Cindi Anderson on mon 8 dec 03
Hi all
1. I did some platters in a class using Soldate 30 and they were fired in a
gas kiln. They sound "brittle" to me. I don't know how to explain it
better, but just stacking them on each other you get the feeling from the
sound of them that they are going to break. (They are thick, solid pieces
so it is not that they are thin and wimpy.) Indeed I just found that after
minor handling, the glaze (Shino) has chipped off in a spot. My guess is
these pieces were overfired? Seem right?
2. I know that you should slow down your firing during key times (quartz
inversion, etc), and also that when you are firing thick sculptures
(especially with uneven thicknesses) you should bisque fire very slowly. My
question is, do you fire very slowly through the whole temperature range, or
mainly at the beginning and during those critical phases such as quartz
inversion? Also is it important to go slower during the glaze firing as
well as the bisque?
Thanks!
Cindi
Fremont, CA
Dave Finkelnburg on wed 10 dec 03
Cindi,
First, 150-degrees F/hour is a good rule-of thumb rate of temperature
climb when "bisque" firing functional ware. For thick sculpture, you want
to slow down, depending on the clay body and the thickness.
If you are firing biscuit or bisque-ware, I think you want to cool
slowly during the key point you mention -- quartz inversion -- and to fire
and cool slowly when you have thick ware or ware with widely varying
thickness. Beyond that, in my very limited experience, few potters fire
fast enough to have found when firing up too fast is a problem. Most bisque
ware can be fired safely at 300 to 600 degrees F per hour. I routinely fire
ware 1/4-inch thick (just over a half centimeter thick) at 600 degrees F per
hour.
Of course if I have large platters, or ware up to one-inch thick, even
if it's bisque fired already, I only raise the temperature about 300 F/hour.
Regarding the Shino-glazed platters, the chipping you describe sounds
like it's caused by shivering. Did the glaze have lithium carbonate in it?
That might lead to localized shivering where the lithium evaporated at the
rim. In general American Shino glazes high in solubles like soda ash or
lithium carbonate will not have a nice ring. The solubles cause the fired
ware to have more of a dull thud.
Good firing!
Dave Finkelnburg
----- Original Message -----
From: "Cindi Anderson"
Sent: Monday, December 08, 2003 11:19 PM
> 1. I did some platters in a class using Soldate 30 and they were fired in
a
> gas kiln. They sound "brittle" to me. I don't know how to explain it
> better, but just stacking them on each other you get the feeling from the
> sound of them that they are going to break. (They are thick, solid pieces
> so it is not that they are thin and wimpy.) Indeed I just found that
after
> minor handling, the glaze (Shino) has chipped off in a spot. My guess is
> these pieces were overfired? Seem right?
>
> 2. I know that you should slow down your firing during key times (quartz
> inversion, etc), and also that when you are firing thick sculptures
> (especially with uneven thicknesses) you should bisque fire very slowly.
My
> question is, do you fire very slowly through the whole temperature range,
or
> mainly at the beginning and during those critical phases such as quartz
> inversion? Also is it important to go slower during the glaze firing as
> well as the bisque?
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