John Baymore on sun 15 oct 00
The very question that seems to be at the core of this whole discussion
seems to me to be the source of a bit of a problem...... in that it sort =
of
implies that there is supposed to be ONE correct answer. "THE" correct
cone to bisque at. But actually, lots of answers work.
Like 99.99 percent of questions about ceramics...... the best (but
frustrating) answer is.................... "It depends." =
There are so many variables involved....from the carbonaceous content of
the particular body to the load density induced temperature lag in the
particular kiln........... that there simply is no one answer that fits
all.
A little experimentation based off of a knowledge of a bit of ceramic
chemistry will lead to workable solutions.
Probably one of the most important variables in bisque firing is the
appropriate flow of oxygen. Miss that and you can set up problems that
will show up later in the finish firing. The current (last 10-15 years o=
r
so) trend of bisquing in electric kilns as opposed to fuel fired kilns ha=
s
contributed to more people having more problems from the lack of good
oxygen flow ...and hence any reaction that requires oxygen to go to
completion can be somewhat retarded. Or CO can inadvertantly be produced=
,
which instead can accelerate vitrification through the develpment of FeO
fluxed glass.
Got an old electric kiln with no local pickup vent system............
S....L.....O......W firing gives plenty of time for oxygen to penetrate
into dense stacks of bisque. Also lets the heat penetrate and even out t=
he
temp. Got plenty of oxygen and good heat penetration into the load by
running a fuel kiln with high velocity burners running an excess air
mix........ you can bisque fast. Got a dirty body with lots of stuff th=
at
needs to be burnt out...... bisque slow. Got a highly refined "paste"
body....bisque fast. Don't know what you've got? Bisque slow and be saf=
e
.
Ivor had a GREAT post on the bisque subject recently.
I'd also like to suggest that in addition to the porosity for the
absorbtion of the glaze slurry you also have to keep track of the gas
permeability of the bisque if you are doing the typical reduction firing
...... so that the reducing agent(s) can penetrate into the claybody
adequately to cause the body to reduce properly with the firing profile y=
ou
typically use. Bisque too high....and you can "loose" the ability to
accomplish "body reduction" to the level you desire without increasing
either the concentration of reducing agent or the duration of the reducti=
on
period in the finish firing. Each body will have it's own maturation cur=
ve
that affects this property.
Best,
.........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"Earth, Water, and Fire Noborigama Woodfiring Workshop August 18-27,
2000"
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