Waltraud Koestler on fri 13 apr 07
Hello,
I'd like to produce a plate with a diameter of about 50 cm.
I tried it several times, but without success.
Each plate cracked during biscuit firing. The crack starts in the middle of
the plate and goes straight to the outside of the plate.
I use clay with about 12 percent grog.
I'm rolling out a plate on the table a little bigger than necessary and cut
the access clay away.
During rolling out the clay I turned the plate bottom up and vice versa.
Additionally I tried to compress the plate on the wheel.
When letherhard, I put a rim made of a coil on it.
The plate has been dried very slowly - coverd with plasticsheet.
I let the kiln cooling down completly before I opened the kiln.
I dont't want to use a different clay, because my glaze is fitting to this
claybody. I have already tried a special clay for plates. The glaze cracks
at the rim and there occur very fine cracks all over the plate. This still
happens after days.
What am I doing wrong?
A friend of mine asked me for such a big plate.
Usually I do only small pieces.
I'd like to make one further try. If it's unsuccessful, I'll give it up.
I hope that someone of you can help.
Best regards
Waltraud
- Germany -
John Fulwood on fri 13 apr 07
Waltraud,
Try dusting your kiln shelf with grog before you load the plates. The grog will act like little ball bearings and allow the plate to move more easily as it shrinks during the firing.
good luck,
john fulwood
-----Original Message-----
>From: Waltraud Koestler
>Sent: Apr 13, 2007 12:01 PM
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Cracks in big plates
>
>Hello,
>
>I'd like to produce a plate with a diameter of about 50 cm.
>
>I tried it several times, but without success.
>Each plate cracked during biscuit firing. The crack starts in the middle of
>the plate and goes straight to the outside of the plate.
>
>I use clay with about 12 percent grog.
>I'm rolling out a plate on the table a little bigger than necessary and cut
>the access clay away.
>During rolling out the clay I turned the plate bottom up and vice versa.
>Additionally I tried to compress the plate on the wheel.
>When letherhard, I put a rim made of a coil on it.
>The plate has been dried very slowly - coverd with plasticsheet.
>I let the kiln cooling down completly before I opened the kiln.
>
>I dont't want to use a different clay, because my glaze is fitting to this
>claybody. I have already tried a special clay for plates. The glaze cracks
>at the rim and there occur very fine cracks all over the plate. This still
>happens after days.
>
>What am I doing wrong?
>
>A friend of mine asked me for such a big plate.
>Usually I do only small pieces.
>I'd like to make one further try. If it's unsuccessful, I'll give it up.
>
>I hope that someone of you can help.
>
>Best regards
>Waltraud
>
>- Germany -
>
>______________________________________________________________________________
>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.
John Fulwood
Kissimmee River Pottery
One 8th St. #11
Frenchtown, NJ. 08825
www.kissimmeeriverpottery.com
Susan Fox Hirschmann on fri 13 apr 07
Have you been drying them slowly and evenly, so that the rim does not dry any
faster than the base of the plate.
Sometimes the trick is in the slow, slow drying. With my large plates...
(and I do them in porcelain as well as stoneware).....I dry them under several
layers of plastic very slowly, changing the newspaper under them every few days.
When I uncover them, it is in a sequence: a few hours the first day, then
cover for the night , then a few more hours of uncovering the next
day......just really slowly. If I notice the rim drying too fast, that gets covered and I
leave the bottom uncovered for one to catch up with the other.
Best of luck! Hope this helps....
Susan
Annandale, Virginia
************************************** See what's free at http://www.aol.com.
Dannon Rhudy on fri 13 apr 07
It is most probable that your plate is cracking during the
heating or cooling of the kiln, when the foot stays hotter
than the rim (or the rim heats faster than the foot). You
did not say if your plate has a ring foot, but whether it does
or not, you should sit it on either a coil of clay made just for
the firing, or put "shims" of clay around the base when firing.
This lifts the piece off the heat-retaining shelf, and permits
the clay to heat and cool more evenly across the piece. Be
sure to let the work cool completely before opening the kiln.
On large diameter objects such as you describe, the differential
between the heating and cooling of the rim and center can be
substantial, and the different rates of expansion create cracks.
regards
Dannon Rhudy
> Hello,
>
> I'd like to produce a plate with a diameter of about 50 cm.
>
> I tried it several times, but without success.
> Each plate cracked during biscuit firing. The crack starts in the middle
of
> the plate and goes straight to the outside of the plate.
>
Michael Wendt on fri 13 apr 07
Waltraud,
One of the easiest ways to fire 20" (50 cm)
plates is:
Load the bottom of the kiln with taller pieces.
Place the plate rim down on shelves that are
level and match each other well.
Surround the plate with either soft fire brick
blocks to create a saggar or kiln posts.
Either will work but be sure to post the
next shelf with 5 mm of clearance from the
saggar so you don't break the shelves.
Load taller items above the plate and fire
in your usual manner.
Why this works:
Large flat items fire perfectly if the heating
from the edge is slowed while heating from
above and below is made as uniform as
possible.
Such a saggar configuration slows heating
and cooling from the edge.
The tall layers improve radiation penetration
to the center of the shelves above and below
the plate.
Please try this and let us know if it helped.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
Waltraud wrote:
Hello,
I'd like to produce a plate with a diameter of about 50
cm.
I tried it several times, but without success.
Each plate cracked during biscuit firing. The crack
starts in the middle of
the plate and goes straight to the outside of the
plate.
I use clay with about 12 percent grog.
I'm rolling out a plate on the table a little bigger
than necessary and cut
the access clay away.
During rolling out the clay I turned the plate bottom
up and vice versa.
Additionally I tried to compress the plate on the
wheel.
When letherhard, I put a rim made of a coil on it.
The plate has been dried very slowly - coverd with
plasticsheet.
I let the kiln cooling down completly before I opened
the kiln.
I dont't want to use a different clay, because my glaze
is fitting to this
claybody. I have already tried a special clay for
plates. The glaze cracks
at the rim and there occur very fine cracks all over
the plate. This still
happens after days.
What am I doing wrong?
A friend of mine asked me for such a big plate.
Usually I do only small pieces.
I'd like to make one further try. If it's unsuccessful,
I'll give it up.
I hope that someone of you can help.
Best regards
Waltraud
Vince Pitelka on fri 13 apr 07
Waltraud -
I have made a great many large platters, up to 30" in diameter, both
wheel-thrown and slab-formed, and learned early on that it's important to
never bisque fire the platter flat on the kiln shelf. Always place it up on
shims or wads, equally-spaced under the foot, or near the outer edge of the
flat bottom if there is no foot. I fire smaller platters on three shims
(fired clay, soft brick, short kiln posts,etc.), but that doesn't work for
large platters, because they need more points of support, and all the points
must touch the platter and the kiln shelf. In that case, I fire them on
wads or on a sun-burst arrangement of coils - perhaps twelve or fourteen
coils radiating out from the center, but not actually meeting in the center,
so that each space is open at the center and at the outer edge.
The wads or coils can be made from any claybody, but I often use a 50-50 mix
of clay and flint, which is cheap, can dry quickly with no fears of it
popping in the firing, and will shrink very little. Make sure that the wads
or coils are of a consistency that is soft enough that the they will
compress slightly when you set the platter down on them, providing perfectly
even support overall, and stiff enough that they will not flatten out
completely. The whole idea is to allow circulation of heat and atmosphere
under the platter. You might not thing it would make that much difference,
considering that it is only a small open space, perhaps 1/8", but it is
enough to make sure that the platter heats and cools independent of the kiln
shelf. I believe that this is where the problem lies. When you fire a
large platter directly on the kiln shelf, the bottom of the platter against
the kiln shelf is insulated from changes in temperature, so the rim of the
platter heats up before the center, and then it cools before the center. I
think that most of the cracking happens in the cooling cycle, when the kiln
shelf holds a lot of heat in the center while the rim is cooling and
shrinking. The crack from rim to center is the common result.
What temperature are you bisque-firing to? That is also an issue. Clay
begins to sinter at low red heat, and is technically fired at that point,
but the sintered connections between particles are very weak. As heating
progresses, the interaction of fluxes and silica begins to form a glassy
phase. Of course, that is also accompanied by shrinkage, and as the clay
gets stronger, it also gets denser. The challenge of bisque-firing is to
get a fired mass that is strong enough to survive cooling in the kiln
without cracking, and the subsequent handling in glazing, and porous enough
to take a coat of glaze. I like cone 08 as a bisque-firing temperature for
all claybodies, but you do have to be careful with any large flat pieces,
never firing them flat against the kiln shelf. At cone 08 the clay is not
strong enough to stand up to much discrepancy in thermal expansion and
contraction.
Hope this is helpful -
- 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/
Vince Pitelka on fri 13 apr 07
> Try dusting your kiln shelf with grog before you load the plates. The grog
> will act like little ball bearings and allow the plate to move more easily
> as it shrinks during the firing.
This technique can be very useful with large sculptural objects, allowing
them to move as they shrink. That's the main thing it accomplishes, as you
state above. But it does still insulate the center of a large flat object
from heating and cooling, and thus there is still the problem of a
discrepancy in expansion or contraction between the outer edge and the
center. I think that using a bed of sand or grog does aleviate the problem
slightly, because the sand is less of a "heat sink" than the solid surface
of the shelf. But for anyone having recurrent problems with cracking on
large platters, I'd certainly recommend firing them on wads or coils. With
large platters or other large flat objects, the overall expansion and
contraction, including firing shrinkage, is far less of a problem than
differential expansion between center and rim, so that's the first thing
that needs to be accommodated if there is problem with cracks from center to
rim.
- 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/
Nancy on sat 14 apr 07
Vince
thank you! I have been toying with the idea of placing some of my large
large bowls on wads but the information I was finding wasn't detailed
enough to try it. Your description makes it so easy now. Do you do
the same thing with you glaze fire?
Thanks again!
Nancy
Vince Pitelka wrote:
> Waltraud -
> I have made a great many large platters, up to 30" in diameter, both
> wheel-thrown and slab-formed, and learned early on that it's important to
> never bisque fire the platter flat on the kiln shelf. Always place it
> up on
> shims or wads, equally-spaced under the foot, or near the outer edge
> of the
> flat bottom if there is no foot. I fire smaller platters on three shims
> (fired clay, soft brick, short kiln posts,etc.), but that doesn't work
> for
> large platters, because they need more points of support, and all the
> points
> must touch the platter and the kiln shelf. In that case, I fire them on
> wads or on a sun-burst arrangement of coils - perhaps twelve or fourteen
> coils radiating out from the center, but not actually meeting in the
> center,
> so that each space is open at the center and at the outer edge.
>
> The wads or coils can be made from any claybody, but I often use a
> 50-50 mix
> of clay and flint, which is cheap, can dry quickly with no fears of it
> popping in the firing, and will shrink very little. Make sure that
> the wads
> or coils are of a consistency that is soft enough that the they will
> compress slightly when you set the platter down on them, providing
> perfectly
> even support overall, and stiff enough that they will not flatten out
> completely. The whole idea is to allow circulation of heat and
> atmosphere
> under the platter. You might not thing it would make that much
> difference,
> considering that it is only a small open space, perhaps 1/8", but it is
> enough to make sure that the platter heats and cools independent of
> the kiln
> shelf. I believe that this is where the problem lies. When you fire a
> large platter directly on the kiln shelf, the bottom of the platter
> against
> the kiln shelf is insulated from changes in temperature, so the rim of
> the
> platter heats up before the center, and then it cools before the
> center. I
> think that most of the cracking happens in the cooling cycle, when the
> kiln
> shelf holds a lot of heat in the center while the rim is cooling and
> shrinking. The crack from rim to center is the common result.
>
> What temperature are you bisque-firing to? That is also an issue. Clay
> begins to sinter at low red heat, and is technically fired at that point,
> but the sintered connections between particles are very weak. As heating
> progresses, the interaction of fluxes and silica begins to form a glassy
> phase. Of course, that is also accompanied by shrinkage, and as the clay
> gets stronger, it also gets denser. The challenge of bisque-firing is to
> get a fired mass that is strong enough to survive cooling in the kiln
> without cracking, and the subsequent handling in glazing, and porous
> enough
> to take a coat of glaze. I like cone 08 as a bisque-firing
> temperature for
> all claybodies, but you do have to be careful with any large flat pieces,
> never firing them flat against the kiln shelf. At cone 08 the clay is
> not
> strong enough to stand up to much discrepancy in thermal expansion and
> contraction.
>
> Hope this is helpful -
> - 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/
>
> ______________________________________________________________________________
>
> 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 sat 14 apr 07
Hi Waltraud,
If the crack is closed after the firing - That is a bisque dunting crack
and it happened during cooling at about 573C because the rime cooled faster
than the food.
Firing on the rim will stop it, as will raising the foot off the shelf a
little, as will slower cooling during the quartz inversion at 573C, as will
keeping the ware at the top of the kiln where it will cool slower.
Rim to rim firing will help but that can result in trapped organics and
black coring.
Even making rims bigger will help.
I would recommend firing to cone 04 - makes the ware stronger for starters.
Some clays have a lot of free quartz in them - porcelain is the worst.
If you are mixing your own clay I can recommend some changes in you clay
body that will help.
RR
>Hello,
>
>I'd like to produce a plate with a diameter of about 50 cm.
>
>I tried it several times, but without success.
>Each plate cracked during biscuit firing. The crack starts in the middle of
>the plate and goes straight to the outside of the plate.
>
>I use clay with about 12 percent grog.
>I'm rolling out a plate on the table a little bigger than necessary and cut
>the access clay away.
>During rolling out the clay I turned the plate bottom up and vice versa.
>Additionally I tried to compress the plate on the wheel.
>When letherhard, I put a rim made of a coil on it.
>The plate has been dried very slowly - coverd with plasticsheet.
>I let the kiln cooling down completly before I opened the kiln.
>
>I dont't want to use a different clay, because my glaze is fitting to this
>claybody. I have already tried a special clay for plates. The glaze cracks
>at the rim and there occur very fine cracks all over the plate. This still
>happens after days.
>
>What am I doing wrong?
>
>A friend of mine asked me for such a big plate.
>Usually I do only small pieces.
>I'd like to make one further try. If it's unsuccessful, I'll give it up.
>
>I hope that someone of you can help.
>
>Best regards
>Waltraud
>
>- Germany -
>
>______________________________________________________________________________
>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
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Dinah Steveni on sat 14 apr 07
Sound advice all round from many folks. Here's a tip to handle the grog on
kiln shelves idea, use a 1/4" layer of grog/placing powder whatever you wish
to call it. Keep a margin of about 1/2" on the edges to avoid grog falling
on pots on shelf below. As you unload the kiln, lift and stack shelves
horizontally on spare props so that the grog stays put. It won't fly around
the studio if you're not trying to scrape it back into a container will it?
Also on big platters I turn two foot rings for stability -- distance apart
depends on proportions -- and I believe this practice relieves potential
drying problems with the base and subsequent stress, and aids firing.
Dinah Steveni
To:
Sent: Friday, April 13, 2007 7:57 PM
Subject: Re: Cracks in big plates
> Waltraud -
> I have made a great many large platters, up to 30" in diameter, both
> wheel-thrown and slab-formed, and learned early on that it's important to
> never bisque fire the platter flat on the kiln shelf. Always place it up
on
> shims or wads, equally-spaced under the foot, or near the outer edge of
the
> flat bottom if there is no foot. I fire smaller platters on three shims
> (fired clay, soft brick, short kiln posts,etc.), but that doesn't work for
> large platters, because they need more points of support, and all the
points
> must touch the platter and the kiln shelf. In that case, I fire them on
> wads or on a sun-burst arrangement of coils - perhaps twelve or fourteen
> coils radiating out from the center, but not actually meeting in the
center,
> so that each space is open at the center and at the outer edge.
>
> The wads or coils can be made from any claybody, but I often use a 50-50
mix
> of clay and flint, which is cheap, can dry quickly with no fears of it
> popping in the firing, and will shrink very little. Make sure that the
wads
> or coils are of a consistency that is soft enough that the they will
> compress slightly when you set the platter down on them, providing
perfectly
> even support overall, and stiff enough that they will not flatten out
> completely. The whole idea is to allow circulation of heat and atmosphere
> under the platter. You might not thing it would make that much
difference,
> considering that it is only a small open space, perhaps 1/8", but it is
> enough to make sure that the platter heats and cools independent of the
kiln
> shelf. I believe that this is where the problem lies. When you fire a
> large platter directly on the kiln shelf, the bottom of the platter
against
> the kiln shelf is insulated from changes in temperature, so the rim of the
> platter heats up before the center, and then it cools before the center.
I
> think that most of the cracking happens in the cooling cycle, when the
kiln
> shelf holds a lot of heat in the center while the rim is cooling and
> shrinking. The crack from rim to center is the common result.
>
> What temperature are you bisque-firing to? That is also an issue. Clay
> begins to sinter at low red heat, and is technically fired at that point,
> but the sintered connections between particles are very weak. As heating
> progresses, the interaction of fluxes and silica begins to form a glassy
> phase. Of course, that is also accompanied by shrinkage, and as the clay
> gets stronger, it also gets denser. The challenge of bisque-firing is to
> get a fired mass that is strong enough to survive cooling in the kiln
> without cracking, and the subsequent handling in glazing, and porous
enough
> to take a coat of glaze. I like cone 08 as a bisque-firing temperature
for
> all claybodies, but you do have to be careful with any large flat pieces,
> never firing them flat against the kiln shelf. At cone 08 the clay is not
> strong enough to stand up to much discrepancy in thermal expansion and
> contraction.
>
> Hope this is helpful -
> - 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/
>
>
____________________________________________________________________________
__
> 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.
mcdenis on sat 14 apr 07
I had the same problem with a couple of big bowls recently. I let them dry
longer next time and covered the rims as Dannon suggested. Plus, in the
firing, I coated the shelves haphazardly with alumina which provides a
sliding surface under the plate/bowl during shrinkage. This was a usual
practice when I fired Cone 10 reduction. I got forgetful the last time when
bisquing at ^06.
Denis Rauchman
----- Original Message -----
From: "Dannon Rhudy"
To:
Sent: Friday, April 13, 2007 1:46 PM
Subject: Re: Cracks in big plates
> It is most probable that your plate is cracking during the
> heating or cooling of the kiln, when the foot stays hotter
> than the rim (or the rim heats faster than the foot). You
> did not say if your plate has a ring foot, but whether it does
> or not, you should sit it on either a coil of clay made just for
> the firing, or put "shims" of clay around the base when firing.
> This lifts the piece off the heat-retaining shelf, and permits
> the clay to heat and cool more evenly across the piece. Be
> sure to let the work cool completely before opening the kiln.
> On large diameter objects such as you describe, the differential
> between the heating and cooling of the rim and center can be
> substantial, and the different rates of expansion create cracks.
>
> regards
>
> Dannon Rhudy
>
>
>> Hello,
>>
>> I'd like to produce a plate with a diameter of about 50 cm.
>>
>> I tried it several times, but without success.
>> Each plate cracked during biscuit firing. The crack starts in the middle
> of
>> the plate and goes straight to the outside of the plate.
>>
>
> ______________________________________________________________________________
> 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.
>
Chris trabka on sat 14 apr 07
When I bisque fire, I rarely bisque any plate/platter by itself. Often they
are stacked 5 high (or more). The bottom one is on the kiln shelf. Three
pieces of soft brick (1/4" thick) separate it from the plate/platter on
top. The pieces of soft brick form vertical columns. I can not remember a
crack caused by this stacking method. The top plate/platter is generally
1/4" from the shelf immediately above. With small plates, I use just a
single piece of soft brick (in lieu of three).
Chris
Vince Pitelka on sun 15 apr 07
Nancy wrote:
> thank you! I have been toying with the idea of placing some of my large
> large bowls on wads but the information I was finding wasn't detailed
> enough to try it. Your description makes it so easy now. Do you do
> the same thing with you glaze fire?
Nancy -
I have not had any problem with large plates and platters cracking in the
glaze firing. I don't even know why. If you have any doubts, you could
certainly wad the piece in the glaze firing, but I find the major time of
risk is the cooling cycle in the bisque firing.
- 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/
Nancy on mon 16 apr 07
vince
I haven't had any cracking problems even in the glaze firing but my
bowls are only 15" wide and I use about 6 lbs of clay so maybe mine
aren't big enough to cause problems. I have been commissioned to make a
birdbath to replace a copper one and have used 15 lbs of clay and it is
about 22" so I feel on these with the weight it may be a problem and
wanted to do whatever I could help through the process. I bisque fire
to cone 04 and since the birdbath is an outdoor feature, I have thrown
them thicker than my normal bowls so they are also much heavier. I am
also firing my first 23" platter since one attempt at the birdbath
became a platter :)
Thanks again I appreciate your help.
N ancy
Vince Pitelka wrote:
> Nancy wrote:
>> thank you! I have been toying with the idea of placing some of my large
>> large bowls on wads but the information I was finding wasn't detailed
>> enough to try it. Your description makes it so easy now. Do you do
>> the same thing with you glaze fire?
>
> Nancy -
> I have not had any problem with large plates and platters cracking in the
> glaze firing. I don't even know why. If you have any doubts, you could
> certainly wad the piece in the glaze firing, but I find the major time of
> risk is the cooling cycle in the bisque firing.
> - 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/
>
> ______________________________________________________________________________
>
> 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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