[Clayart] Oversize chimney due to altitude

Hank Murrow hmurrow at efn.org
Thu Jul 13 15:38:38 EDT 2017

> On Jul 13, 2017, at 10:20 AM, jonathan byler <jebyler2 at gmail.com> wrote:
> I wouldn’t worry about constricting the whole chimney.  It may be much more effective to constrict the flow at the flue opening, and again with the damper.  I think it’s better the whole chimney is a little too big than too small, since too much draught is an easy problem to fix - you simply remove material from the top of the chimney and make it shorter.  Or you could (and maybe should) have a “passive damper”, which allows extra air from outside into the chimney, which will lower the volume of air getting pulled through the kiln, and also cool the flue gasses somewhat slowing things down a bit.

Hank in Eugene replies; 

I have found that pulling bricks to create a 'passive damper’ tends to encourage the flows through the kiln in rivers with not much mixing while in the chamber. Consequently, when i fire large wood kilns, I ask if we can reduce the kiln with active plate-type dampers near the lower part of the chimney. In my experience, nearly all the dozen or so big wood kilns I have fired have active dampers built in(if not used in practice). Recently, i was firing with John Dix in Sasayama, Japan where i suggested reduction with kiln shelf dampers and John agreed to try it after i explained why I thought it better for more even reduction. the result was more even reduction throughout the large chamber, and another suggestion was that we stoke with wet wood near the end of the firing, and following each stoke, toss in a pint or more of water onto the coals. My suggestion was based upon the belief that water introduced in the front of the kiln wold absob excess heat from the front and give it up to the rear of the chamber as it was leaving. john wondered about that, but kindly tried the experiment, and sure enough, the kiln dropped three cones in the back while not advancing beyond the desired C12 in the front, and the hydrogen released from the H2O added more reduction during this final hour of the firing.

Five days later, when we opened the kiln, it was marvelous for john to find that the nrmally coler rear of the chamber looked melted and great, and the entire load was pretty evenly reduced. As I remeber it, he claimed that usually he lost about a quarter of each load bfore trying this new régime. Perhaps he will add his views here if he is listening.
> All of the kilns I have fired and built are down draught types and have the burners beside the chimney pointing towards the front opening of the kiln.  I’ve seen people do a burner in front and a burner in back, and don’t know if there is a real advantage to one or the other.  My thought has been that if the burners come in symmetrically from the back, it keeps side to side differences in temperature minimal, and your target brick sets the heat distribution front to back.  Our soda kiln is a downdraught design, and works pretty well in this regard, but after a number of firings, I wished that it would be a cross draught kiln instead, so that the effects of the soda on the ware would be more directional.  I’m hoping one day to have the time to reconfigure things and see how that goes.
> Good luck with this, I hope it all fires off well when you’re done!

Hank adds his congratulations to Helen, and hopes for a great outcome in the mountains!
>> On Jul 13, 2017, at 10:34 AM, Helen Stone <helenestonepeony at gmail.com> wrote:
>> Hi folks, thank you for your responses re the type of iron to place along
>> the side of the skew bricks of my sprung arch soda kiln.  Iron work is
>> looking good, and Paco my welder is making me safety-wise burners.
>> After reading all of your different kind remarks about the size of the
>> chimney - I live at about 5,000 feet in the Ecuadorian Andes - I opted to
>> make the chimney deeper by the width of one brick up to a certain point
>> above the damper , and then to constrict the bricks in to make a 9 inch X 9
>> inch standard chimney interior.  Presently the chimney is only built to
>> just above the damper.  I have the opportunity to restrict the lower
>> interior portion of the chimney.  My idea is to put into place inside the
>> chimney, only in the area where it is one brick deeper than normal, a stack
>> of loose bricks, one of top of the other.  Then, upon firing, if this
>> system works I can just leave the bricks inside the chimney.  If I need
>> more draught, I can climb into the kiln and reach my hand into the chimney
>> opening and remove the bricks one at a time (with some struggle).  What say
>> you to this crafty plan?  Am I nutty?
>> So far I have made the two burner ports, which are diagonal to each other,
>> one in front and one in back, a bit wider than normal.  I can always insert
>> a brick piece to close the ports a bit if required.  The chimney flue is
>> the usual size i.e. 9 X 5 if I remember.
>> Helen in southern Ecuador
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