L. P. Skeen on mon 8 sep 03
I have been making tiles from a Standard white sculpture claybody. They =
are 3/8" thick and 6x6". I stack 'em about 10 high to bisque in my =
12cube electric oval kiln with computer controller.
YEsterday a friend fired a stack of these for me in her NON computer =
kiln. Same temperature bisque (06). The centers of these tiles are =
BLACK. They look like somebody held a match right in the middle of the =
tile. Any idea what caused this? IT's a white claybody, and NONE of =
the tiles I bisqued in MY kiln has come out like this. Weirdness......
L
L. P. Skeen www.living-tree.net
Living Tree Pottery, Summerfield, NC
"There are three kinds of men. The ones who learn by reading, and the =
few who learn by observation. The rest of them have
to pee on the electric fence."------- Will Rogers (att.) =20
Louis Katz on mon 8 sep 03
Tile manufacturers use this property to make tiles go from red edges to
black. The black is unburnt carbon. In red clays it is probably also
reducded iron bearing clay. Only the edges of your tiles got enough air
to burn off the everpresent organic material. I would rebisque.
Louis
----------------------------------
> The centers of these tiles are BLACK. L. P. Skeen
>
piedpotterhamelin@COMCAST.NET on mon 8 sep 03
She burnt her kiln for a shorter duration of firing and cooling than your
kiln.
> I have been making tiles from a Standard white sculpture claybody. They are
> 3/8" thick and 6x6". I stack 'em about 10 high to bisque in my 12cube electric
> oval kiln with computer controller.
>
> YEsterday a friend fired a stack of these for me in her NON computer kiln. Same
> temperature bisque (06). The centers of these tiles are BLACK. They look like
> somebody held a match right in the middle of the tile. Any idea what caused
> this? IT's a white claybody, and NONE of the tiles I bisqued in MY kiln has
> come out like this. Weirdness......
>
> L
>
>
>
> L. P. Skeen www.living-tree.net
> Living Tree Pottery, Summerfield, NC
> "There are three kinds of men. The ones who learn by reading, and the few who
> learn by observation. The rest of them have
> to pee on the electric fence."------- Will Rogers (att.)
>
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Snail Scott on mon 8 sep 03
At 12:43 AM 9/8/03 -0400, you wrote:
>I have been making tiles...I stack 'em about 10 high to
bisque...>YEsterday a friend fired a stack of [tiles]...bisque (06). The
centers of these tiles are BLACK. They look like somebody held a match
right in the middle of the tile. Any idea what caused this? IT's a white
claybody...
Remember that one of the purposes of bisque is
to burn out organic impurities. (Operative word
here: burn!) ;) Firing a thick stack of tiles
makes it harder for the carbon (produced by the
burning) to escape, so it's stuck to the tiles,
except in the areas close to the edge where it
was able to escape.
This is a case where a slower firing (or little
soaking time) would be useful, or else don't
stack them quite so tightly and so deep next time.
If you re-fire them, the carbon will burn off OK.
Just give it time. Although we call electic firing
'oxidizing', it's not, really. At best it's a
neutral atmosphere, and if you are burning off a
lot of stuff like this, I'd bet it's actually a
mildly reducing environment. Without lots of loose
oxygen (true 'oxidation'), it'll take a little
while for the carbon to be removed, especially
when it's trapped between layers of flat tiles.
(It's similar to the effect of 'carbon core',
when a thick-walled piece of clay is gray in the
middle (visible if broken) because the carbon
did not escape completely.)
Some clay bodies have more organic material in
them than others, and the color of the processed
wet clay is no indication.
-Snail
Stephani Stephenson on mon 8 sep 03
L>P>Skeen wrote:
Yesterday a friend fired a stack of these for me in her NON computer
kiln. Same temperature bisque (06). The centers of these tiles are
BLACK. They look like somebody held a match right in the middle of the
tile. Any idea what caused this? IT's a white claybody, and NONE of
the tiles I bisqued in MY kiln has come out like this. Weirdness......
L
Lisa.. when you say 'The centers' of the tile, I am inferring that you
mean, a dark circular area when you look down at the face of the tile?
My best guess is that you have areas of localized reduction. when tiles
are stacked like that, air does not circulate as easily to those center
areas. organic materials in the clay do not have a chance to fully
combust in presence of oxygen. . I am not familiar with your particular
claybody or the firing schedule or stack of the kiln you refer to, so
can only speak in generalities.
In addition to incomplete combustion , and subsequent changes in the
clay due to lack of oxygen at the center, I think compounds which were
unable to combust may also become deposited in the middle of the tile
. ..I had a claybody which I intentionally stacked in this manner as I
would get center halos of color on the tile/ In later glaze firing the
center would react differently to the glaze, so it it seemed to be more
than just simple reduction... though these are just my observations and
I cannot say for sure.
This seems the opposite of conventional thinking, which would have
soluble salts being deposited wherever the water last leaves the tile...
(through the circumference of the tile rather than the center, in this
case). I don't quite understand the chemistry here, but I have observed
noticeable differences in tile center with certain claybodies when
stacked this way, though not as dramatic as the ones you describe.
rebisquing the tile will likely rid you of the black....though there may
be a concentration of trace elements in the area which may still show up
in some way in the later glaze firing or refiring
. I usually fire 6X6 tile on edge. If this is a continued problem I
would suggest firing on edge and even putting spacers between every tile
or every 4th tile or so..... Also it may be that there was not adequate
ventilation during the firing in the first 1000 degrees F or so.. .. I
always keep lid propped for first part of firing...and peeps open
throughout the bisque firing. (my kiln is well insulated and has small
peepholes...). I even keep the lid propped about 1/4 inch during the
entire firing when I have a full load of greenware.
sincerely
Stephani Stephenson
steph@alchemiestudio.com
Ron Roy on tue 9 sep 03
Hi Lisa,
Yes -as so many are saying - it's unburned carbon - I wonder if you have a
vent running and the other kiln did not - that would make a big difference.
Speed of firing will also make a difference.
It would not have happened if the tile were not stacked up of course.
RR
>I have been making tiles from a Standard white sculpture claybody. They
>are 3/8" thick and 6x6". I stack 'em about 10 high to bisque in my 12cube
>electric oval kiln with computer controller.
>
>YEsterday a friend fired a stack of these for me in her NON computer kiln.
>Same temperature bisque (06). The centers of these tiles are BLACK.
>They look like somebody held a match right in the middle of the tile. Any
>idea what caused this? IT's a white claybody, and NONE of the tiles I
>bisqued in MY kiln has come out like this. Weirdness......
>
>L
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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