisque temperature to ^010 to ^08 (we were doing our bisque at
about ^06). Originally, we did this to help preserve the burnished finish on
the pots, but I think it helps with the cracking as well. BTW, despite the
low bisque temp, all of our pots ring when thumped unless there is a crack
somewhere.
2) Add thermal shock resistant material to the clay body. Specifically, we
have added pyrophyllite and talc. We tried wollastonite (thinking that the
needle-like crystals would act like rebar), but it did not help much. We
have had a lot of difficulty with the additives, as they decrease the
(already low) plasticity of the clay. I suggest that you try other methods
before messing with additives.
3) Throw and trim thin. The outside of the pot cools faster than the inside
and, as a result, the outside of the pot goes into tension. The thicker the
piece is, the greater the temperature difference between the inner and outer
walls, hence more cracking. The thickness of our pot walls is usually about
1/8 inch.
4) Put up a wind block to minimize breezes. Variations in surface
temperature due to one part of the pot cooling faster than another can lead
to cracking.
One unfortunate aspect of horsehair decoration is that the temperature range
at which the decoration is done straddles the quartz inversion temperature.
So you have a pot cooling at a tremendous rate that also has to undergo the
beta to alpha quartz transition. If you have the opportunity to create your
own clay body, I suggest that you use as little free quartz (flint, e.g.) as
possible. I have not had the opportunity to do any extensive testing of clay
bodies formulated specifically for minimizing thermal shock due to quartz
inversion.
I hope that helps. Just another note - we pull our pieces to a banding wheel
that has a piece of refractory ceramic fiber board on it for insulation. The
banding wheel allows us to turn the piece easily while applying the hair and
the insulation minimizes thermal shock. Even with everything that we do, it
is still a two person process. I handle the hot pieces while Lynne does most
of the decoration. We find that we have just a little over a minute in which
to complete the decoration. With our current methods we rarely suffer any
cracking. We lose perhaps one pot in 20.
Bruce "sometimes compared to a different part of a horse" Girrell
____________________________________________________________________________
__
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.
Paul Taylor on tue 6 aug 02
Dear carole
You are right about the clay being more accepting of carbon but
conversely if the clay is smooth it does not need to take much black before
it shows a shiney black coat.
The amount of grog needed to make any thing but a rounded pot in Raku is
a lot depending on the size and flatness. A french potter visiting (forget
name ) recommended 75% and more grog, and coarse grog at that for any thing
flatish or sculptural. Yet you can get some stoneware to Raku if the shape
is small and enclosed. He was right but the clay for the flat ware is very
short I am going to add the grog to slip to see if I can improve the
plasticity.
Its is a bother making different bodies for different size pots. Also it
depends on the type of clay you are getting the plasticity from. so any body
recipes can be less than help full. What you do is put coarse grog (coarse
not coarse to dust) in, until it stops cracking in the kiln. And if that
does not work start with another base clay or coarser grog. My base clay is
not very plastic. I suspect I could get more grog into a more plastic clay,
say a ball clay, but it might prove the same in the end . I think it helps
if you glaze as much of the pot as possible inside and out so the glaze does
not pull the pot to bits.
Another trick is instead of dunking the whole thing in water - fire the
Raku kiln in reduction and wrap the hot pot in news paper and then foil to
secure the reduction and or make the clay black (try not to set fire to your
self) - that stops it cracking.
But again for flat ware the thing to do is add more grog than you can
imagine would be necessary and when that cracks add more. Tell me how you
get on.
As an after thought its obvious that Raku is dangerous; lighting kilns made
out of old dust bins and shoving red hot pots in sawdust with rusty tongs. I
expect we only get away with doing it because such actions are beyond the
imaginations of the regulatory authorities.
--
Regards from Paul Taylor
http://www.anu.ie/westportpottery
The boy who announced that the king had no clothes was tortured for the
names of his confederates and then shot.
> From: Carole Fox
> Reply-To: Ceramic Arts Discussion List
> Date: Mon, 5 Aug 2002 09:31:52 -0400
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: raku thumping and horsehair question
>
> I know it's late to jump on the thread about the lack of a ringing when
> thumping on raku but I've been away and only now just caught up with a weeks
> worth of clayart. I have always thought that one of the reasons for using a
> clay that will not be vitrified at raku firing temperature is that the clay
> will be more accepting of the carbon...to get a good black. Please correct me
> if I am mistaken.
>
> I've been trying to make some large horsehair raku pots but am getting much
> cracking. Is there a limit to the size of a pot that will successfully take
> this treatment? I have been putting small pieces of kiln shelf in with the
> firing to place the pots on upon removal but this does not do the trick. It is
> starting to get a little depressing.
>
> Carole Fox- in Elkton, MD where it's hot and dry- not great for firing, but at
> least you don't have to mow the lawn too often!
> cfox@dca.net
>
> ______________________________________________________________________________
> 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.
michael Cave on wed 7 aug 02
Hi, can someone share with me as to how you thow a
horse hair pot in the first place??
--- Cantello Studios wrote:
> This is an easy one, after I lost 50% to 60% of my
> Horse Hair production
> over a six mounts period.
> I started running two kilns, a raku kiln to get the
> pots to temp and a small
> up draft running
> around 500 F to decorate in. This kiln also works
> like a annealing oven,
> which keeps the pots from cracking.
> It's quit a dance at times but when your loss rate
> drops down to almost 0
> It's a dance I'm loving.
> Another thing I found is the pots must be washed
> after the firing process is
> over and
> Before a wax past or what ever is put on. If not
> this protective coating
> will soon come off. Why??
> The carbon from the hair will act as a release. This
> is something I learned
> in Glass casting.
>
> Happy to Help. Chris Cantello -
> www.cantellostudios.com
>
> -----Original Message-----
> From: Ceramic Arts Discussion List
> [mailto:CLAYART@LSV.CERAMICS.ORG]On
> Behalf Of Bruce Girrell
> Sent: Monday, August 05, 2002 10:29 AM
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Re: raku thumping and horsehair question
>
> Carole Fox wrote:
> > I've been trying to make some large horsehair raku
> pots but am
> > getting much cracking. Is there a limit to the
> size of a pot
> > that will successfully take this treatment? I have
> been putting
> > small pieces of kiln shelf in with the firing to
> place the pots
> > on upon removal but this does not do the trick. It
> is starting to
> > get a little depressing.
>
> Oh, boy, can I empathize with this. Our first
> attempts at horsehair
> decoration were met with a frustratingly large
> percentage of cracks. Just to
> make things more difficult for ourselves, we chose
> to use a porcelain clay
> because of its whiteness and smooth texture. Often
> we lost more than half of
> our pieces.
>
> People talk about the thermal shock inherent in the
> raku process, but raku
> is nothing compared to horsehair decoration. Raku
> pots get thrown in a nice
> cozy reduction bin surrounded by burning material,
> while horsehair pots sit
> shivering out in the open air, over 1000 degrees
> hotter than their
> surroundings.
>
> Here are some suggestions based on what we have done
> to address this
> problem:
>
> 1) Lower the bisque temperature to ^010 to ^08 (we
> were doing our bisque at
> about ^06). Originally, we did this to help preserve
> the burnished finish on
> the pots, but I think it helps with the cracking as
> well. BTW, despite the
> low bisque temp, all of our pots ring when thumped
> unless there is a crack
> somewhere.
>
> 2) Add thermal shock resistant material to the clay
> body. Specifically, we
> have added pyrophyllite and talc. We tried
> wollastonite (thinking that the
> needle-like crystals would act like rebar), but it
> did not help much. We
> have had a lot of difficulty with the additives, as
> they decrease the
> (already low) plasticity of the clay. I suggest that
> you try other methods
> before messing with additives.
>
> 3) Throw and trim thin. The outside of the pot cools
> faster than the inside
> and, as a result, the outside of the pot goes into
> tension. The thicker the
> piece is, the greater the temperature difference
> between the inner and outer
> walls, hence more cracking. The thickness of our pot
> walls is usually about
> 1/8 inch.
>
> 4) Put up a wind block to minimize breezes.
> Variations in surface
> temperature due to one part of the pot cooling
> faster than another can lead
> to cracking.
>
> One unfortunate aspect of horsehair decoration is
> that the temperature range
> at which the decoration is done straddles the quartz
> inversion temperature.
> So you have a pot cooling at a tremendous rate that
> also has to undergo the
> beta to alpha quartz transition. If you have the
> opportunity to create your
> own clay body, I suggest that you use as little free
> quartz (flint, e.g.) as
> possible. I have not had the opportunity to do any
> extensive testing of clay
> bodies formulated specifically for minimizing
> thermal shock due to quartz
> inversion.
>
> I hope that helps. Just another note - we pull our
> pieces to a banding wheel
> that has a piece of refractory ceramic fiber board
> on it for insulation. The
> banding wheel allows us to turn the piece easily
> while applying the hair and
> the insulation minimizes thermal shock. Even with
> everything that we do, it
> is still a two person process. I handle the hot
> pieces while Lynne does most
> of the decoration. We find that we have just a
> little over a minute in which
> to complete the decoration. With our current methods
> we rarely suffer any
> cracking. We lose perhaps one pot in 20.
>
> Bruce "sometimes compared to a different part of a
> horse" Girrell
>
>
____________________________________________________________________________
> __
> 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.
>
>
______________________________________________________________________________
> 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.
__________________________________________________
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Marcia Selsor on wed 7 aug 02
michael Cave wrote:
> Hi, can someone share with me as to how you thow a
> horse hair pot in the first place??
Well, the tail hairs really cause the clay to twist and slice it up. Chopped hair
holds it together really weel.
No, no, no. I am teasing. You throw a pot for decorating with horse hair after a
bisque and another firing to 1200 F. I make mine very simple shapes , round and
buffed with terra sig.
Please excuse my teasing but I couldn't resist.
marcia in Montana
Rikki Gill on wed 7 aug 02
Hi Marcia, I recently saw the documentary film, "Rivers and Tides" about the
work of Andy Goldsworthy. It was just wonderful. To a potter, perhaps the
best part was about rebuilding an internal church wall in France. They dug
up local red clay, cleaned and removed all extraneous matter, added human
hair they had obtained from the floor of a local beauty parlor in Andy's
home town in Scotland for its strength, and applied it directly to the wall.
As the clay dried and cracked it showed a pattern from decorative supports
behind the clay. It seemed very beautiful. I don't remember if they
applied any finish, but I don't think so. What, to me, was wonderful about
his work was the fact that the entire world was his medium.
Anyway after that, the horse hair thread fitted in very nicely. Rikki
----- Original Message -----
From: "Marcia Selsor"
To:
Sent: Wednesday, August 07, 2002 8:02 AM
Subject: Re: raku thumping and horsehair question
> michael Cave wrote:
>
> > Hi, can someone share with me as to how you thow a
> > horse hair pot in the first place??
>
> Well, the tail hairs really cause the clay to twist and slice it up.
Chopped hair
> holds it together really weel.
>
> No, no, no. I am teasing. You throw a pot for decorating with horse hair
after a
> bisque and another firing to 1200 F. I make mine very simple shapes ,
round and
> buffed with terra sig.
> Please excuse my teasing but I couldn't resist.
> marcia in Montana
>
>
____________________________________________________________________________
__
> 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.
>
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