Alisa and Claus Clausen on tue 25 apr 00
------------------
Dear Clayart,
After reading Cindy=3B Tim's and Paul's posts, I think I can better sort out=
what
happened in my firing and
why the pots had such pronounced bloating.
As I check over my notes, I should say that I was firing to 1240c not 1250c,
just for the record.
According to Orton cones, that would be a cone 7.
All of the clay bodies combined have a firing range suggested as 1220-1260c
oxidation and up to
1300c in reduction (which at this time I do not do). In oxidation, the =
range is
cone 6 to cone 8.
All of the glazes I use have the same range of 1220c1260c with the exception=
of
one that has the
suggested range of 1250-1280c. In the firings I have done with that glaze, =
I
found 1240c to
be sufficient to melt it uniformily. However, at 1250, some of my =
=22universal
under and over
glazes=22 get weak or the greens burn out. I have just decided to omit the
1250-1280c range
glaze and see what the results will be with my next glaze firing lowered to
1230c.
I was thinking that because I stacked the kiln too tightly, that the heat =
may
have been held in,
retaining a high temp. for a long time. All my wrong theories. But Cindy =
says
that that a tightly
packed kiln makes for a cooler kiln. I am beginning to get it..
Tim brings in the entire idea of heatwork, something I knew very little =
about
but am reading
rapidly up on.
I did not have a witness cone because I simply ran out and forgot to get =
more
when I just at
my supplier. But usually I use them and the kiln does fire hotter on top, =
but
that is where I have
been placing the hirer firing (mat) glazes. I absolutely agree that a =
witness
cone is essential.
Mike Bailey was really helpful a ways back helping me decide on a controller=
or
sitter. His advise
was well taken, that nothing is completely dependable and one should be =
there
for the firing or at
least around the time the kiln shuts off.
Cindy says that 14 hours is too long to fire. That is what it is taking =
now,
with the ramp program
I got from Tim. If I do the math, the program should take ca. 10 hours. =
But my
kiln is 30 years old
and I do not know if the elements or some of them are as old.
I believe after evaluating all the factors of what could have been wrong, I
think it was the heatwork.
I have other pots of mixed up cone 6-8 clays that I will fire in a hopefully
normal firing next time,
just to see if that was not the probem. I think it was too much heat.
Thanks everyone for your input. It has helped me to sort out most likely =
what
went wrong.
I am looking forward to Tim's answer about cool down and restarted a failed
firing.
Best regards,
Alisa in not such good pots
Denmark
But I rethrew most of them yesterday and today with one nice, smooth,
light grey, good consistency, lots of tooth, all in all , lovely claybody.
How quickly we forget the pain..
-----Oprindelig meddelelse-----
Fra: Cindy Strnad =3Cearthenv=40gwtc.net=3E
Til: CLAYART=40LSV.UKY.EDU =3CCLAYART=40LSV.UKY.EDU=3E
Dato: 24. april 2000 19:17
Emne: Re: Toil and kiln trouble
=3E----------------------------Original message----------------------------
=3EAlisa,
=3E
=3EGlazes tend to be a bit more forgiving of overfiring than claybodies are.
=3EThey may run off, or lose color, but glazes can usually be fired higher =
than
=3Etheir designated cone, in my experience.
=3E
=3EI'm no expert, but if you truly mixed only =5E6 bodies, I doubt that was =
your
=3Eproblem. If I'm wrong, I'll gladly concede the point.
=3E
=3EIt's never a good idea to pack your kiln too tightly, but this generally
=3Eresults in lower, rather than higher firing temperatures, and slower
=3Efirings.
=3E
=3EFourteen hours is a long time to take getting to =5E6. Is your kiln =
capable of
=3Ereaching temperature faster, and you just fire slowly because you want =
to?
=3EMine used to take that long and longer and still never quite make
=3Etemperature, but this is not a normal or acceptable situation. Slow =
firing
=3Ecan make some beautiful pottery, but it should be a choice rather than a
=3Enecessity.
=3E
=3EI suspect that the pre-firing did more heat-work than you realized, and =
that
=3Eyour clay body may have reached more like the equivalent of =5E7 or even =
=5E8.
=3EMany =5E6 glazes can take this in their stride, but a true =5E6 body =
won't like
=3Eit at all.
=3E
=3EFinally, did you use a witness cone? It's possible that your kiln reached=
a
=3Ehigher temperature than the thermocouple was =22willing=22 to admit. =
These
=3Egadgets are wonderful, great, excellent tools to have, but you can't take
=3Ethem too seriously.
=3E
=3ECindy Strnad
=3Eearthenv=40gwtc.net
=3EEarthen Vessels Pottery
=3ERR 1, Box 51
=3ECuster, SD 57730
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