Cameron Harman on sun 16 nov 97
Craig,
Black coring is more likely in any kiln having a deficiency
of oxygen and a lot of organic material in the body. Black
coring comes from the organic material taking oxygen from
the iron oxide because that is its next target for oxygen
when it can't get any more free oxygen. The slightly reduced
iron oxide then reacts with silica to produce iron silicate
which is black and glassy.
If you fire in a any kiln that has lots of oxygen, but is
fired very fast in the first 900 degrees F, the oxygen may
not diffuse into the thick wall of a piece of ware. The
organics will still react with oxygen it can find and will
behave as described above. sometimes there is no iron oxide
present, but the organics still must react and will produce
ware having some other sub-oxides. Those sub-oxides produce
a white core in some cases. That white core is also weak and
can cause delayed cracking (hours, days or weeks after
firing) or can even cause pieces to split apart. Talc is one
of the materials that can get involved in this same type of
chemical process.
So, remember, it is the organics that are the engine driving
the creation of the problem. You must give the organics
enough oxygen from the outside of the ceramic and that
usually means giving enough time for the oxygen to diffuse
through the ceramic walls.
In an electric kiln there is usually sufficient oxygen,
sometimes not, however. If you have very thick walled
ceramic or have a lot of ceramic tightly packed in the load,
you could get the black coring in either a gas or an
electric kiln (it happens all the time) if there is also
enough organic material present. You could get black coring
in the center of the load and not on the outside in either a
gas or an electric kiln.
On the other hand, if you had such a load, but you had
sufficient fresh air coming into the kiln and allowed enough
time, the problem would disappear. Either an electric kiln
with some circulation or a gas kiln firing with excess air
would work.
Now, I'm going to seem really mean, but I'm just trying to
be technically accurate .. You can also get sub-oxides in
reactions at higher temperatures (about 1800 F) due to other
material breakdown. Whiting turns into CaO and CO2 with heat
and reacts with the ceramic matrix . At this point the
explanations can get too complicated and are not usually
mentioned because most of the problems we get are from the
lower temperature reactions mentioned above.
By the way, a lot of the red clay used for pottery and for
brick making contains a lot of organics. Some clay is
cleaner than others which is why the problem turns up
sometimes more than others, even in clay with lots of iron
oxide.
One final word, I am not trying to mess with anyone's heads
here, just trying to give you a straight picture of what's
taking place. The subject is much larger and more
complicated than presented here; so you can pick apart some
arguments that I have made on purely technical grounds.
These points are purposely simplified so that they have
meaning and are not just a lot of technical jargon. If I
have failed in this mission I know you will tell me, and for
that I apologize in advance.
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
see our web site at http://www.kilnman.com/potters.html
THE place for total kiln and drier support
**********************************************************
| |
|
