Wood Fired Pottery Kiln

[jessopottery]

I searched for "kiln" in the forums, and didn't see any topics related to pottery kilns, so here it goes...

I've been interested for awhile now in designing a pottery kiln that's run on alternative fuels (ie: not LP, natural gas, or electric). I thought that this forum would be a good place to pick some brains.

I'm quite familiar with traditional wood fired pottery kilns. They usually are made with "hard brick" (dense hard clay) as opposed to insulating fire brick (IFB) which is almost like pumice in its weight and texture. This is because the superheated ash from the wood burn (the kiln must reach an internal temperature of 2350 F) contains a "flux" of sodium and/or potassium which "sticks" to the walls of the kiln and melts and erodes the bricks inside as well as the shelves that the pottery are stacked on. Hard brick is cheaper, as the kiln must be rebuilt less often and with less expense.

I'm curious about some of these wood fired boilers that heat the wood to form a gas, before sending the gas into the combustion (pottery?) chamber for a super efficient fire that reaches over 2000 F. The problem with a pottery kiln (the reason I'm posting here) is that I get the feeling that in a traditional pottery kiln, the combustion reaction is much less efficient than the modern boilers out there. The inside of the kiln is a super dense thermal mass that must be heated (the quicker the better...) to an ultra hot temperature. To get the kiln (and the pottery) to this temp, wood must be hand fed into the kiln every few minutes, constantly, for 24-48 hours. I get the feeling that this constant process of opening the stoking door(s) lets unnecessary air into the combustion chamber, resulting in an inefficient fire. (not to mention that whenever a new (cold) piece of wood (usually 4"x4" or so) is added, black smoke comes out the chimney as the combustion reaction loses heat and is overwhelmed by excess fuel for a few seconds...)

My idea is to combine the ultra refined and engineered wood burning boiler with the fringe "artsy fartsy" millenia-old kiln technologies of ancient Japan, Korea and China. (Google "anagama" or "noborigama" kilns of ancient China)

Where should I go to find schematics of the wood combustion reaction, blueprints of current wood burning boilers, and details details details. I want to know what material lines the combustion chambers of these newfangled boilers, and do they eventually get lined with ash deposits (shiny glass-like deposits on the brick). Would it perhaps be possible to convert a boiler's burner mechanism to feed the fire directly into a larger combustion chamber such as a kiln?

 

[begreen]

Hopefully Adios Pantelones will be chiming in shortly. He's a wood fired kiln kind of guy.

 

[myzamboni]

Adios Pantalones will be along shortly to help.

 

[Adios Pantalones]

Oh boy! This is exciting stuff!
I have thought a fair bit about gasification for preheating my kiln. I may make a miniature rocket stove for this application, or even build a gasifier as is used for fueling a wood burning car, but I like some ash effect and the wood fired look.

OK- so a few things first- you need to define if you wnat a lot of "wood fired" effect on pots- flashing and melty ash. If you don't, then you can make a Fast Fire/ Olsen / Fast Freddy kiln with fireboxes underneath the chamber. For anything from say 10 cu ft up to over 100 cu ft stacking space, these are the most efficient kilns out there. You will absolutely burn less wood than other styles, though it's a lot of work and bending over to keep it fed. You will get a little atmospheric effect, but not a huge amount. These are often built with IFB's, and since little ash makes it into the chamber the bricks won't spall. Hard brick (super duty) are still used in the firebox and throats on any wood burning kiln (or even gas) because soft IFB's will wear too easily.

They will produce some smoke. This can be reduced by the addition of a collection box before the chimney that gathers the gases. If passive dampers are used here, then air will mix with the heated gases and burn off most of the smoke.

I could be wrong, but I think that you would have a very difficult time building a gasifier that would fire a fast-fire on less wood than the standard firebox design unless you have the whole gasification unit in the firebox. The simple reason for this is that the first process- turning wood into gas- releases energy. Just burning the wood in the firebox, that energy goes into the kiln. If the gasifier is outside the firebox, some is lost to the atmosphere. The difficulty, of course, is that any metal put in there will be destroyed in a few fires or less at those temps.

In the firebox or out- you would need more than one gasifier running at once (of conventional dimensions) to produce anough power for a kiln, and that means more blower power and control. Too much air and you will cool the kiln and not get reduction when you want it. Too little, and your efficiency drops and you may not get secondary burn like you want.

If you do want ash and flash, or if you want to reduce your work- I'd suggest building Bourry-box style. Look at Sidestoke.com for several great examples, and some plans. Also- see Steve Harrison's "Laid Back Woodfiring" for lots of designs.

This design uses 'hobs'- bricks that suspend the wood charge above the ash pit (coal bed). Air is drawn DOWN through the fuel supply and therefore burns from the bottom. Because you can fill up the firebox (in reduction), it is easy for one person to stoke this design alone (once called 'the semi-automatic wood grate'. Mine produces almost no smoke- even at top temps (cone 12+), unless I really overstuff it on purpose for heavy reduction.

There is no reason that you could not run- say a Bourry box- PLUS some gasifier as an assist. It's not uncommon to see used engine oil burners or gas assist on wood fired kilns- you would just be using wood gasification. You may be able to run a gasifier by convection (using chimney draft to draw air), but you may then have to mortar more of the kiln.

If you want a lot of 'oomph' out of a gasifier- you will need one bigger than is used in a boiler. Look at the "wood fired car" thread in the "green" forum here. There are schematics etc for building any size gasifier in a link there.

 

[Adios Pantalones]

I'd like to add- spraying ITC coating on IFB's will also reduce spalling and make them stand up to ash better. Some people even report that it has helped them build a SALT kiln out of IFB's ITC100 will also reflect heat- some well respected potters ascribe magic powers to the stuff's ability to even out heat and reduce firing times when coating IFB's.

Whatever you do- if using IFB's- go 2600F inside and 2000F outside, double wall. The double wall reduces movement, and the 2600F IFB's are much harder than 2300F's. I fired a couple times in a crossdraft wood kiln with uncoated 2600's inside, and they looked pretty good after several firings.

Further- put in a hardbrick floor with 2 courses of IFB's (can use 2300's under high heat duties) or the floor will suck the heat out of the kiln like you wouldn't believe. You don't need that in a fast fire, as the heat is generated under the kiln. If you want to build a fast fire- let me know- I have a few references that show designs brick-by-brick! Really makes life easier. One shows a gas/wood design; you could substitute a gasifier for the gas jets.

 

[fossil]

Oh well...maybe someone else who knows something about it will chime in. :lol: Rick

 

[Adios Pantalones]

sunuva...

 

[KWillets]

Will that IFB coating work on a steel stove? I was thinking about that for the "burning driftwood" thread, but I know essentially nothing about it.

 

[Adios Pantalones]

ITC100 is used on refractories. ITC also makes a coating for metal- I think it's ITC213.

(broken link removed to http://www.bigceramicstore.com/Supplies/ITC.htm)

I can't vouch for the material personally but have heard good reviews. Worth a look.

 

[Jags]

Slow down Adios - your starting to froth at the mouth. ;-)

 

[renewablejohn]

Jessopottery

Have a look at Hamech woodchip burners I think they are imported into US by New Horizon and are fully automatic so are ideal for pottery or glass kiln applications.

 

[jessopottery]

Wow Audios Pantalones! Thanks!

So you're saying that having the main combustion chamber baffled off from the rest of the stacking space (firebox made from hardbrick) should reduce the ash deposits on the main kiln walls, as well as reducing any smoke coming out of the chimney? The theory being that the combustible smoke is combined with oxygen before it gets to the rest of the kiln, reducing any ash buildup on the softbrick (or kaowool blanket/fiberboard?) kiln lining. This should result in little or no deterioration of the kiln, right?

My situation is that I'm in suburbia Portland, and I want to build a kiln. I was thinking of using an oil burner with a blower, or even an ethanol carburator/blower type thingee to mix the fuel and gas, but then I learned about these gassification wood burners, and I remembered firing wood kilns at U of Oregon. My biggest problem is the stink. I need something besides LP/gas that doesn't produce smoke. And while I really like the atmospheric effects of wood, I can't afford the heavy reduction (extra smoke) that it seems this will produce. The fire department might just show up to shut me down. I'm going for an ultra efficient (I guess it doesn't have to be the fastest fire), ultra clean fire that I can throw into slight reduction at the right times for half an hour at a time or so, just to get the glaze effects that I want. I'm also interested in a fire that will prolong the life of my kiln, as this is the kind of investment that I can't just throw down every six months for fun...

I want to have ultimate control over input air, and an automated wood stoking feature would be super cool. so... what say you now?

 

[Adios Pantalones]

In the fast fire design- the fireboxes (2) are baffled off a bit, but it's also UNDER the ware chamber, so only a little ash fights gravity getting in there, but the heat flows nicely and evenly into the chamber.

In the Bourry box, as soon as ash is created and falls, it gets swept up in the convective air flow and drawn into the chamber. I don't have a step between the firebox and ware chamber, so a LOT of ash comes in, and the bottom has to be stacked really tight or the flame gets drawn directly across the chamber and the top doesn't get enough heat. In fact- because I single-fire, the shrinkage of pots near the floor opens up enough air channel to cause an issue (took a few firings to learn this all), so I need to load bricks in the bottom to choke it up and get the flame to the top. A step/ bagwall would alleviate this problem, but reduces ash. If you want it to scream "woodfired" from across the room, without building an anagama- you could do worse than a Bourry without a step- it has almost the effect of a train kiln up front, with other areas of the kiln great for glazes etc- a nice variety when you learn where the effect is going!

My Bourry box fired kiln has a collection box, and I use passive dampers rather than an active damper- it produces very little smoke unless I want it to (hey- it's a little fun to do this).

If worried- a number of people have installed blowers/burners in the chimney/collection box to destroy any leftover smoke. It's a minimal fuel expense, IIRC. The Bourry box gives great woodfired effect.

A little ash

Or a lot up front (there were some with a lot more ash)

 

[Adios Pantalones]

By the way- as an alternative heat source- you might look into sawdust burners/blowers. They blow the sawdust out so that it's mixed with air before combustion. Should be very efficient/low smoke. They have big ones for industrial use, but I'm not sure if you wouldn't need to experiment and build one yourself for powering a kiln. Chip burners are another possibility.

You can guestimate the power needed if you go to www.wardburner.com and look through the site. They have some rough rules on determining the energy by volume and wall type.

 

[Jags]

Bravo Adios - cool looking stuff.

 

[Adios Pantalones]

I put together some pics from the last firing and the first firing (late last fall) in the picture forum- some good shots of FIRE coming out the chimney, hippies in hammocks, me with a machete- you know- average weekend at the Pantalones homestead.

https://www.hearth.com/talk/threads/22752/

 

[Redox]

That actually looks like fun! I'm just a pyro with a wood stove; you look like you are having WAAAAY too much fun for one person. Maybe that's why it's a team effort?

BTW, your dog has a strange growth around its body... ;-)

Chris

 

[kb richard]

Nice thread.
Am trying to figure out how to make a pellet "kiln" (2300f) for 24/7 operation and be safe enough to be unattended to melt some glass with. (190 Lbs.)

Just heard about pulverizing the pellets and blowing that. Any plus side to that stuff?

Planning on recouping exaust heat for burner air