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Catenary Wood Fire Kiln Stalling? Eveness?


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What causes of tthe proverbial stall when wood firing?

 

What is going on when a kiln stalls due to rainy weather?

 

We had broke 2100 f and makinprogress, I expected rise to slow but once the rain started. We were struggling just to maintain 2000F

 

While the question applies to any kind or wood -fired kiln This was a catenary kiln.

 

This was my first time helping fire a Cat'. What are strategies to get even temp top to bottom? Loading, firing, and or design wise....... Bottom level was cooler . And didn't want to come up.post-25544-0-65180600-1461465001_thumb.jpg

post-25544-0-65180600-1461465001_thumb.jpg

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Air flow clear ---  I could see good movement at all intakes, and coal bed was maintained not to block,  if you look at picture  you can see auxillary air partially opened this is on both sides of kiln.  Primary air and secondary stoke area on front of kiln. Ill try an add picture of front of kiln.   The top door we would sometimes stoke make wood hobs, and make “wood door†( fill with stacks of 1x6’s)  other wise closed.   Bottom doors are  primary air.

 

Altitude  basically sea level   technically a whopping +17 feet

 

Stack draws well, on little stokes and big stokes .. I didn’t see it during early phase candle to 1000f

 

Damper was at approx. 1.5- 2  inches most of firing,  but tried up to wide open

 

I had steady progress to 1800.  A bit slower to 2000,   had to work a bit to break 2100,   crew and weather change ….. STALL plus weather change  to intermittent showers to RAIN.

If you look close on picture 2 thermocouples just below mid level and low on kiln.   mid thermo.  Bit the dust @2000f.   but at this point we were stoking on feel, sound, taste.   

 

9 was down and 10 was soft  in upper cone packs,  8 down and soft 9 in middle,  6 down and soft 8 on bottom packs , If memory serves, before light salting and sealing her up. 

 

My wild guesses   … I read elsewhere @ stall to give more air and more frequent smaller stokes,  I didn’t check but possibly kiln floor is not thick enough. 

post-25544-0-52676000-1461500296_thumb.jpg

post-25544-0-52676000-1461500296_thumb.jpg

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"I read elsewhere @ stall to give more air and more frequent smaller stokes"

 

 

I agree with this. The flames coming out from the around the stoking door indicate that you are overloading with wood per stoke. All of the flame should be drawn towards the chimney, rather than blowing backwards out the door.

 

With my (somewhat limited) experience with wood firing, the hot top/cool bottom issue is a stacking issue. Kilns are stacked from the bottom up, so it's natural to pack it tight at first, then start running out of pots near the top. The bottom shelves should be tall and fairly loose.

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That picture with flames out of box was due to a long piece of wood and couldn't close door. And hours before

stall .... Otherwise it was drawing fine......

 

I didn't load... But I think bottom was fairly dense .... I'll post photos of kiln packing later....

Are saying les dense at bottom is better?

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That picture with flames out of box was due to a long piece of wood and couldn't close door. And hours before

stall .... Otherwise it was drawing fine......

 

Gotcha, never mind that comment.

 

 

I didn't load... But I think bottom was fairly dense .... I'll post photos of kiln packing later....

Are saying les dense at bottom is better?

 

Yes, as in a 9 inch tall space, with an inch or more between pots. The middle levels can be packed as tightly as you want. The top will automatically be "loose" because of the head space above the pots on the top shelf. The heat/flame will seek an easy path, if there is only one easy path at the top, that's where it goes. If there are two easy paths, top and bottom, that "sandwich" of heat will heat the middle levels too.

 

Again, my wood firing experience is about once a year for the last 12 years or so, in community/classroom kilns. There are probably others with more detailed advice about this.

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Wood kills are all about air flow

The shorter and smaller wood at end should supercharge the heat climb.

Stalls can be from as noted already to tight stacking that slows air flow or stack that does not draw

Wet or to large of wood at end of fire

To much ash build up or to much dampering

Always end the fire with lots of dry small wood that has lots of surface area to explode in flames and burns very quickly hence the short stokes

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You might try lowering the bagwall(s) a brick, or opening up the second from the top layer.  In my wood kilns and in my oil-fired salt kiln, it seemed to me that tuning the bagwall was a pretty important part of getting an even firing.

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Firing a crossdraft wood kiln is no different than firing a gas kiln. You have fuel, air, and pressure (chimney draw). If it stalls out, them something is out of whack. I would say that the load was not too tight. It looks like you have plenty of open space for the kiln to breath. Usually it's all about the air- either too little or too much, but usually too little. In the wood kilns I have dealt with, the problem was usually that the ash pit was clogged up, in which case you had to let it burn down or scoop it out. To burn it down, I'd put in a few big soaker logs to keep stuff burning without creating coals too quickly, and increase the chimney draw to pull more air across the ash pit to burn it down. Stirring the pit helps, too. Sometimes there was a little loss of temperature, but it was quickly regained once everything was burned down.

 

Also, most of the wood kilns I've fired have a point at which they stall out, usually around cone 5ish, sometimes for an hour or more, after which they rocket up. I think there's a point at which the bricks and ware are still soaking up the heat, and the kiln just can't get any hotter until everything catches up.

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