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rocket mass stove kiln?  RSS feed

 
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Can rocket mass stove technology be applied to firing ceramics? Has anyone tried this, if so how?
 
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At Dragon Heaters, we kiln our products using a large box lined with fireclay brick we made and a 6" Dragon Heater. However, we are not trying to get the heat to a certain cone level or anything else very particular. The exhaust goes directly into the box and the smoke discolors the products in a random (it seems to me) manner. This would probably not be acceptable for firing ceramics. So, you might have to do some air to air heat exchange (so that the smoke is excluded) to get consistent results. You would also have to really feed the fire continually to heat up all the bricks.
 
pollinator
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Theresa Zelazny : With a Rocket mass heater that has come up to temperature a downstream kiln should see little if any smoke ever ( unless a little smoke was
simply not objectionable), which Is what Cindy Mathieu meant I Think !

It was recently pointed out to me that cyclone dirt catcher will work with air flow and it does not matter if the air flow is natural draft-pushing, or vacuum assisting
pulling, this should reduce any airborne fly ash greatly, again, for some 'objects d'art' that might be a plus !

FIRE BRICKS I really need to start a thread called fake fire bricks > > >

The best Fire brick as we configure bricks in Rocket stoves are generally 9'' x 4.5'' x 2.5/3'' and weigh 28-30 ounces Less than a quart of milk, the are very
insulating returning or reflecting, much of the heat energy of our Burn Tunnel and Heat Riser / Combustion Chamber back into the 'heart of the fire' !

The second best brick to use to surround and make our Burn Tunnel/ Combustion chamber is 100 yr old soft, red/red orange House Brick!!!

There is a 2nd kind of 'FIRE BRICK" that comes in the same size as regular Lightweight Firebrick and is the same nominal size as well as many others, and
generally weighs more that 7 Pounds, almost a gallon of milk.

This 2nd class of brick is very hard and dense, and is used in industrial kilns because it is extremely durable absorbs and holds heat like the Dense Thermal
Mass it is, and functions to hold the kiln at high temperatures to allow ether further annealing or hardening to take place !

As a choice for use in our Rocket burner core it is a poor choice, not worth its premium price and should be watched for and avoided !

Any one having access to these Hard Dense fire bricks for free or very cheep will recognize that they fit the bill exactly for a good type of Thermal Mass for
our Cob Bench or to line the inside or the outside of a ''Rocket Bell' I I hope this was clear as many people have gone to Lowes' or HO-DE's or a big box
store and got the wrong thing ! For the good of the craft ! Big AL
 
Cindy Mathieu
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Some clarification:

About the smoke, if you are not watching the fire very closely, it will become smokey as the bulk of the wood gets burned up. I believe its because there is too much air for the amount of combustion going on which cools the fire. Cooler fire...volatiles don't get burnt up... smoke is produced. This could be alleviated if you were right there watching it and adding more fuel or closing off some of the feed tube, etc. However, I have admitted that this is not how this kiln is being used. It is outside and not closely supervised, not in our living room.

About the fireclay bricks, we are using the dense refractory kind as the walls of the kiln. The bricks do build up heat and hold it for several hours. Our burn tunnel is cast refractory and is insulative. In addition, it is designed to be surrounded by 2" of perlite on all sides.

As with all refractory materials, there is a range of density and other qualities available suiting different purposes in a hi-temperature environment. This does not mean either kind of fire bricks are fake; it just means they were designed either for insulating or for conducting (and storing) heat. Allen's concept of weighing the bricks will indicate generally whether the bricks insulate or store heat.

We have one pyrometer in there so we know the approximate temperature, but coning is another matter. Our kiln does not have any windows and if we opened it up to check the cones, a lot of heat would leave.

Our kiln box is mounted up off the ground so that the exit from the heat riser flows directly into it.
 
allen lumley
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Cindy Mathieu : I plead guilty of (on an other post ) using the term ''fake fire brick '' It was Grand Standing at its finest, Meant only to grab the attention of people
browsing the rocket and wood stoves forum Threads and make sure that they were not purchasing the heavier Fire brick at a premium price, when it is the lighter
Sub 2 pound type or the 'Split-' sub 1 pound type (equal to a pint of milk) that they should be using if constructing D.I.Y. units, Only time will tell if my grab for
sensationalism works for good, my primary intention, (or not!)

If my best intentions of spreading the word about the inadvertent use of the wrong fire brick works to create disinformation, and poorly built rocket stoves on any
level or form I am sure that we will both be working to restore the correct order to The Rocket/Dragon Worlds. BIG AL !
 
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Hey Cindy, would you mind posting a picture of that kiln?
 
Cindy Mathieu
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Here are 2 pictures of the kiln. The first one shows it after we had fired some pieces and were starting to unload. You can see some discoloration on the burn tunnels. They go in gray with no brown.

The second one shows the 6" combustion system on the ground on the right hand side. The feed tube is inside a short 13" x 13" chimney flue liner. The heat riser is right under the kiln in another chimney flue liner. You can see the old pyrometer (with a handle) on a cement block in front with a wire going inside which is the thermocouple.

The sides and top are wrapped with 5" thick Roxul and then Hardie siding on the outside. There is a piece of metal roofing coming off the building to which it is adjacent to keep the rain off. This was a big project to build...lots of heavy fireclay bricks to lay, built a form to do the arch on the top, etc.
Kiln_Front_open_web.JPG
[Thumbnail for Kiln_Front_open_web.JPG]
Kiln_Front_closed_web.JPG
[Thumbnail for Kiln_Front_closed_web.JPG]
 
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thanks for asking the question Theresa, I was wondering the same thing as there are already wood fire kilns for pottery.
 
Miles Flansburg
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Thanks Cindy !
Are the big steel doors insulated with anything?
 
Cindy Mathieu
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Yes, the big steel doors have 1" ceramic fiber blanket on the inside. The blanket has to be attached with special anchors welded on to the steel plate.
 
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Just curious, but does anyone know if you could build a pottery kiln that would work effectively using only cob (not fire bricks) and build in a rocket stove feature right in? I mean like building a cob pizza oven (only bigger) with like a rocket stove type firebox built into it. I don't know if I'm making my self clear here or not.
 
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I've never built one, but the "groundhog" style kilns look to me to be the best candidates for a rocket heat source. Perhaps these will inspire you.

https://www.google.com/search?q=groundhog+kiln&safe=off&es_sm=93&tbm=isch&tbo=u&source=univ&sa=X&ei=5CXcU8amLpSeyATD1oG4Dw&ved=0CB4QsAQ&biw=1534&bih=83
 
Katrin Kerns
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Joe Braxton wrote:
I've never built one, but the "groundhog" style kilns look to me to be the best candidates for a rocket heat source. Perhaps these will inspire you.

https://www.google.com/search?q=groundhog+kiln&safe=off&es_sm=93&tbm=isch&tbo=u&source=univ&sa=X&ei=5CXcU8amLpSeyATD1oG4Dw&ved=0CB4QsAQ&biw=1534&bih=83



I'm not sure if this is quite what I was thinking of but it certainly gives me some ideas. Thanks for posting the link Joe.
 
allen lumley
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Katrin K. : I see a lot of assumptions/presumptions of what you mean by using the word 'KILN' , can you come back and tell us do you want to bake- bread, wood, or clay ?

Or something else altogether ? While each of these projects require a "KILN" , there is considerable difference in the need to hold ether/both high and constant temperatures !

Hopefully we will get a diverse group of craftsmen to weigh in on your next answer ! For the good of the craft ! BIG AL !
 
Katrin Kerns
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allen lumley wrote:Katrin K. : I see a lot of assumptions/presumptions of what you mean by using the word 'KILN' , can you come back and tell us do you want to bake- bread, wood, or clay ?

Or something else altogether ? While each of these projects require a "KILN" , there is considerable difference in the need to hold ether/both high and constant temperatures !

Hopefully we will get a diverse group of craftsmen to weigh in on your next answer ! For the good of the craft ! BIG AL !



So sorry that I wasn't clear enough BIG AL, I meant a pottery kiln for firing clay stone ware and bisque ware. I'm interested in finding out if one could be built out of cob (not brick) but with like a rocket stove type design.
 
allen lumley
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Katrin K. : O.K., Lets try this, to start with there are three types of ' COB ' - Structural COB, with hay, straw, animal hair, material that helps lock the cob into a
monolithic single piece ! Much, if not most of a COB House is made this way ! Generally we use equal parts non structural COB (no binders in the mix ) and local dense
rock or possibly urbanite to make up the bulk of the Thermal Mass of the Rocket Mass Heater RMH!

Preferably the second to last layer or shell created to surround both the Rocket Mass Heater and its Thermal Mass is a structural layer to give some physical protection
to the Whole RMH/Thermal Mass against knocks, jars, and things falling on it ! The Extreme outside layer also contains fiber and additional waterproofing materials !

It is very important that the layer of COB closest to the core of the RMH be insulating, or physically holds insulating materials against the "Firebrick" that surrounds the
actual combustion zone of a RMH, as such this Cob can contain various amounts of Perlite or Zonolite, along with its normal Sand and Clay materials,* with enough
thickness this cob can be highly insulating !

Even more insulating are various amounts of pure clay slip mixed with Perlite or Zonolite, however this mixture is extremely fragile and does not take the bumping into
common in unloading and loading a Pottery Kiln !

So structural cob is insulating and will help hold heat within the Thermal mass, slowly letting heat energy 'leak out from the core of the thermal mass !

Regular cob -mostly sand in a sand clay mixture that while not as dense as Rock, will provide a Solid core transporting the heat energy easily through itself to the Dense
Rock that can both hold and transport large amounts of Heat energy !

Finally, there are types of highly insulating cob that have additional materials in them to help create and maintain the high Temperatures 2000dF + , that allow us to burn
the wood cleanly, efficiently, and Totally !

A quick review of "Fire Brick'', which comes in two types and we can wrap this up !

"Soft Fire Brick " is highly insulating and we say it comes up to temperature quickly " Hard Fire Brick " absorbs a lot of heat this is because it is more Dense than the
other Soft(er) Fire Brick ! Both get their name from the fact that neither one expands or contacts much as it absorbs heat, or as it cools, not how much 'heat it can take '

In 'firing' Pottery^ the higher the temperature the Clay can survive in, the more durable (Generally ) the final product is !

The hotter the temperature that the Pottery is 'fired to' the slower you want the whole KILN to cool down !

Most Commercial Kilns have their interior walls lined with 'Hard Fire Brick'' that will absorb large amounts of Heat Energy at High Temperatures, and these Dense bricks
are glued, Cemented, Sealed to the interior walls and then between them and the Kilns Exterior there are large amounts of insulating materials to help retain the Heat
Energy trapped within or High Temperature Hard/Dense "Fire Brick " !

While you certainly can build a workable kiln out of COB, and fire it with a Rocket Mass Heater RMH, Conventional/Traditional Wisdom says that You Need ether a Dense
/Heavy" Fire Brick " or other dense stone to maintain the High Core Temperatures you need over the extended period necessary to reliably fire and Save your Pottery
pieces !

For further reading I can recommend www.traditionaloven.com ! I hope you found this to be Timely and helpful ! For the Crafts !

Think like fire, Flow like a Gas! Don't be a Marshmallow ! As always, your comments, or questions are Solicited and Welcome ! Big AL !

* sawdust too has been used to make a very light insulating material, when 1st fired it will turn to charcoal and in subsequent firings much will burn away, leaving empty
'cells' where the sawdust was ! This too is very fragile !

^ The Fire bricks are themselves fired this way ! Buy fire bricks rated for the temperature you expect to fire at ! Higher Temperature fire bricks are not worth the extra
cost ! A.L.
 
Katrin Kerns
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Hi Allen.

Thank you for the information. I'm a complete novice and haven't actually done any pottery or kiln building before. It's something that I'm highly interested in and actually hope to do some day though. I like the artistic way that cob can be shaped and used and hate the traditional large square brick oven style of kiln. Yes, I know that there are also round-ish kilns, but all of the one's that I've seen are very small in comparison to what I was envisioning. I was just wondering if it would be at all possible to build a large kiln out of cob because it's shape-able and I could be creative with the design and size. But then I wondered if you could build a wood fired cob kiln and that of course led me to the question of whether or not it was possible to build an all in one cob RMH type of kiln. LOL, you never know what you can or can't do if you don't ask, so I asked. You have given me a great deal of information to work with and an excellent link. So thank you very much for helping to educate a novice such as myself. Take care,

Kat
 
allen lumley
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Kat K. : A rising tide floats ALL Boats ! We are all in this together, keep coming back, with over 25,000 Fellow Members World Wide you should be able to come here 24 / 7 and
talk to someone who wants to talk about what you want to talk about ! For the good of the Krafts ! Big AL
 
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Katrin - If as you say you have never even done any pottery, let alone built a kiln, you need to start there first. Find a community studio or something where you can take classes and see all the operations. A good teacher (live) will help you avoid innumerable errors and frustrations. There is also a considerable amount of expensive equipment you can use before buying your own.

Once you have some experience, you can think about building a kiln. You said you wanted a big kiln, but think about how much work you need to make to fill it up (a half-empty kiln fires inefficiently and poorly). Unless you get into production quantities (making a living at it), it will be unreasonable to have a kiln much bigger inside than a three-foot cube or the equivalent.

As for actually building a kiln, I have annually for the past decade or so built and fired an experimental version of an English medieval kiln, using local clay with dried grass temper... essentially cob. The usual location has very silty, sandy native clay which fires out easily, but my local native clay (gravelly with little sand) has also worked well.
The mechanics of a kiln require lots of heat input at a steady and controlled rate. Too fast at the start and your pottery no matter how dry will explode as its residual water boils... this stage is very sensitive. Earthenware/bisque temperatures are relatively easy to get, but stoneware needs much higher temperatures and specialized design skills. Also, native clays may not be able to stand stoneware temperatures, and you might find the inside of your kiln melting. My local clay makes porous earthenware at cone 06, is semi-vitrified at cone 6 and turns to a runny glaze at cone 10.

As a rocket heater is designed to burn fast and hot, it is inherently at odds with pottery kiln requirements. I have heard of research involving multiple rocket heaters for a large kiln, with baffles and staging to attain the necessary slow temperature rise. It would be interesting to find out more about the subject. I encourage you to go ahead... with your eyes open.
 
Katrin Kerns
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Thanks Glenn. Very good advice all.
 
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Years ago I saw a video of a rocket stove brick kiln made in the Phillipines I believe. I searched but cannot find the video. I do remember that they switched to using rice husks and also used a blower to get the heat up. It seemed to work. They had the riser go into the kiln about a third up and then exhaust through two ports on the bottom of the bell/kiln then up the chimney. I would think you could build one up on a platform and have a 6 feet heat riser and make it a ten inch system. According to what I heard from Ernie say on a podcast that should make the temperature extremely hot and give you the airflow you might not need a blower.

The kiln might function better if you could twist the riser as it goes up and then shape the top of the kiln to direct the flow in a spiral so it will swirl around the ceramics and evenly heat the kiln.

Just a thought.
 
Jason Learned
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Okay I drew a crude sketch-up picture of one of the kiln ideas(sorry, I don't have a huge amount of time to make it perfect). The idea is the tube is slanted to get the flow to go in on the tangent to get the spiral going around the clay then the middle is hollow to let the exhaust go out through the center to the chimney. If this would not work. I would have the riser go straight up through the bottom and then have two exhaust ports on the tangent going out to two chimneys of the appropriate diameter to match the in flow. This should get the hot gases to flow around the kiln in a similar way that they do in a blast furnace with a blower. If you want to melt metal you blow the hot air around the crucible from the bottom to the top. With this I would try to get it to flow down for a bit to trap as much heat in the kiln. Also I would add a small port where you could aim a laser thermometer to check your temps.

So to see the bad scale. The inside of this kiln is 6' diameter and 6' high. That is about how big the one I saw in Asia was, but it was square.
kiln.jpg
[Thumbnail for kiln.jpg]
 
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https://picasaweb.google.com/Jonnygms/RocketStoveKiln#

I posted this link in another thread, but it seems that you are looking for the same answer. These pictures show the build of an inexpensive kiln made with cob materials and a dual rocket stove setup. There were some problems but they address these in the photo captions.

Does anyone see any issues with building a pottery kiln of this design? I am not experienced with cob or rocket stove building, so I would like some input on this design.
 
Glenn Herbert
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Not "issues" as deterrents, but a kiln built this way will require careful firing to start the temperature rise slowly and evenly (as does any wood-fired kiln). It is extremely easy to get the fire running away and getting hot too fast, causing the residual water in the dry but unfired pots to flash to steam and explode. This is unlikely to be a personal hazard, but a large part of the pottery load can be destroyed this way either directly or by shrapnel. The character of the pottery clay will influence this greatly; porous clay with considerable sand or grog may let the water escape easily, while porcelain or other dense, tight clay bodies require extreme care.

A kiln built from local clay is likely to be unable to withstand stoneware temperatures, as the one in the example shows. 1000 C (1830 F) which damaged that kiln is equivalent to cone 06, or earthenware bisque temperature. Obviously they are firing those stoves to a very low earthenware temperature which would not be suitable for any reasonable American style use. But if you build a kiln with material tested to withstand stoneware temperatures, there should be no reason you can't use the rocket firing technology with appropriate care.
 
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Theresa Zelazny wrote:Can rocket mass stove technology be applied to firing ceramics?  Has anyone tried this, if so how?



Wow, this was from 2013!  So I will attempt to upload a pic. I'm located in Chehalis, WA and a noob to this pottery thing, too.  We just moved to a new place and I have so far a few.different types of clay around here on the property.

1. On my hill, lots of sand, iron oxide so yellow. Quite sticky. Not so awesome for making pottery, but makes excellent cob. Mix in a little hay and it's good to go. I made a rough kiln pit of this over a 3 ft deep hole in the ground dug out of the same stuff. The fires I built in it make the surrounding hole into terra cotta. It holds water like a pot. Just is crumbly, hence not.good.for pottery. The hay in cob solved the crumbly issue when using it to build my kiln. Acts as "rebar" to make it stay together long enough to burn hard.

2. My earthenware type.clay, excellent for pots, grey in color or sometimes iron oxide yellow.  It is in little pockets on a hill in my field and is almost pure.

3. Brown stuff lower down in the field that has lovely,  fine powder texture and feels so smooth for working with in making pots, but it cracks so badly and disintegrates in fire.  Apparently too much organic matter. I even added grog, and it's a big nope. Sigh... it makes such fine details in things like little roses...

4. Stuff in the creek, grey or yellow iron oxide. Like No. 2 just harder to get to and more gravel in it.

So I made the pottery, made the kiln, and I can at the very least bisque or make terra cotta. I don't have a thermometer yet. Is there an affordable one anyone recommends? Was looking on Amazon.   I bought some low fire simple clear.glaze and a white shiny one as well from Amazon.  Haven't gotten the kiln to glaze pieces correctly yet, just make pottery, but then the kiln clay isnt all the way dry yet and smokes out the many cracks on top. I have burned 3 fires in it so far, drying the kiln itself a d testing one little cup w glaze on it. The kiln top is a good 4 feet away from the fire, with the racks that the ware sits on being dispersed in between, and I fill the whole 3 ft hole below with fire.  It is so hot it is difficult to stand there and look how things are going when I crack the door open. The door is currently a piece of galvanized metal.from an old garbage can, not the cement board in the picture- at least until I get aomethimg better. Have to argue w hubby on everything, including how to make the dumb door on a mud kiln! The top of the kiln, which is 2 inches thick in clay cob..  is hot enough on the outside that it's too hot to touch,
but not  glowing on the inside or out, just down in the firepit up to ground level glows red.    I had some loose metal greenhouse hoop material (steel? it has memory and doesn't bend easily,  4" gap wire metal mesh, see pics) inside that I slapped the cob onto for structure... some was sticking out on the inside of the kiln here and there, and while it did not burn off, it did melt significantly and sag down.
I think it's safe to say its a plenty hot fire for pottery.  Still working the bugs out.  I pit fired things also in that 3 ft hole before I built the cob kiln on top and it glowed red and made pottery, but I had problems with accidentally breaking the handles off of my coffee cups when adjusting the wood and stuff. As far as glaze goes, one side would.glaze correctly,.and the other side would be matte colored still- uneven heating even though I buried the stuff in coals.  At the cost of my cup handles,  sadly.  But this is all a learning curve.  I was using my wood stove in the house, too, with similar issues. I can see that I'm gonna have to slap.clay on that thing 3 or 4 inches thick and put some kind of sealer on it for the rain. Have worked with cob a bit now, and at my former house. Found out that on an indoor woodstove, cob makes.an excellent insulator and makes a fire VERY efficient burning, no waste, just ashes. Also, cement can cover it,  but be careful bc cement and clay shrink and absorb moisture at different rates. Where it wasn't quite so.close to the actual stove, like the backstop part, I brushed on cement w a paintbrush in layers w no issues and I had a surface I could use, like put a teapot on or your wet mittens, cat slept on it...   But closer to the heat, it would crack bc of the different shrinkage rates. Also learned that cob corrodes your wood stove and chimney very badly, so if you want to keep the metal nice, then maybe don't put it right on the metal. Use a wire frame and build it a half inch put away from it or so.  I did a dome over the top of the wood stove (a large tile on the actual top of the stove bc In sure that metal top.glowed.good and red) in cob with a tiled flat surface on top of that that I kept a teapot warm with. And inside the dome I baked bread and made.casseroles or fried things in a cast iron skillet. Cob insulates VERY well on a fire source.

My goal is cups and bowls you can drink out of, that's it. So.if anyone has recommendations for glazing or a temp gauge, I'm all ears. Burnishing is a major pain in the butt.  I hope this post doesn't get lost in the archives. Google doesn't seem to care how old it was. lol

My.next attempt, hubby bought me a large terra cotta flower pot and we are gonna try putting  that over a piece either setting on that rack, or down in the hole and build the fire all on top of it and around it.

I have seen videos of.native pottery where they put the piece inside a metal oil barrel, put sticks on top of it, set it.on fire (and somehow it does not neat up too fast and break) , and then add more sticks, then leaves to make it blacken (chimes off the oxygen from the fire). They build a fire underneath the barrel, which has a few holes in the bottom to catch the kindling inside. A piece of sheet metal covers most of the top.  The guys name is Joe... something on YouTube,  traditional Cherokee Native potter.

So it can be done, I guess it just takes some finessing.
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Side view
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Shows the grid used to build.cob on
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The shelving inside is just some pieces from an old garden cart.
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Lighting the fire in it.
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Front view, lit, door off, w all the clay on it.
 
P Colvin
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--" Obviously they are firing those stoves to a very low earthenware temperature which would not be suitable for any reasonable American style use. "---

Is this bc the clay is still porous? Is this a health issue that I've been reading about "foodsafe" issues, or is it more of whether or not it can withstand more thermal shock or be microwavable?  Bc I have seen available in Mexico, and I think Turkey...  seen on Amazon and Google for sale....basically casserole pots that are terra cotta, used to cook in in the oven or over a fire. Also, traditional Native Cherokee stew pots look like they are low temp fired in a pit fire or an old oil barrel w a cover on it . They designed them w a round cone bottom so it could be nestled down onto the fire coals and you cook your stew on it that way. Saw another video of Tibetan black pottery fired in a big bonfire, then treated w a mixture of barley water or something to treat it for food.use?  I read elsewhere, its something about plugging the pores so it doesn't "weep" out the liquid.   Cherokee Indians and Mexico potters burnished their pots (pain in the butt, but very pretty) to make them hold water.
 
Cindy Mathieu
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As far as a thermometer goes, you need a thermocouple. You have choices as far as how high the thermocouple probe can read; the higher the temp, the more expensive the thermocouple. The thermocouple probe is on the end of a wire and the display is on the other end of the wire outside the kiln. You can order them off ebay from China, wait a few weeks, and save some money.
 
Glenn Herbert
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A thermocouple with a temperature gauge attached is nice for learning what various temperatures look like, but I prefer to just use "cones" - little triangular pointed sticks of clay that are made to soften at specific temperatures, or more accurately, specific amounts of heat work. This is more accurate than a thermocouple in telling how hard your pottery is fired to. Within limits, you can get the same result with a slightly lower temperature for a longer time, or a higher temperature for a shorter time (around the peak of firing). The cones behave the same way the pottery does. You can get them from any pottery or ceramics supplier. For starters, I would recommend boxes of cones 010, 07, 06 and 05, to work with standard bisque or earthenware firing. Here is a link that describes cones in more detail: Temperature Equivalent Chart for Orton Cones

If you are using commercial low-fire glazes, you probably want to get to cone 06. Common practice is to place a cone pack (a set of cones that brackets the desired temperature range) where you can see it during firing without opening a big door. Watching this tells you when you are approaching the right amount of firing for your wares. You will learn what the right temperature looks like so you can make approximate judgements during the firing on your own.

Wire racks are not going to work for long when firing to any glaze temperature; they will soften and bend under the load. Blacksmiths shape iron and steel at lower temperatures than you will get inside a kiln. You need ceramic shelves or ledges to stack the pots on. You can make clay arches over the fire with flat top surfaces to hold the pots.

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looking down into ware chamber, firebox below
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building the firebox, green sticks for armature
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adding dirt for insulation
 
pollinator
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I have been experimenting for a few years with rocket stoves and been able to make them successfully with clay mixed with fresh cow manure as well as some straw creating a traditional building material which the Indians have been using in villages for hundreds of years and continue to do so even to this day.. but when I got into pottery and wanted to use the same principal for firing pottery I realized that such a big structure for such high temperatures may not be feasible to do with just cob, but by using firebrick and insulating materials like ceramic fibre etc. I was able to build one with very good airflow, and able to easily reach bisque temperatures.. I will try another firing with a pyrometer soon to check actual temperatures but the first run bisqued about thirty mugs, bowls at a quick 8 hour run with minimal wood and no breakages inside.
Checkout the kiln burning six hours into the firing!
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Glenn Herbert
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Nice looking kiln! And nice work.

It depends on the properties of your local clay, but I have found in two different regions in the northeast US that native clay kilns can be used to fire up to around cone 01 to cone 1 (around 2000 to 2100 degrees F), possibly more. Certainly potters in some southern US traditional potting regions were able to fire to extreme stoneware temperatures (cone 12) using nothing but native clay, but that is in areas where the clay can be used for stoneware pottery right out of the ground.

My native clay shrinks and distorts on the surface as it reaches cone 01, vitrifies at cone 6, and melts before cone 10.
 
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