Wanda Holmes at Alistia on tue 18 dec 01
I recently fired some 6.5x6.5" tiles, 3/8" thick, and almost all of them
came out of the bisque firing with s-cracks right down the center. Hamer
and Hamer refers to s-cracks as the result of uneven shrinkage in the base
versus the walls of a pot. But it's unclear to me how this applies to a
tile.
I was in a rush and I dried the tiles out in an oven at 150F for 24 hours
before firing. Could the cracks be due to moisture? To drying too fast?
I stacked the tiles in the kiln and I did notice that the top tile on the
stack did not crack while each of the tiles below the top did. I've stacked
before and never had a problem, but this does seem curious enough to perhaps
be a clue.
I fired at 90F/hr to 212F, then 212F/hr to 1745F, held for 2 hours, then
108F/per hour to 1945 (cone 04), in an electric kiln with an envirovent on
during the entire firing. The kiln cooled naturally to 114F before I opened
it, with the envirovent remaining on during the cooling.
I've seen this once before, some months ago, but it seemed a freak occurence
and I didn't pursue it. Now that it's happened again, I realize I need to
find the source of the problem. One final bit of information is that I
generally work with smaller tiles - 4x4" - and the only other time this
occurred, it was also with a larger tile. Is there something about the size
of the tile that I need to be aware of and handle differently?
Any help any of you can provide will be greatly appreciated.
Wanda
In central Texas where the Christmas bird count yesterday in the Balcones
Canyonlands was spectacular.
vince pitelka on tue 18 dec 01
> I was in a rush and I dried the tiles out in an oven at 150F for 24 hours
> before firing. Could the cracks be due to moisture? To drying too fast?
> I stacked the tiles in the kiln and I did notice that the top tile on the
> stack did not crack while each of the tiles below the top did. I've
stacked
> before and never had a problem, but this does seem curious enough to
perhaps
> be a clue.
Wanda -
I prefer not to fire tiles stacked directly on top of one another because of
problems arising from uneven heating of flat objects. But I do not think
that was the problem here. If that had been the problem, you would have had
cracks extending in from the outside edge. Since your cracks were confined
to the center of the tile, I think you can assign all the blame to
shock-drying, which almost always causes problems. In this case, the tiles
dried around the edges before drying in the center. When they dried in the
center it built up stresses, which developed into cracks in the firing. It
is very similar to S-cracks in thrown wares, but in that case the stresses
are caused by greater shrinkage of excessively damp or un-compressed clay in
the center of the piece. You were the unfortunate victim of that pitfall
which strikes so many of us, where we are in too much of a hurry, and end up
damaging or destroying the work.
Best wishes and good luck redoing the tiles -
- 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/
linda blossom on tue 18 dec 01
Dear Wanda,
I really don't think the drying method had anything to do with the s cracks.
I have dried tiles in the sun and on a shelf over a wood stove in a matter
of a few hours and never had a problem. Could you tell me how you made the
tiles? The only time I ever saw s cracks was a long time ago when I thought
it would be clever to cut them off a bagged pug of clay by slicing across
the short distance. Every one cracked this way and it seems to me that the
particle alignment had something to do with this since the particles are
parallel to the direction the clay is pugged. If I have this right, the
particles are on end in a tile if the clay is cut perpendicular to the long
side. If moisture in the stacking was a problem I would expect cracking and
breaking from moisture having difficulty escaping. I wouldn't stack
horizontally - I have stacked vertically with a coil of clay pressed across
the top surface to keep them separate.
Linda
Ithaca, NY
> I was in a rush and I dried the tiles out in an oven at 150F for 24 hours
> before firing. Could the cracks be due to moisture? To drying too fast?
>
> I stacked the tiles in the kiln and I did notice that the top tile on the
> stack did not crack while each of the tiles below the top did. I've
stacked
> before and never had a problem, but this does seem curious enough to
perhaps
> be a clue.
>
> I fired at 90F/hr to 212F, then 212F/hr to 1745F, held for 2 hours, then
> 108F/per hour to 1945 (cone 04), in an electric kiln with an envirovent on
> during the entire firing. The kiln cooled naturally to 114F before I
opened
> it, with the envirovent remaining on during the cooling.
>
> I've seen this once before, some months ago, but it seemed a freak
occurence
> and I didn't pursue it. Now that it's happened again, I realize I need to
> find the source of the problem. One final bit of information is that I
> generally work with smaller tiles - 4x4" - and the only other time this
> occurred, it was also with a larger tile. Is there something about the
size
> of the tile that I need to be aware of and handle differently?
>
> Any help any of you can provide will be greatly appreciated.
>
> Wanda
>
> In central Texas where the Christmas bird count yesterday in the Balcones
> Canyonlands was spectacular.
>
>
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Paul Lewing on tue 18 dec 01
Wanda, I agree with Vince that it was probably the forced drying that caused
your cracking this time. It's also true, as you say, that the bigger your
tiles are, the likelier they are to crack.
One thing that has caused tiles to crack for me, though, is wedging the
clay. I just slice them off a pugged block of clay and run them through the
roller. And making tiles out of the scraps that I trim off tiles and wedge
up will virtually always result in this kind of cracks.
And when I bisque mine, I stand them up on their sides and leave a bit of
air between them.
Good luck,
Paul Lewing, Seattle
Martin Howard on wed 19 dec 01
A simple way of slow drying the tiles is to lay them on a wooden table and
then lay another table sized piece of wood on top. I use an 8 feet by 4 feet
panel of plywood or a similar panel of flooring material.
For a week I work on the top of that second table, while the tiles are
drying underneath. It keeps them flat and they don't crack.
Martin Howard
Webbs Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
01371 850 423
martin@webbscottage.co.uk
http://www.webbscottage.co.uk
Updated 13/12/01
Marsh Pottery on wed 19 dec 01
For we "little old ladies" who cannot lift whole sheets of plywood may I
suggest smaller pieces of dry wall works just as well & the plaster inside
helps absorb the water in the clay.
Marsha Koenig
----- Original Message -----
Subect: S-cracks in tiles:
A simple way of slow drying the tiles is to lay them on a wooden table and
then lay another table sized piece of wood on top. I use an 8 feet by 4 feet
panel of plywood or a similar panel of flooring material.
For a week I work on the top of that second table, while the tiles are
drying underneath. It keeps them flat and they don't crack.
Martin Howard
Webbs Cottage Pottery
Russel Fouts on thu 20 dec 01
Martin
>> A simple way of slow drying the tiles is to lay them on a wooden table and then lay another table sized piece of wood on top. I use an 8 feet by 4 feet panel of plywood or a similar panel of flooring material. For a week I work on the top of that second table, while the tiles are
drying underneath. It keeps them flat and they don't crack. <<
An ingenious solution but with the way I hand build, pounding, dropping,
slapping, flapping, throwing, etc, those tiles would end up as flat
powder. ;-)
Are the tiles the only support for the 2nd table top?
Russel
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Mes Potes & Mes Pots
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