Erin Hayes on mon 3 dec 01
Hi Friends,
I have been making slab work of late, and I have noticed that I get some =
rather spectacular cracking in the second firing - to Cone 3 ox. From =
the looks of things, I am thinking that the shrinkage taking place in =
the firing may be combining with the surface area of the piece in full =
contact with the shelving, causing friction and cracks. I use alumina =
hydrate on the shelves as a thin bed, but that doesn't seem to help =
consistently.
My questions are - is my hypothesis correct that the friction between =
the shelf and the work may be a factor? and has anyone used thin fiber =
blanket as a cushion for slab work? I use newspaper between my slab =
work and ware boards when I build, and I thought maybe a thin fiber =
blanket might sort of act the same way in the firing. Please feel free =
to shoot that full of holes if you know better!
Thanks for any advice you can give. If you'd like to contact me =
off-list, please try earthandsky@earthlink.net, as my @home connection =
is a daily questionmark since the declared bankruptcy.
Erin.
vince pitelka on tue 4 dec 01
You wrote:
"My questions are - is my hypothesis correct that the friction between the
shelf and the work may be a factor? and has anyone used thin fiber blanket
as a cushion for slab work? I use newspaper between my slab work and ware
boards when I build, and I thought maybe a thin fiber blanket might sort of
act the same way in the firing."
Flat slabs are the most difficult shapes to fire. Whenever possible, they
should be fired standing on edge. But if they are fired flat, a bed of
alumina, sand, or fiber blanket is no better than firing the piece flat on
the shelf, and may even be worse. Movement is not the problem. Uneven heat
and shrinkage are the problem, and all of these solutions INSULATE the
center of the piece from heat, so that the outside edge heats up first.
When that happens, a large flat piece will almost always crack.
To overcome the problem, fire the piece on parallel rows (spaced every 2")
of thin coils of mixture of 50/50 clay and silica (same recipe as basic
shelf wash), and glaze fire at normal speed. Do not criss-cross the coils,
because you want air circulation beneath the piece. This mixture is similar
to the wadding you use in salt, soda, and wood kilns, but in that case you
would use alumina rather than silica. In this case silica works as well,
and is far cheaper. You can heat this mixture from damp with a torch and it
will not pop.
Put the piece in place right after you prepare and place the coils, and
nestle it down gently on the coils so that it is well supported overall.
Don't push down too hard, because as I said, you want air circulation
beneath the piece.
Good luck -
- Vince
Ceramic Design Group on tue 4 dec 01
on 12/3/01 10:07 PM, Erin Hayes at ephayes@HOME.COM wrote:
> Hi Friends,
>
> I have been making slab work of late, and I have noticed that I get some
> rather spectacular cracking in the second firing - to Cone 3 ox. From the
> looks of things, I am thinking that the shrinkage taking place in the firing
> may be combining with the surface area of the piece in full contact with the
> shelving, causing friction and cracks.
It sounds to me that the cracking is not "joint" related cracking.
These could be dunts.
There is more surface contact with the kiln shelf and the base of your work,
and this area retains heat longer than the rims, edges, walls, etc. These
outside areas are cooling quicker than the inside areas, so what we have is
a classic T1 and T2 (temperature 1 and temperature 2) differential.
A solution is to fire the work using a series or refractory stilts to
elevate the ware off the kiln shelf, but this may cause some warping if not
stilted correctly. I have seen some triangular bars made of a refractory
material. This might help, as might some larger size grog so that not only
is the friction reduced as the piece shrinks inwards, but it will also
elevate the ware off the kiln shelf.
It could also be a glaze related issue if the ware is in an overly high
state of "squeeze" from the glaze compression, but there would be shivering
also from this, but you didn't mention this as occurring. Could also be the
shape of the ware also which may add some stress during the cooling state.
The cracks would also produce a visual clue if they are heating related
cracks or cooling related cracks. If the glaze has rolled over the edge over
the edge of the crack and into the crack, it is a heating crack. If the edge
of the crack is sharp and has no glaze migration into it, it is a cooling
dunt.
Jonathan
--
Jonathan Kaplan
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
infor@ceramicdesigngroup.net www.ceramicdesigngroup.net
(use PO BOX for all USPS correspondence)
Plant Location
1280 13th Street
Steamboat Springs CO 80487
(use PLANT LOCATION for all UPS, Common Carrier, and Courier deliveries)
Cindy Strnad on tue 4 dec 01
Dear Erin,
Friction from the shelf can cause cracking, though
of course I don't know if that's your problem.
Uneven heating caused by the heat not having
access to the base of your pieces can also be a
factor. There are a number of ways to solve this
which are less messy and expensive than fiber
blanket, however. Make yourself some little clay
roller-logs and situate them under your form. I
don't anticipate you'll have trouble with slumping
at ^3, but then I haven't seen your forms. If
you've got large, flat surfaces to support, use
lots of roller-logs.
Meself, I use silica sand, but that is messy. Not
as messy as fiber blanket. The silica sand will
reduce friction, but has limited application in
evening out the heat that flows under your piece.
It won't cause your pieces to slump, however, so
you might want to try this if the roller-logs
don't work for you.
Best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Marcia Selsor on tue 4 dec 01
Dear Erin,
I tried the fiber blanket and cracked every slab in the kiln. Now I fire them upright. If that doesn't work for the shape, use a heavier grog bed for more movement. I think the fiber "insulates " (duh!) the piece causing uneven heat and resulting in cracks. -my theory anyhow.
Good luck,
Marcia in Montana
Erin Hayes wrote:
> Hi Friends,
>
> I have been making slab work of late, and I have noticed that I get some rather spectacular cracking in the second firing - to Cone 3 ox. From the looks of things, I am thinking that the shrinkage taking place in the firing may be combining with the surface area of the piece in full contact with the shelving, causing friction and cracks. I use alumina hydrate on the shelves as a thin bed, but that doesn't seem to help consistently.
>
> My questions are - is my hypothesis correct that the friction between the shelf and the work may be a factor? and has anyone used thin fiber blanket as a cushion for slab work? I use newspaper between my slab work and ware boards when I build, and I thought maybe a thin fiber blanket might sort of act the same way in the firing. Please feel free to shoot that full of holes if you know better!
>
> Thanks for any advice you can give. If you'd like to contact me off-list, please try earthandsky@earthlink.net, as my @home connection is a daily questionmark since the declared bankruptcy.
>
> Erin.
>
> ______________________________________________________________________________
> 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.
Richard Jeffery on tue 4 dec 01
I have had large slabs (in propane raku kiln) crack - found solution - if
you piece allows it - to fire on a thin layer of sand/crushed soft
brick/whatever. seems to allow the movement, rather than fighting it.
Richard
Bournemouth UK
www.TheEleventhHour.co.uk
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Ceramic Design Group
Sent: 04 December 2001 15:50
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Question re: firing slab work
on 12/3/01 10:07 PM, Erin Hayes at ephayes@HOME.COM wrote:
> Hi Friends,
>
> I have been making slab work of late, and I have noticed that I get some
> rather spectacular cracking in the second firing - to Cone 3 ox. From the
> looks of things, I am thinking that the shrinkage taking place in the
firing
> may be combining with the surface area of the piece in full contact with
the
> shelving, causing friction and cracks.
It sounds to me that the cracking is not "joint" related cracking.
These could be dunts.
There is more surface contact with the kiln shelf and the base of your work,
and this area retains heat longer than the rims, edges, walls, etc. These
outside areas are cooling quicker than the inside areas, so what we have is
a classic T1 and T2 (temperature 1 and temperature 2) differential.
A solution is to fire the work using a series or refractory stilts to
elevate the ware off the kiln shelf, but this may cause some warping if not
stilted correctly. I have seen some triangular bars made of a refractory
material. This might help, as might some larger size grog so that not only
is the friction reduced as the piece shrinks inwards, but it will also
elevate the ware off the kiln shelf.
It could also be a glaze related issue if the ware is in an overly high
state of "squeeze" from the glaze compression, but there would be shivering
also from this, but you didn't mention this as occurring. Could also be the
shape of the ware also which may add some stress during the cooling state.
The cracks would also produce a visual clue if they are heating related
cracks or cooling related cracks. If the glaze has rolled over the edge over
the edge of the crack and into the crack, it is a heating crack. If the edge
of the crack is sharp and has no glaze migration into it, it is a cooling
dunt.
Jonathan
--
Jonathan Kaplan
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
infor@ceramicdesigngroup.net www.ceramicdesigngroup.net
(use PO BOX for all USPS correspondence)
Plant Location
1280 13th Street
Steamboat Springs CO 80487
(use PLANT LOCATION for all UPS, Common Carrier, and Courier deliveries)
____________________________________________________________________________
__
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.
iandol on wed 5 dec 01
Dear Erin Hayes,
I have two suggestions.
1 In place of aluminium hydrate I would substitute 100# alumin sand. =
this is better at permitting motion as things expand and contract =
whereas alumina hydrate is a very fine powder and will not assist =
movement.
2 You seem to be firing at a temperature well below the pyroplastic =
deformation point of your clay. This will allow you to raise your work =
on shallow supports or low wads which will allow hot air and gas to =
circulate beneath the floor of each pot ensuring even heating. This will =
avoid stress caused by differential thermal expansion due to temperature =
variations within the fabric of your pieces.
Best regards,
Ivor Lewis
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