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Kiln 2.0: melting glass, baking pottery, etc. - design

paul wheaton

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this is a place holder so that design discussion can begin

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Lisa Orr

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From my experience, kilns achieve high temperatures by the firebrick absorbing more and more of the heat given off by burning. For glass, it needs to be at least 1500F, for the lowest temp sintered clay (say, for flowerpots with or without lead glaze--the lowest firing and most forgiving but toxic glaze), around 1800F needs to be achieved. In higher firing applications like porcelain, heat needs to be built up to 2400F or so. I am wondering how we can use rocket technology to get temperatures up to 2000F (to fuse low-fire clays) or even to 2400 for stoneware or porcelain? Usually wood firing for ceramics uses SO much wood and creates SO much smoke and is SO inefficient. Historically, potters have been shifted away from city centers from ancient greece, to germany to great britain due to smoke. However, the beautiful effects of wood firing are admired worldwide. Can we use rocket technology to get high heat, sublime surface with less wood and low smoke? Thanks anyone with thoughts. Lisa, who is working to create a permaculture pottery paradise in MA.

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Awesome timing. I just started looking at this, too.

I acquired a couple broken electric kilns just to get "the box" of refractory and lid. One is a small hobbyist one I want to convert to wood and/or propane. I think it could sit on top of a J tube and work just fine. The larger one is a classroom/small commercial size and will need something a little more thought out.

There is a potter on YouTube named Simon Leach that has done the electric to gas conversion, it will get you good ideas on air/flame travel through the kiln and how to modify it.

Another issue is being able to control the throttle on the rocket. It is important to start slow to prevent cracking and warping of the clay. For the larger one I was thinking of having multiple rockets. At least a small warming rocket and a big main rocket.

I also want to capture the waste heat in a bench or water to heat the shop in the winter.

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Chris Kott

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Great discussion.

I just want to add, trapping heat in a ground-based heat loop, even one as simple as an air exchange system taking exhaust heat from the roof peak of a greenhouse or hot shop, would be like trapping the heat in a bench, but bigger and longer-lasting.

My much better half is a glassblower when she's not engraving things with sharp spinny metal. This is my plan for waste heat reuse. I think my secondary use will involve powering a retort to make biochar.

Incidentally, the dirty exhaust that, as a byproduct, creates lovely patterns in woodfired kilns can also negatively affect the colour of glass, especially clear glass. That's why wood gas exhausting from the retort will not be fed back into the furnace, but will, instead, fire a secondary burn under the retort on its own, probably using a one-way pressure valve and a venturi tube/manifold from a barbeque.

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Lisa Orr

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Yes getting an old electric burnt out "R2D2" type kiln as a shell is a great idea, and these have been converted to wood or gas before, but not sure if ever really researched with rocket technology. This is my new passion. Old burned out electric kilns are usually free, and contain expensive refractory K23 bricks already assembled into a circle, with a floor and a lid! Wonderful! Wonder if we would want to get a discarded one of those before the Jamboree to really give it a go? I am flying from MA and these weigh 150-200 lbs for the average school size of 6.7 cu'-- not super easy to bring. I do know some potters in MT, maybe they know someone ready for out their burnt-out electric to get hauled off for free? Thoughts? Lisa

Lisa Orr

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Oh and regarding throttle-- yes somehow pottery needs a slow warmup to release physical and chemically combined water around boiling point, and to get it through the first quartz inversion-- around 500F. Pottery LOVES to develop cracks at this stage if it is subjected to uneven heat due to rapid temperature climb.

One interesting thing I noted while studying the techniques of potters using simple lowfire roman style/mexican style wood kilns (circle of bricks topped with shards and firing chamber underneath) is that they pre-heated their wares atop a regular woodfired kitchen stove (outdoors) and later inside oven to really drive off all H2O prior to loading inside their capricious larger regular kiln. That way, stuff would for sure not blow up and stick little shards on everything else. This could be one way to manage water, but not thermal shock.

Thanks all,
Lisa again

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R Scott wrote:

I also want to capture the waste heat in a bench or water to heat the shop in the winter.

and Chris Kott expanded the thought and I will back it up further.

It's one thing to get the heat you want/need, it's a second thing to find a way to do so as efficiently as possible, and it's a third thing to recapture that heat in other useful ways. This last bit is so often ignored in our current mind-set - which just wants the stuff we've finished with to go away a quickly as possible, rather than thinking about how we could make it a useful byproduct.

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R Scott

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If you look at a noborigama kiln a certain way (like staring at clouds), the multiple chambers look a lot like stratification chambers.

I can't find a good link, but a noborigama kiln is really a series of small kilns going up the side of the hill with the exhaust of one feeding the next one up the hill. Each has its own burn chamber as well. Basically the exhaust of the first kiln pre heats the rest and when it is done, they start feeding the second burn chamber, etc. Each chamber was a down draft to boot.

Lisa Orr

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Love the idea of the noborigama climbing kiln. I dont know much about them other that seeing the dreadful amounts of smoke puffing out chimneys. Fantastic design for firing decent amounts of ware, but can a pool warmer, bun warmer, long warm horizontal bench, etc at the end maybe mitigate this issue? Extremely curious and thinking out loud, Lisa

paul wheaton

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Can people post pics of broken kilns that we might attempt to retrofit?

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paul wheaton

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My general thought is to have an 8 inch j-tube that will feed a well insulated box. And then use the box as a stratification chamber. For this to work, the box will need to be extremely well sealed and insulated. And the exhaust will need to run down pretty low to add to the stratification effect.

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paul wheaton

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I'm open to the idea of re-using a kiln that is no longer working. On the other hand, I kinda like the idea of making something really awesome from scratch. If nothing else, we have a couple pallets of that orange insulated firebrick to use.

Maybe this can be something where we connect a "rocket engine" to a portable kiln? Or maybe we need something rather permanent?

I think we want something that will last 20 years, and it will also be not terribly big because we would still be experimenting.

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Kirk Mobert

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I think that an 8 inch stove is going to be too small.
We're trying to get a space above or to the side of the RS up around 2000F. We see this regularly in a roughly grapefruit sized area, down inside the elbow but I've NEVER seen that kind of temperature above the riser. Our biggest hurtle is going to be in having a stove belch that temp at the top. It's a REALLY tall order!
My inclination would be to build a 12 inch rocket or even bigger and see what that produces. If it can't, then move up and see, if that cant, then we'll need a complete rethink.

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Lisa Orr

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Here is one from Facebook marketplace. It has an 18 inch interior diameter by 18 inches tall.

5CC19951-1A39-45FB-AF2A-40682493F851.jpeg

paul wheaton

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How about this: we build an 8 inch j-tube system with lots of insulation and a really deep stratification. Something that could be a small kiln. And then we put some cones in there and see how hot we got it.

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Lisa Orr

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I don’t know enough about fire dynamics. However I do know that heat the heat of the flame going into the Kiln can get trapped/held in the bricks if they are the ancient to pre 1960s high fire hard brick. And then it’s a matter of continuing to add flame for a long period of time over a number of hours thus causing the interior temperature to rise. Anagama and noborigama japanese traditional wood kilns take up to a week to fire and puff loads of smoke. Japan is out of wood for these traditional kilns and imports wood for these— they are large and wasteful, though everyone loves the effects of the ash left on the surface. How to get the temperature (2000-2400f) but engineer out the wasted carbon and put it to better use? Maybe there needs to be several rockets like 2-4 feeding the kiln interior if it is made out of the light weight refractory brick that does not absorb the heat readily? The good news about lightweight expensive fire brick (k23s or k26s) is it reflects the heat and keeps heat energy inside the interior space and wares. But when the source of heat stops, those bricks don’t continue to hold the heat like ancient hardbrick kilns.

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That (insulated 8 inch plus insulated stratification) could be a good first step but I have little faith that will do.
I think we need a space where we can do fast iterative design. We will need to be able to build, tear down, build again very quickly. This is a very thorny problem that will require serious head scratching and experimentation to get done.

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Kirk Mobert

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Multiple stoves might help quite a lot. One of our baseline limits will be just how much energy can be produced per unit time. This translates directly into how much wood can we eat at a time?

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Kirk Mobert

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We can reduce the amount of wood used by burning it more efficiently but we still need to produce a huge amount of output, regardless.

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R Scott

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Here is a picture of the kind of kiln that you can find free/cheap. Because they quit pottery, outgrew the kiln, or it broke. Common (Smallest) size is about a 30 gallon drum diameter.

Converting it to gas is simply putting a weed burner or forge burner in the bottom and a chimney out the top. Not so simple with wood because ash destroys the soft fire brick. You need to add a stucco layer with hard riser type refractory.

When you look at kiln net efficiency, even the best are maybe 10% efficient because the kiln itself has so much mass to heat vs. the pottery payload.

A09EAF87-7608-4F8D-B65E-1F6F1095E2C8.jpeg

paul wheaton

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I just ordered new cones. 04 (2008 degrees Fahrenheit), 06 (1873 degrees fahrenheit) and 016 (1517 degrees Fahrenheit)

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Kirk Mobert

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A Rocket J-tube typically reaches 2500°F inside, though that temperature measurement is about the size of a grapefruit (depending on system size).

One could theoretically set up a system to fire one object (pot) at a time inside the heat riser. The volumes and shapes of the riser would need tweaking to accept the thing to be fired, without choking flow. Perhaps the piece can be slowly lowered to gradually turn up the heat and avoid explosions and cracks.

This can be made to work relatively easily but only one or maybe 2 small things can be fired at a time and its highly likely that the exact same shape and size objects would need to be fired each time as well. Changing object type or size would change flow characteristics and need a new design tweak or efficiency will be thrown off.

I'm not advocating that this be built. It's a thought experiment of a type of system that I know will work. It will NOT be convenient at all, but it will work.

We need instead to have a box with heat piped to that we can place any old thing (or collections of things) into and go. I imagine heat control can be achieved by valving on or off inputs from one or more combustion units or venting away heat from the firing box or some combination of all of the above..

I'm thinking aloud here, so please jump in and correct or add where you see it.

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Kirk Mobert

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A second hand kiln body for a firing box can work really well. An issue with this is they tend to be top loaded. RS combustion units are tall. Placing a top loading kiln body above one or many J-tubes can easily put the loading door out of reach.

An easy-ish solution is to get one of those kiln bodies that is made from sections stacked. One section can be modified or removed and replaced to accept heat inputs. Because treating the firing box as a stratification chamber seems like a good idea, I'd input heat high in the system and chimney away from the bottom.
This will drop the working end by a lot. The rest we may gain by burying the feed box in the floor, with its opening (to put in wood) level with the floor.

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Lisa Orr

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Excited to learn that part of the J tube gets up to 2500! If we can move and collect that heat in the larger box kiln box/cylinder to achieve high temps and somehow combust all the smoke/carbon etc that would be amazing! Right now I am still thinking that we need to have several j tubes feeding one kiln to get the whole thing up to 2000-2400 but excited to learn what is possible.

Lisa in MA

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Here is a video talking about mashing wood fired kilns more efficient. He gets almost twice the efficiency, but when he is talking about TONS of firewood, you understand just how inefficient it is.

I am seeing the kiln and forge potentially converging.

Staff note (Glenn Herbert) :

Note that he is giving temperatures in Celsius. 1300 degrees = 2372 F, 1500 degrees = 2732 F.

2340 F is cone 10, common high-fired stoneware temperature. 2379 F is cone 12, common porcelain temperature. The Orton cone chart does not have anything to measure above 2530 F.

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Lisa Orr wrote:Excited to learn that part of the J tube gets up to 2500! If we can move and collect that heat in the larger box kiln box/cylinder to achieve high temps and somehow combust all the smoke/carbon etc that would be amazing! Right now I am still thinking that we need to have several j tubes feeding one kiln to get the whole thing up to 2000-2400 but excited to learn what is possible.

Lisa in MA

Moving that temperature upwards and into a kiln box is going to be rather challenging. The conditions inside RS are fairly special and we do it exactly for the purpose of burning off all the fuel. This is currently burning all the smoke in well built stoves. Were seeing averages of 93% efficiency outside of the lab, in normal conditions, which is practically unheard of.

Anyway, burning the fuel completely and delivering the heat, in rocket stoves, are 2 separate puzzles. The first of these has largely been solved, the second will be our major target in the kiln project. We need not just a large amount of heat, we need to concentrate it in a relatively small space. The kind of heat density we need is a sticky problem.

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Talyn Crafts

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Here is a potential inspiration , coming from Japan !

The video is only 6 mins long or so , no words - great ASMR sounds though.

I like how they have built a kiln large enough to fire essentially 6 months worth of work all at once.

Reminds me of a giant whale almost...or Earth Fish.

Also might be inspiring for setting up a small space (like the solarium) for potter's wheels --- not sure what their energy needs are, but I imagine they could be hooked up to a battery charged by solar.

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Lisa Orr

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Love it. Thanks for posting this video! He gets nice effects. I wish he would divulge his efficient design though! I'd be happy not using a ton of wood and not getting those high temps, but getting more efficiency. There is another kiln posted somewhere else on these threads that is a number of years old of some Cindy Mathieu that fired their kiln with jtube at corner of the kiln box for their firings. They reported good results-- not sure what temps they achieved!

https://permies.com/t/30535/rocket-mass-stove-kiln

I remain in favor of the J shaped feeder to provide high heat and wish to find a way to design in some sort of use for the heat headed up and out of the chimney. Recycle it back to kiln maybe? Heat a tub? Blow some glass? jk! (Though some very ancient glass pieces were formed in the sides of pottery kilns) Wondering if a horizontal tube or chimney that is bench like is not good for the higher kiln temperatures in some way.

Here is a diagram of a "smokeless wood kiln" in Canada: https://ceramicartsnetwork.org/daily/clay-tools/ceramic-kilns/the-sasukenei-smokeless-kiln-a-wood-kiln-that-produces-little-smoke-and-great-results/

Here is the only article in english that I could find about these low fire smokeless kilns that are being constructed in pottery towns and villages around Mexico in order to keep the tradition of wood firing but alleviate all the pollution and breathing of the smoke from the older traditional kilns that are much more simple and open. https://mexiconewsdaily.com/news/smoke-free-wood-burning-potters-kilns-under-construction-in-oaxaca/

Here is a youtube of a smokeless wood kiln installed in Oaxaca its in spanish but the visuals are good:

Maybe it would be helpful to examine these kiln plans and see what they are doing correctly that we might learn from? Or maybe a Rocket kiln is completely different?

Just ideas,
Thanks, Lisa

Lisa Orr

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Here is another one of the Mexico smokeless kilns with more visual detail-- great video of how proportionally tall the chimney needs to be to achieve smokelessness, apparently, because a shorter one will still have draft but more obvious waste like smoke and flames shooting out.

Someone tell me if this is the same effect as the RMH exit chamber tunnel that burns up the creosote that would stick in a regular stovepipe or chimney?

Or, where does the extra smoke get burned in a RMH? In the j tube? In the barrel bell or eventually in the chimney/bench/mass chamber?

Thanks!
Lisa

Lisa Orr

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Says here on this "smokeless" wood kiln design that the firebox is designed to burn the fire upside down, with the help of a metal grate and tall chimney. They made the ware chamber 25% of the usual size of the typical japanese wood kiln. This seems in the direction of what we might be wanting to do and what we might achieve. Here is another account of building one of these: http://rudymeyer.blogspot.com/2010/05/building-sasukenei-smokeless-kiln.html

Lisa again.

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Kirk Mobert

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Lisa Orr wrote:.
Someone tell me if this is the same effect as the RMH exit chamber tunnel that burns up the creosote that would stick in a regular stovepipe or chimney?

Or, where does the extra smoke get burned in a RMH? In the j tube? In the barrel bell or eventually in the chimney/bench/mass chamber?

Smoke is unburned fuel.
RMH burn it in the heat riser.

That smokeless kiln is really cool but I'm not sure if any of it can be brought in to inform a rocket stove kiln. They're just so different...

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t talianis

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Hey everyone,
I'm also in MA.... ceramist who took a break from clay in the past 5 years because of what it takes to mine, glaze, fire etc. But I MISS it. If there was a way to make an efficient RMH kiln, I would love to get in on it and do it. I built a downdraft gas kiln before but not a RMH. Trying to understand how to get it up to cone 6, even cone 04 with sticks. Thanks for starting this thread-- definitely into it!!!

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Not only are the temperature needed for a kiln crazy high, they also need to be fairly even and slow-changing on both heat-up and cool-down to reduce the chances of your piece cracking due to one part expanding/contracting faster than another. This is a situation where big is better than small, both because thermal mass is your friend and because it improves the surface area: volume ratio.

To minimize heat loss through the surface, how about surrounding the highest temperature oven (the kiln) with lower temperature ovens that can be fired up individually? These surrounding ovens could be used for lower-temperature functions, like a forge, annealer, low-fire kiln, or even a pizza oven. If you fill the central kiln with unfired pottery first, and then fire up the exterior ovens for a day or so before starting up the main oven it should help the pottery pre-heat before the main event. It will also reduce any interior temperature gradient due to losses from the surface by sheltering most of the surface from the (relatively cold) ambient air, surrounding it instead with other hot things.

Lisa Orr

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t talianis wrote:Hey everyone,
I'm also in MA.... ceramist who took a break from clay in the past 5 years because of what it takes to mine, glaze, fire etc. But I MISS it. If there was a way to make an efficient RMH kiln, I would love to get in on it and do it. I built a downdraft gas kiln before but not a RMH. Trying to understand how to get it up to cone 6, even cone 04 with sticks. Thanks for starting this thread-- definitely into it!!!

I am in Northborough-- you would be welcome to come practice this with me with my new old burned out electric. Starting small. Have all I need. Had some success in at Wheaton-- we reached a solid 04 in kiln area of the chamber in 2 hours. The firing chamber/heat riser builds up and hangs on to heat. Our stuff was bisqued previously or preheated and nothing blew up. Glazed stuff was very oxidized. I'm working at my place in MA on this after asparagus valley tour next week. Be in touch here if you wish.

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Jennifer Pearson wrote:Not only are the temperature needed for a kiln crazy high, they also need to be fairly even and slow-changing on both heat-up and cool-down to reduce the chances of your piece cracking due to one part expanding/contracting faster than another. This is a situation where big is better than small, both because thermal mass is your friend and because it improves the surface area: volume ratio.

To minimize heat loss through the surface, how about surrounding the highest temperature oven (the kiln) with lower temperature ovens that can be fired up individually? These surrounding ovens could be used for lower-temperature functions, like a forge, annealer, low-fire kiln, or even a pizza oven. If you fill the central kiln with unfired pottery first, and then fire up the exterior ovens for a day or so before starting up the main oven it should help the pottery pre-heat before the main event. It will also reduce any interior temperature gradient due to losses from the surface by sheltering most of the surface from the (relatively cold) ambient air, surrounding it instead with other hot things.

I like the function-stacking idea, and the "insulating effect" of nested chambers.
Thinking of Kirk"s observation about just how much fuel this thing can/will need to 'eat'... (and rather than seat-of-the-pants it, Btu/hr fuel consumption for a gas kiln firing, or kWh for electric, could be converted to tons of wood/hour?)
Maybe some of the surrounding chambers could be bell chambers with charcoal retorts inside. The resulting wood gas could be fed into the J-tube to further the process. A side benefit would be an up-front loading of the fuel, so that stoking might be reduced? possibly even timed to allow for a period of unattended operation (overnight?), or phased to provide wood gas from each retort in series for a steady supply. Retort bells could be in in series such that the first one would be pyrolyzing, and the next would be coming up to temperature, and others driving off steam. The charcoal produced could be used as fuel for another process like a forge, or glass kiln, or cooking fuel.

Nails are sold by the pound, that makes sense.
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adam wrate

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Location: Mohave Desert

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posted 2 years ago

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Jennifer touched on the issue I think is pretty critical and cannot be overlooked, the cool down. Did anybody record, or have any idea how many degrees per hour the temperature dropped after firing during PTJ'22? Sometimes, I'll fire down after reaching temperature for the first 4 or 5 hours if the kiln gets a little drafty. I like the idea of nesting two kilns as mentioned above.

The kilns that are made of Hard Firebrick are satisfying to operate. They insulate really well, and increase the cool down time. Also, the more experience you have with loading, the tighter you can stack them and that helps the insulating factor as well. Not necessarily all the same size pieces but different levels of 3d pieces that fit together like a puzzle does...let the flames do the rest.

Geoff Lawton's Greening the Desert Project

A feeble attempt to tell you about our stuff that makes us money

permaculture and gardener gifts (stocking stuffers?)

https://permies.com/wiki/permaculture-gifts-stocking-stuffers