John Baymore on thu 25 jun 98
------------------
(clip)
I'm having a problem firing the reduction kiln at a studio
situation........we
are trying for some consistantancy and are not able to reach
success.....one
firing we'll have copper reds in many areas of the kiln....the very next
firing, the copper reds are clear and runny, crackled also????? I would be
so
very pleased if a reduction potter would post a firing schedule on this
list,
we were told our kiln is in reduction the entire firing. We have some black
smoke at the peeps from the time cone 7 is down in the front of the
kiln......we have one peep on each of the sides towards the back of the
kiln.....we then watch the cones in that area....when 9 is bending, we
close
the damper and primary air on each of two burners, reduce for 15 minutes,
black smoke is pouring out of every crevise and peeps.......then we open
the
damper and primary airs for a few moments to clean the kiln atmosphere and
turn off the gas. We close the ports, close the damper and of course the
peeps have been plugged, allow the kiln to cool for a week.........opened
it
this morning to find some runny glazes..most successful.........but no
reds=21=21=21=21=21 Everyone is so very disappointed when this happens....I =
need some
professional advice.........please......and thank you.
(snip)
Emily,
For the really fast short answer, get an oxyprobe, read the instructions,
install it correctly, and quickly solve a lot of your problems (won't solve
poor kiln design issues.....only measures the atmosphere at the point the
probe is located...see below). Even the most advanced, experienced
firepersons find these tools a great asset. They are well worth the =24600
or so they cost in reduced wasters and decreased firing costs.
If you just can't spring the bucks for the electronic version, get a Fyrite
Flue Gas Analyzer. This is a wet chemical method, and it works. Sells in
the =24250 range or so. I wrote the users manual that went with the one =
sold
by Cutter Ceramics way back in ancient history. Had the opportunity to do
a lot of playing with them. Still much better than the =22by eye=22 =
approach,
but not as convenient as the modern digital magic.
Guaging reduction levels accurately by eye from firing to firing requires a
LOT of firing experience. Even then it is tough to do with much precision.
An oxyprobe will show you how much variation there can be. Have an
expereinced person set the level of reduction on firing =22A=22. Measure =
the
setting via the probe. The next firing, have the same person set the
reduction to what he/she thinks is the same as last time by eye. Then take
a measurement on firing =22B=22. Compare the oxyprobe readings from firing =
=22A=22
and =22B=22. They might suprise you. And the experienced person doing the
settings =3Cg=3E.
If a particular effect you desire has a narrow range of conditions that
produce it, and the accuracy of the reduction setting being able to be
duplicated by eye is not that tight a tolerance, then reliability for that
effect goes out the window. In more scientific terms this variable ... the
possible error factor........ is often stated as the =22plus or minus
factor=22. It shows the tolerance of the system.
An example of this using another piece of kiln instrumentation: Cheap type
=22K=22 (chromel/alumel) analog pyrometers often are accurate to +/- 2 =
percent
of FULL SCALE. Full scale is 2400F on the meter, so the accuracy of any
reading is plus or minus 48 degrees F. If you have a firing effect that
requires you to hit a temperature within a passband of only 25 degrees F
and you try to use this meter to duplicate results...... you are out of
luck.
If your glaze effect needs an accuracy of plus or minus 5 percent on the
variation in reduction agant levels and you can only guage 10 percent by
eye (random numbers for example) ...... you are out of luck.
The real key to good consistent firings is to understand the firing process
pretty well. To do that you need to undersand a bit about clay and glaze
chemistry, combustion, and kiln design. Add to that some good
instrumentation, (an oxyprobe, accurate guages on gas and air flow, a
calibrated damper, maybe a stack draft manometer, and the like) and keeping
accurate records, and you'll get pretty relaible results.
One thing that is important to understand in regard to firing in
general........ cookbook firing is NOT really a satisfactory approach to
the issue of firing your particular ware. By that I mean that firings are
individualized more than you think. A good cook uses a recipe from a
cookbook maybe as a starting point to some exploration or as a tool to
learn from.... then goes out and creates a new recipe to meet his/her
specific needs and tastes. As alluded to in my recent post about pots
blowing up in the bisque.... there is no such thing as a =22one size fits
all=22 firing cycle for anything. Your cycle has to be developed to match
your clay and glazes and your kiln design.
This comes from knowledge of materials amd processes. It takes study and
time.
Reduction is a chemist's phrase. It refers to the removal of oxygen from a
compound thus =22reducing=22 it to what is refered to as a lower state. It =
is
purely coincidental (and serindipitous) that it can be accomplished by
=22reducing=22 the amount of the air going into a potters kiln =3Cg=3E. As =
an
over-simplefied example, FeO (black iron oxide) in the presence of high
levels of heat and a reducing agent (something that =22wants=22 the oxygen =
more
than the iron atom does ....like CO), can be =22reduced=22 to the raw =
iron....
Fe, and gives off CO2 in the process. The carbon monoxide (CO) in this
case has been oxidised=3B has oxygen added to it. This concept is often
refered to as REDOX reactions. One thing is oxidised and the other is
reduced.
To produce reducing agents in a fuel fired kiln you run the burner mixture
rich on fuel. This can be accomplished by either increasing the fuel flow
and keeping the air the same, or reducing the air flow and keeping the fuel
the same. Controls on various kilns vary for doing these things. In kilns
that are highly dependant on secondary air, closing the damper cuts down
the air supply, and that can produce reduction just like turning down the
air flow to the mixing unit of the burners. The more accurate the control
systems on the kiln, the more accurate and consistent the results firing to
firing.
Instead of combustion going to completion and producing mostly carbon
dioxide (CO2) and water vapor (H2O) out of the hydrocarbon fuel and the
oxygen from the air, you get some forms of intermediary combustion products
(complex partially combusted hydrocarbons), carbon monoxide (CO), and
hydrogen (H2) along with some proportion of CO2 and H2O vapor. The amounts
of each of these formed depends on a number of factors such as the
composition of the gas being burned, the temperature of the firebox and the
refractories in close proximity to the flame front, the ratio of primary
air to fuel, the ratio of seconday air to partially aerated fuel/air
mixture, and so on.
To simplify the chemical soup produced in a reducing kiln a bit we can
reasonably assume that in a fuel fired kiln the primary reducing agent is
carbon monoxide and secondarily is hydrogen. CO is a good marker to follow
in practical usage, anyway. These are both gases, and as such they exist
at a very small sub-microscopic size level and can penetrate into
microscopic pores in clay and glaze layers. As long as the material that
you want to reduce is gas permeable, the CO and H2 can get into close
association to the reducable materials and do their magic.
On the other hand, carbon (the black soot in the smoke) is a particulate
solid. As a visible particulate, it is pretty large in size. Particularly
when compared to a single gas molecule =3Cg=3E. Therefore it is not
particularly reactive at the molecular level, since it cannot easily
contact the molecules that we want to =22reduce=22. Smoke is NOT an =
indicator
of good reducing conditions. It is an indicator of poor combustion, poor
reducing conditions, wasted fuel, and so on. This is a common
misunderstanding among potters.
If you monitor on a graph the precentage of CO in a atmosphere sample from
the kiln as you decrease the flow of air, you will see a steady increase up
to a point as the air is decreased. The maximum reading is determined by
the composition of the fuel gas..... the more carbon it has relative to
hydrogen, the higher the CO2 level can be. Then at some point, you will
see a DECREASE in the percentage of CO. If you were to look carefully at
the kiln as the CO started decreasing, you would see visible smoke starting
to appear. This is because the carbon in the fuel is becoming particulate
carbon as opposed to carbon monoxide. So....... if you see smoke (on a gas
kiln), you are not in optimum reducing conditions. The more smoke, the
LIGHTER the reduction effects. Smoke indicates that you have gone well
PAST optimal reducing conditions.
(Carbon trap glaze effects are another story here.... and a particular
specialized aspect of firing cycles.)
Another factor at work here is the function of the design of the kiln. The
kiln SHOULD circulate the well mixed combustion by-products evenly through
all parts of the chamber. Then a sample taken at one point is
representative of all points in the chamber. This often isn't so in
potters kilns=21 There are a lot of poor designs circulating out there that
really don't work so well. You know...... the ones that a potter will tell
you work GREAT..... except there is this place in the back bottom that
never gets reduced.... and the front bottom that is always cold.... and so
on. (Don't get me started on this one =3Cg=3E.)
So....... is the smoke and backpressure coming out that top rear spyport
indicative of what is happening in the rest of the chamber or is it just
showing the circulation to that part of the kiln? Ditto for the reading on
the Fyrite Analyzer or on the Oxyprobe. Probes, and sensors, and pickups
only are sampling at one specific point. They don't assure that other
points are the same. In many potters kilns circulation is very different
in different parts of the kiln. So you can have heavy backpressure, flame
jumping out of spy ports, and still have other areas that are burning
cleaner, or even oxidising.
A condition that kind of typifies this situation is closing the damper a
lot (or god forbid, all the way) and seeing a lot of flame belching out of
the upper to mid spy ports and out the cracks at the top of the door.
Looks like heavy reduction. An Oxyprobe located in the top of the chamber
arch will show reduction. But when unloaded, the lower 1/2 of the chamber
shows light or no reduction=21=21=21=21 That is because the circulation of =
the
gases throughout the chamber were choked off, and the flame front was just
rolling up the walls and into the arch area, and leaking out all the spy
ports and cracks in the door and the like...... the kiln became sort of an
updraft =3Cg=3E. Nothing was being pulled down and through the bottom =
areas,
so they never had reducing agent circulating through them.
Another factor in effective reduction is the gas permeability of the ware
at the time the reducing agent is present. If the clay body and glaze are
vitrified enough so that the CO and H2 cannot get back inside the layers,
then the only effect is on the outer surfaces. Those outer surfaces are
exposed to excess oxygen on the cooling cycle.... so are ALWAYS re-oxidised
anyway (unless you fire down in reduction or fill the kiln with an inert
gas).
If you haven't thought of this..... what color is the oxidised state of
iron? Red. Reduced iron is grey or black. So why are nice reduced
stoneware clay bodies brown instead of grey? Cause the outer surface is
reoxidised during cooling...... break them open and they are grey where the
oxygen couldn't get back inside cause the body was no longer gas permeable
on the cooling cycle. The warmer general tone of the body is because the
reduced iron becomes a powerful flux in the body matrix glass, and colors
the glass that is forming in the claybody. More of this is visable on the
outside clay surface once it has migrated from areas back inside the clay
wall.
Reduction agents need permeability to get the job done. The lower the body
or glaze permeability, the longer the reducing agent needs to be present,
or the higher the concentration per unit time to get the same level of
effect. Past a certain point of low permeability, no increaseed time or
level will get back into the clay body or a sealed layer of glaze. This is
why a refire of a piece to get =22body=22 reduction on a vitrified body =
doesn't
really work.
So you have to start reduction at a point where the body is still
=22receptive=22 to it. Different clay bodies have different ranges at which
they begin to tighten up. Also the glaze layer over the surface can start
to vitrify and seal up. So the particular glazes can alter the necessary
starting point too. Some glazes and claybodies cannot be effectively used
in the same firing. You have to learn your materials, and adjust your
firing cycle to match their demands.
It is important to note that there are certain reactions in the clay and
glazes that REQUIRE excess oxygen be present to go to completion.
Particularly if they have been short changed in this regard in the bisque
firing =3Cg=3E. Beginning reduction too early can prevent these reactions =
from
happening correctly, and cause defects. Carbon coring is one of these....
the carbonaceous materials and carbonates cannot decompose and get out of
the clay....... so they form carbon and/or start decomposing at high
temperature causing bloating and the like. So you shouldn't be reducing
for the whole firing.
General reduction firing cycles call for a period of reduction to begin
sometime in the high bisque range and continue until the body is beginning
to tighten. Prior to this oxidation is typical. or if bisque has been done
very well oxidised, then running tight to =22on-ratio=22 (just enough air =
for
combustion.....neither reducing nor oxidising) maximises fuel economy.
Then reduction can be lightened (for increased climb rate) until the glazes
are a bit below losing gas permeability. At that point reduction is
increased a bit until the glazes are pretty well fused. At which point the
reduction can be backed off. A period of oxidation at the end of the cycle
(once glaze and body are no longer gas permeable) is a common practice.
The critical temperature for reduction for your copper reds is much lower
than cone 7 to 10. The exact glaze formulation and body formulation
determines the exact points. The =22magic=22 is probably happening =
somewhere
in the cone 06 to cone 7 range.
I did a lot of controlled experiments with celadons many years ago. Found
that reduction had to happen in a specific range (for the particular glazes
and claybodies) of cone 04 to cone 6. Before and after you could oxidise
the heck out of them and they still came out great. Percentage of
deficiency of oxygen in that range was a factor too and could tighten up
the cone range a little more. But let's not complicate the dsiscussion
=3Cg=3E.
It is neigh on to impossible to diagnose these types of things by remote
control =3Cg=3E. Meaning from afar without seeing the kiln, the formulas, =
the
firing charts, and sometimes the actual firing. That being said,
............
Try running the damper much more open (minimum of maybe 20=25 or so) to
promote better circulation throughout the chamber, but increase the total
volume of effluent generated (a little higher burner gas/air settings).
Close off the primary air on the burners as the main determinant of
reducing conditions. Adjust the damper to constrict flow until you JUST
get slight positive backpressure at the lowest spyport, and start this
process at maybe cone 04. No smoke=21 Just fluffy flame (color determined
by fuel type....... what are you burning?). Calibrate all kiln controls so
that you can accurately duplicate settings. Keep detailed records of what
you did and specific results.
So .... a long dissertation on some aspects of reduction firing. Sorry
about the length. Hope it is of use solving your problems.
Best,
.........................john
PS: There is ONE space now open in the August climbing kiln woodfiring
workshop.
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
=22Professional kiln design, consulting, and construction since 1972.=22
DIANA PANCIOLI, ASSOC. PROF. on fri 26 jun 98
I fired copper reds by reducing gently and steadily all the way up from
body reduction to finish. No need for smoke.
Diana
On Thu, 25 Jun 1998, John Baymore wrote:
> ----------------------------Original message----------------------------
> ------------------
> (clip)
> I'm having a problem firing the reduction kiln at a studio
> situation........we
> are trying for some consistantancy and are not able to reach
> success.....one
> firing we'll have copper reds in many areas of the kiln....the very next
> firing, the copper reds are clear and runny, crackled also????? I would be
> so
> very pleased if a reduction potter would post a firing schedule on this
> list,
> we were told our kiln is in reduction the entire firing. We have some black
> smoke at the peeps from the time cone 7 is down in the front of the
> kiln......we have one peep on each of the sides towards the back of the
> kiln.....we then watch the cones in that area....when 9 is bending, we
> close
> the damper and primary air on each of two burners, reduce for 15 minutes,
> black smoke is pouring out of every crevise and peeps.......then we open
> the
> damper and primary airs for a few moments to clean the kiln atmosphere and
> turn off the gas. We close the ports, close the damper and of course the
> peeps have been plugged, allow the kiln to cool for a week.........opened
> it
> this morning to find some runny glazes..most successful.........but no
> reds!!!!! Everyone is so very disappointed when this happens....I need some
> professional advice.........please......and thank you.
> (snip)
>
> Emily,
>
> For the really fast short answer, get an oxyprobe, read the instructions,
> install it correctly, and quickly solve a lot of your problems (won't solve
> poor kiln design issues.....only measures the atmosphere at the point the
> probe is located...see below). Even the most advanced, experienced
> firepersons find these tools a great asset. They are well worth the $600
> or so they cost in reduced wasters and decreased firing costs.
>
> If you just can't spring the bucks for the electronic version, get a Fyrite
> Flue Gas Analyzer. This is a wet chemical method, and it works. Sells in
> the $250 range or so. I wrote the users manual that went with the one sold
> by Cutter Ceramics way back in ancient history. Had the opportunity to do
> a lot of playing with them. Still much better than the "by eye" approach,
> but not as convenient as the modern digital magic.
>
> Guaging reduction levels accurately by eye from firing to firing requires a
> LOT of firing experience. Even then it is tough to do with much precision.
> An oxyprobe will show you how much variation there can be. Have an
> expereinced person set the level of reduction on firing "A". Measure the
> setting via the probe. The next firing, have the same person set the
> reduction to what he/she thinks is the same as last time by eye. Then take
> a measurement on firing "B". Compare the oxyprobe readings from firing "A"
> and "B". They might suprise you. And the experienced person doing the
> settings .
>
> If a particular effect you desire has a narrow range of conditions that
> produce it, and the accuracy of the reduction setting being able to be
> duplicated by eye is not that tight a tolerance, then reliability for that
> effect goes out the window. In more scientific terms this variable ... the
> possible error factor........ is often stated as the "plus or minus
> factor". It shows the tolerance of the system.
>
> An example of this using another piece of kiln instrumentation: Cheap type
> "K" (chromel/alumel) analog pyrometers often are accurate to +/- 2 percent
> of FULL SCALE. Full scale is 2400F on the meter, so the accuracy of any
> reading is plus or minus 48 degrees F. If you have a firing effect that
> requires you to hit a temperature within a passband of only 25 degrees F
> and you try to use this meter to duplicate results...... you are out of
> luck.
>
> If your glaze effect needs an accuracy of plus or minus 5 percent on the
> variation in reduction agant levels and you can only guage 10 percent by
> eye (random numbers for example) ...... you are out of luck.
>
> The real key to good consistent firings is to understand the firing process
> pretty well. To do that you need to undersand a bit about clay and glaze
> chemistry, combustion, and kiln design. Add to that some good
> instrumentation, (an oxyprobe, accurate guages on gas and air flow, a
> calibrated damper, maybe a stack draft manometer, and the like) and keeping
> accurate records, and you'll get pretty relaible results.
>
> One thing that is important to understand in regard to firing in
> general........ cookbook firing is NOT really a satisfactory approach to
> the issue of firing your particular ware. By that I mean that firings are
> individualized more than you think. A good cook uses a recipe from a
> cookbook maybe as a starting point to some exploration or as a tool to
> learn from.... then goes out and creates a new recipe to meet his/her
> specific needs and tastes. As alluded to in my recent post about pots
> blowing up in the bisque.... there is no such thing as a "one size fits
> all" firing cycle for anything. Your cycle has to be developed to match
> your clay and glazes and your kiln design.
>
> This comes from knowledge of materials amd processes. It takes study and
> time.
>
> Reduction is a chemist's phrase. It refers to the removal of oxygen from a
> compound thus "reducing" it to what is refered to as a lower state. It is
> purely coincidental (and serindipitous) that it can be accomplished by
> "reducing" the amount of the air going into a potters kiln . As an
> over-simplefied example, FeO (black iron oxide) in the presence of high
> levels of heat and a reducing agent (something that "wants" the oxygen more
> than the iron atom does ....like CO), can be "reduced" to the raw iron....
> Fe, and gives off CO2 in the process. The carbon monoxide (CO) in this
> case has been oxidised; has oxygen added to it. This concept is often
> refered to as REDOX reactions. One thing is oxidised and the other is
> reduced.
>
> To produce reducing agents in a fuel fired kiln you run the burner mixture
> rich on fuel. This can be accomplished by either increasing the fuel flow
> and keeping the air the same, or reducing the air flow and keeping the fuel
> the same. Controls on various kilns vary for doing these things. In kilns
> that are highly dependant on secondary air, closing the damper cuts down
> the air supply, and that can produce reduction just like turning down the
> air flow to the mixing unit of the burners. The more accurate the control
> systems on the kiln, the more accurate and consistent the results firing to
> firing.
>
> Instead of combustion going to completion and producing mostly carbon
> dioxide (CO2) and water vapor (H2O) out of the hydrocarbon fuel and the
> oxygen from the air, you get some forms of intermediary combustion products
> (complex partially combusted hydrocarbons), carbon monoxide (CO), and
> hydrogen (H2) along with some proportion of CO2 and H2O vapor. The amounts
> of each of these formed depends on a number of factors such as the
> composition of the gas being burned, the temperature of the firebox and the
> refractories in close proximity to the flame front, the ratio of primary
> air to fuel, the ratio of seconday air to partially aerated fuel/air
> mixture, and so on.
>
> To simplify the chemical soup produced in a reducing kiln a bit we can
> reasonably assume that in a fuel fired kiln the primary reducing agent is
> carbon monoxide and secondarily is hydrogen. CO is a good marker to follow
> in practical usage, anyway. These are both gases, and as such they exist
> at a very small sub-microscopic size level and can penetrate into
> microscopic pores in clay and glaze layers. As long as the material that
> you want to reduce is gas permeable, the CO and H2 can get into close
> association to the reducable materials and do their magic.
>
> On the other hand, carbon (the black soot in the smoke) is a particulate
> solid. As a visible particulate, it is pretty large in size. Particularly
> when compared to a single gas molecule . Therefore it is not
> particularly reactive at the molecular level, since it cannot easily
> contact the molecules that we want to "reduce". Smoke is NOT an indicator
> of good reducing conditions. It is an indicator of poor combustion, poor
> reducing conditions, wasted fuel, and so on. This is a common
> misunderstanding among potters.
>
> If you monitor on a graph the precentage of CO in a atmosphere sample from
> the kiln as you decrease the flow of air, you will see a steady increase up
> to a point as the air is decreased. The maximum reading is determined by
> the composition of the fuel gas..... the more carbon it has relative to
> hydrogen, the higher the CO2 level can be. Then at some point, you will
> see a DECREASE in the percentage of CO. If you were to look carefully at
> the kiln as the CO started decreasing, you would see visible smoke starting
> to appear. This is because the carbon in the fuel is becoming particulate
> carbon as opposed to carbon monoxide. So....... if you see smoke (on a gas
> kiln), you are not in optimum reducing conditions. The more smoke, the
> LIGHTER the reduction effects. Smoke indicates that you have gone well
> PAST optimal reducing conditions.
>
> (Carbon trap glaze effects are another story here.... and a particular
> specialized aspect of firing cycles.)
>
> Another factor at work here is the function of the design of the kiln. The
> kiln SHOULD circulate the well mixed combustion by-products evenly through
> all parts of the chamber. Then a sample taken at one point is
> representative of all points in the chamber. This often isn't so in
> potters kilns! There are a lot of poor designs circulating out there that
> really don't work so well. You know...... the ones that a potter will tell
> you work GREAT..... except there is this place in the back bottom that
> never gets reduced.... and the front bottom that is always cold.... and so
> on. (Don't get me started on this one .)
>
> So....... is the smoke and backpressure coming out that top rear spyport
> indicative of what is happening in the rest of the chamber or is it just
> showing the circulation to that part of the kiln? Ditto for the reading on
> the Fyrite Analyzer or on the Oxyprobe. Probes, and sensors, and pickups
> only are sampling at one specific point. They don't assure that other
> points are the same. In many potters kilns circulation is very different
> in different parts of the kiln. So you can have heavy backpressure, flame
> jumping out of spy ports, and still have other areas that are burning
> cleaner, or even oxidising.
>
> A condition that kind of typifies this situation is closing the damper a
> lot (or god forbid, all the way) and seeing a lot of flame belching out of
> the upper to mid spy ports and out the cracks at the top of the door.
> Looks like heavy reduction. An Oxyprobe located in the top of the chamber
> arch will show reduction. But when unloaded, the lower 1/2 of the chamber
> shows light or no reduction!!!! That is because the circulation of the
> gases throughout the chamber were choked off, and the flame front was just
> rolling up the walls and into the arch area, and leaking out all the spy
> ports and cracks in the door and the like...... the kiln became sort of an
> updraft . Nothing was being pulled down and through the bottom areas,
> so they never had reducing agent circulating through them.
>
> Another factor in effective reduction is the gas permeability of the ware
> at the time the reducing agent is present. If the clay body and glaze are
> vitrified enough so that the CO and H2 cannot get back inside the layers,
> then the only effect is on the outer surfaces. Those outer surfaces are
> exposed to excess oxygen on the cooling cycle.... so are ALWAYS re-oxidised
> anyway (unless you fire down in reduction or fill the kiln with an inert
> gas).
>
> If you haven't thought of this..... what color is the oxidised state of
> iron? Red. Reduced iron is grey or black. So why are nice reduced
> stoneware clay bodies brown instead of grey? Cause the outer surface is
> reoxidised during cooling...... break them open and they are grey where the
> oxygen couldn't get back inside cause the body was no longer gas permeable
> on the cooling cycle. The warmer general tone of the body is because the
> reduced iron becomes a powerful flux in the body matrix glass, and colors
> the glass that is forming in the claybody. More of this is visable on the
> outside clay surface once it has migrated from areas back inside the clay
> wall.
>
> Reduction agents need permeability to get the job done. The lower the body
> or glaze permeability, the longer the reducing agent needs to be present,
> or the higher the concentration per unit time to get the same level of
> effect. Past a certain point of low permeability, no increaseed time or
> level will get back into the clay body or a sealed layer of glaze. This is
> why a refire of a piece to get "body" reduction on a vitrified body doesn't
> really work.
>
> So you have to start reduction at a point where the body is still
> "receptive" to it. Different clay bodies have different ranges at which
> they begin to tighten up. Also the glaze layer over the surface can start
> to vitrify and seal up. So the particular glazes can alter the necessary
> starting point too. Some glazes and claybodies cannot be effectively used
> in the same firing. You have to learn your materials, and adjust your
> firing cycle to match their demands.
>
> It is important to note that there are certain reactions in the clay and
> glazes that REQUIRE excess oxygen be present to go to completion.
> Particularly if they have been short changed in this regard in the bisque
> firing . Beginning reduction too early can prevent these reactions from
> happening correctly, and cause defects. Carbon coring is one of these....
> the carbonaceous materials and carbonates cannot decompose and get out of
> the clay....... so they form carbon and/or start decomposing at high
> temperature causing bloating and the like. So you shouldn't be reducing
> for the whole firing.
>
> General reduction firing cycles call for a period of reduction to begin
> sometime in the high bisque range and continue until the body is beginning
> to tighten. Prior to this oxidation is typical. or if bisque has been done
> very well oxidised, then running tight to "on-ratio" (just enough air for
> combustion.....neither reducing nor oxidising) maximises fuel economy.
> Then reduction can be lightened (for increased climb rate) until the glazes
> are a bit below losing gas permeability. At that point reduction is
> increased a bit until the glazes are pretty well fused. At which point the
> reduction can be backed off. A period of oxidation at the end of the cycle
> (once glaze and body are no longer gas permeable) is a common practice.
>
> The critical temperature for reduction for your copper reds is much lower
> than cone 7 to 10. The exact glaze formulation and body formulation
> determines the exact points. The "magic" is probably happening somewhere
> in the cone 06 to cone 7 range.
>
> I did a lot of controlled experiments with celadons many years ago. Found
> that reduction had to happen in a specific range (for the particular glazes
> and claybodies) of cone 04 to cone 6. Before and after you could oxidise
> the heck out of them and they still came out great. Percentage of
> deficiency of oxygen in that range was a factor too and could tighten up
> the cone range a little more. But let's not complicate the dsiscussion
> .
>
> It is neigh on to impossible to diagnose these types of things by remote
> control . Meaning from afar without seeing the kiln, the formulas, the
> firing charts, and sometimes the actual firing. That being said,
> ...........
>
> Try running the damper much more open (minimum of maybe 20% or so) to
> promote better circulation throughout the chamber, but increase the total
> volume of effluent generated (a little higher burner gas/air settings).
> Close off the primary air on the burners as the main determinant of
> reducing conditions. Adjust the damper to constrict flow until you JUST
> get slight positive backpressure at the lowest spyport, and start this
> process at maybe cone 04. No smoke! Just fluffy flame (color determined
> by fuel type....... what are you burning?). Calibrate all kiln controls so
> that you can accurately duplicate settings. Keep detailed records of what
> you did and specific results.
>
> So .... a long dissertation on some aspects of reduction firing. Sorry
> about the length. Hope it is of use solving your problems.
>
> Best,
>
> ........................john
>
>
> PS: There is ONE space now open in the August climbing kiln woodfiring
> workshop.
>
> John Baymore
> River Bend Pottery
> 22 Riverbend Way
> Wilton, NH 03086 USA
>
> 603-654-2752
> JBaymore@Compuserve.com
>
> "Professional kiln design, consulting, and construction since 1972."
>
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