clennell on fri 14 apr 06

Sour Cherry Pottery

Sherry...
Ahh...I have played the "lets rebuild the firebox" game too! That was
actually my biggest improvement - movement of the primary air from the top
of the box to the upper front of it. My conclusion was that too much cold
air was entering the kiln to get to top temps. What I think would be the
best solution, which I did not do before, is enlarging the box to have a
larger hob area. I think that's the real issue. Wish I had thought of that
the last time I did this, but I didn't. Next time I build one of these I
will err on the "too large" end for the firebox instead of being
conservative with it. I was trying to save bricks when I built this
kiln...and I should've just coughed up the extra cash to do it right. Yet
another lesson learned the hard way! Interesting that we had a similar
problem though.

Best of luck...
Andy

andy and sherry: An article that I recommend you both read is by sometimes
lurker Gary hatcher of Texas entitled Chariot of Fire: Principles. gary is a
kiln geek and knows his stuff as to the physics of wood firing.
I will try in my unorthadox manner to explain what I think happens at around
Cone 4-6 in a wood kiln. Every 690degrees F in temperature the volume of gas
doubles. that means at about 3 times 690 or about 2170 or cone 4 ,3 times
the gas is trying to get the hell outta the kiln. this is when many wood
kilns stall and I try to burp or fart the kiln. It can't eat anymore wood
with a good burp or fart. Once you let off on the wood and allow it to let
wind it will start to consume more wood again and look out for it's headed
for the promised land of Cone 10.
Many potters that have tried to shorten my beloved train have forgotten that
the chamber is actually a horizontal chimney with pots in it. By shortening
the chamber you have shortened the chimney and hence the draft. The volume
of gases trying to get out of this shorter/narrower chimney is like a
sparrow eating a turkey. Constipation, gas big time and I can't eat no more.
At your stall, stoke less and probably smaller wood and let the kiln have a
good loud rip. When it has given off gas it'll eat wood like a starving man.
Best,
Tony

Ruth Ballou on sat 15 apr 06

After reading all these posts about Bourrys, I have to add my $.02.
Been there, done that and raked the coals over it.

After my experience with my first Bourry, and hearing of others'
problems, I'm coming to the conclusion that a Bourry with one firebox
risks being under powered. Maybe this is a leap and there are
Bourrys out there that fire well with a single fire box. But think
these would be smaller kilns. Steve Harrison's "Laid Back Wood
firing" is about 30 cu. ft. I think. And it certainly fires very
well. But I think there are some practical limits as to how large a
single firebox can be. You may end up having to climb a ladder to
stoke. And there's a limit to how much air can be drawn through a
single box, before it just becomes cold air and drops the temperature.

I played all kinds of games with the first kiln... rebuilt the
chimney, firebox, raised the floor, etc, etc. Nothing worked. Hit a
solid wall at around cone 3. Flames everywhere, backburning out the
firebox. Let it rest.... Temp plummets. Painfully regain lost ground.
Stoke slower, stoke faster, big wood, small wood. Now I could've
just maintained that temp for a looooong time, and maybe I'd have
reached
something resembling ^10. Finally, I rebuilt the kiln... after
obtaining plans for one with a track record.

What a difference! Same size chamber, two fireboxes, one on either
side. Solves lots of problems. Essentially it's another burner. More
fuel and more air. Chimney moved around to back to make a true
downdraft instead of a crossdraft. Generous flue channel under the
first set of shelves. First firing I was awed to see the flames
burning upside down in the firebox, they way I'd heard they burn in a
Bourry. Takes about 20 hours or so to fire, but there's never any
question of which direction it's headed in. Steady upward progress.
With two fireboxes, alternate stoking allows a nice rhythm to
develop. One side peaks as the other burns down. Last firing, slowing
down became the issue.

Ruth Ballou
Belgium

I haven't fired this kiln in the 3 years I've been in Belgium and now
I'm selling the property. And likely not to a potter. Gonna go kiss
the kiln good bye next week.




On Apr 15, 2006, at 3:28 AM, clennell wrote:

> Sour Cherry Pottery
>
> Sherry...
> Ahh...I have played the "lets rebuild the firebox" game too! That was
> actually my biggest improvement - movement of the primary air from
> the top
> of the box to the upper front of it. My conclusion was that too
> much cold
> air was entering the kiln to get to top temps. What I think would
> be the
> best solution, which I did not do before, is enlarging the box to
> have a
> larger hob area. I think that's the real issue. Wish I had thought
> of that
> the last time I did this, but I didn't. Next time I build one of
> these I
> will err on the "too large" end for the firebox instead of being
> conservative with it. I was trying to save bricks when I built this
> kiln...and I should've just coughed up the extra cash to do it
> right. Yet
> another lesson learned the hard way! Interesting that we had a similar
> problem though.
>
> Best of luck...
> Andy
>
> andy and sherry: An article that I recommend you both read is by
> sometimes
> lurker Gary hatcher of Texas entitled Chariot of Fire: Principles.
> gary is a
> kiln geek and knows his stuff as to the physics of wood firing.
> I will try in my unorthadox manner to explain what I think happens
> at around
> Cone 4-6 in a wood kiln. Every 690degrees F in temperature the
> volume of gas
> doubles. that means at about 3 times 690 or about 2170 or cone 4 ,3
> times
> the gas is trying to get the hell outta the kiln. this is when many
> wood
> kilns stall and I try to burp or fart the kiln. It can't eat
> anymore wood
> with a good burp or fart. Once you let off on the wood and allow it
> to let
> wind it will start to consume more wood again and look out for it's
> headed
> for the promised land of Cone 10.
> Many potters that have tried to shorten my beloved train have
> forgotten that
> the chamber is actually a horizontal chimney with pots in it. By
> shortening
> the chamber you have shortened the chimney and hence the draft. The
> volume
> of gases trying to get out of this shorter/narrower chimney is like a
> sparrow eating a turkey. Constipation, gas big time and I can't eat
> no more.
> At your stall, stoke less and probably smaller wood and let the
> kiln have a
> good loud rip. When it has given off gas it'll eat wood like a
> starving man.
> Best,
> Tony
>
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