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Clay saggar for traditional woodfired raku muffle kiln

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Do you want to only build the saggar/muffle, or the kiln itself? Honestly I had to google search the term muffle in relation to a refractory/furnace/kiln etc; had never heard that one before. From what I can glean a muffle refers to a saggar which is a chamber inside of another heated chamber (typically a kiln). Typically the saggar is where all the "good stuff" occurs in a saggar style firing; bannana peels, seaweed, pine needles, saw dust.....etc is placed around the wares, inside the saggar, inside of the kiln, and the whole thing is fired off, and allowed to cool naturally. All of the reduction and flashing of your clay/glazes occurs in the chamber.

When you say raku, do you mean a western raku, where you open the hot kiln chamber, extract your vessels, and put them into some kind of combustibles (placed on ground, in trashcan chamber, etc), or do horsehair, or alcohol redux....Raku means a lot of things, to a lot of different people. Can you clarify what your actual intentions are? Do you plan on having more than one pot in the saggar at a time....3 pots, 5 pots..?

If you are planning on opening the kiln, and then consequently your saggar/muffle chamber (which is inside your kiln) to extract your vessels from inside the saggar, its going to have to have some unique design features. Typically a saggar is a sealed chamber, so there's going to be a lid; you need to make the lid easy enough to remove while the saggar is hot. Likewise, it should have a large opening, so you can squeeze a set of tongs inside, to grab your wares, and not have to "thread the eye" so to speak. Ideally, I would make this saggar shorter, and wider, than narrower and taller. A downside is that wider objects take thermal shock not as well as smaller objects, where the transfer of heat/cold can occur more evenly across the surface of the object; with that in mind, if you plan on using this saggar more than once, I might have a kaowool "bin" set up where you can drop your hot lid, throw a piece of wool over it, to prevent it from cracking/shattering. Likewise, if this saggar is sizeable, I would plan on being able to seal your kiln back up as soon as you have extracted your pots from the saggar, so the large saggar doesnt crack.

Another thing to consider; if you are loading the interior of the saggar full of combustible materials, sealing it from oxygen, and then opening that said sealed chamber, inside of a hot chamber, expect a sizeable ball of fire/backdraft to come flying out at you when you open this saggar.....open sagger lid, stand aside, and allow combustibles to ignite....save your eyebrows.

What kind of kiln are you firing this in? Typically wood fired kilns are built from hard brick, as the fly ash will eat into soft brick and wool refractories, which will in relatively short time, destroy the kiln. Hard brick are much heavier, and in terms of "opening" a kiln during the firing, I've not seen a method to open a hard brick kiln, quickly. You might have to build the hard brick kiln, and have a space which is sealed with a kaowool "plug" which will be your opening during the firing to extract your pots from the saggar. Some high temp wire could be used to bind the plug together, and act as a handle to lift your plug out of place.

Lastly, in regards to heating it with wood; two things come to mind; one, the fly ash, over time, will coat your saggar in a pseudo like glaze; at low fire temps this buildup will take a long time to occur, but at high temps, it will be relatively fast, especially if the firebox is close to the chamber. Eventually the "glaze like" coat of fly ash will make the saggar a little less capable of handling the thermal shock, and/or it could seal your saggar shut during the firing. Second, wood is a fuel source which is relatively slow to build heat with; a small chamber (just big enough for your saggar), with relatively well insulated walls, may take something on the order of 4-8 hours to get to low fire temps (cone 06-04, where most "raku" occurs); this of course will depend a lot on your kiln design, and how you stoke the kiln.....could be a lot faster, or slower.......keeping a good balance of oxygen with your fuel will give you the most heat from your fuel....dont just "load" it up.....little pieces at a time, in a rhythm....the kiln will start to "breathe" and you will build heat quite quickly.

As far as what kind of clay; choose a body that handles thermal shock well; a raku or sculpture body usually have a wide mix of particle sizes, and grogs/fillers which allow it to expand/contract better. If you can find a body which has pyrophillite in it, or if you mix it in yourself, this too will help with the thermal shock. You could also mix up a "castable" like you would for kiln building, and build your saggar from this. Make your walls relatively thick..approx 1/2-1" thick.

Hope all this answers your questions!

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Thank you for your thorough and comprehensive answer.

It will be a simple wood fired kiln.  More simple traditional ware than  complicated or involved reduction practices.

Multiple pots in the saggar, no combustibles  included.  Yes , taking the pots out as soon as  glaze has matured. (so there is a possibility for post firing reduction if someone wants)

Old hard bricks and hard firebricks. I built a simple downdraft kiln that went past 1100C. I will be dismantling that and using the bricks. I think this will only need to get to around 900C?

I think I will fashion some kind of bellows system for extra air.

I am very interested in the idea of making a "castable" ... Can you elaborate please.

I am based in Finland so supplies and materials may be a bit limited.

Something based on this... Possibly with charcoal packed around the  saggar.  (also thought that adding some rings to support some king of chimney during initial heating might speed it up?)

Any other thoughts? Cheers Roderick


Edited by roderick
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Oftentimes when building a wood kiln many folks will use a castable layer of refractory as the cold face of their kiln. It essentially a mixture of fireclays and other refractory materials....lots of different recipes out there. Google searching will turn up a myriad of results, as will searching on this forum. If you are planning on firing this saggar to 1100-1200 C (high fire range) then a castable saggar may not be the best option. Castables are generally used as a cold face where the temps are not as severe; I believe that at low fire temps, typical for raku processes, that a castable would withstand the rigors of your process better.

Based on the drawing you provided that would be quite simple to build the kiln, and have access to your saggar's lid. However I might take the "chimneys" layer of hard brick up above the lid of your saggar; cold breezes or uneven temps across the lid of your saggar could lead to it cracking.

A bellows could be made from a small squirrel cage blower and some metal pipe, a leaf blower on idle or low, or a small box fan. I would be gentle with the amount of air you add in the beginning so you dont heat the saggar TOO fast and crack it.

I wouldnt pack coal around the saggar; the area in your drawing where the saggar sits is basically the chimney for your kiln; if you dont have ample draw, you wont build enough heat, and you'll get a lot of smoke in your face. Blocking the airflow is not what you want to do. You could use coal as a part of your fuel source though. Generally when building a kiln, you want your chimney to have the same amount of cubic inches that your inlets (firebox, burner ports, etc) are. I would allot at least a 1" space in between your chimney walls and your saggar; thats a large diameter circle, so a 1" space might be all you'd need to draw properly, and still retain heat on your saggar. You could go through and do the calculations to confirm, which would give you the most accurate setup, or since this is a very easy kiln to alter, you could just do it on the fly. If you wanted to have more speed to your kiln, a larger/taller chimney could be added, however if you wanted to access the saggar it would need to be something you could remove quickly. A sheet of kaowool, wrapped into a cylinder, and held with high temp wire on inside/outside of the cylinder could increase the draw of your kiln==speed, but also be removed with tongs/gloves to access the saggar. A piece of non galvanized chimney pipe will work as well, but you'll need to find a LARGE diameter pipe (most chimney pipes are 10" and under), and accept that the pipe wont last but maybe a couple dozen firings before it begins to degrade.

What kind of glazes are you planning on using? Most traditional "glazes" used in gas/electric/wood kilns wont perform well in a "raku" style firing. Generally "raku" style glazes are formulated to handle the stresses of thermal shock better. There's no reason why the saggar lid could be left once you reached your maturation point, and the saggar lid/chimney covered over with a piece of kaowool and allowed to cool; it would basically make this saggar into a more "traditional" kiln....essentially the saggar would provide you an atmosphere clean of fly ash and debris, and would allow you to fire "normal" glazes in oxidizing atmospheres.

I would bisque the saggar before its initial glaze firing; if you have access to an tradtional electric or gas kiln id bisque in that if you can, otherwise go SLOW in your kiln depicted above.

Sounds like a fun project and you've got some kiln building/firing with wood experience which should make this a breeze for you!

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it looks a lot like my wood fired raku kiln from the 70s except I had a barrel on pulleys lined with fiber. 

It was made of scrap bricks and was torn down regularly by neighborhood kids. The drawing of the design is in "Raku ; A Practical Approach , 2nd edition

p 113.

We split 2 x 4 scrap to fire. It did fire fast once we got the temperature for the first batch. My classes fired all day. The barrel sat on top of the cylindrical brick chamber. Mine was more tapered.






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Thank you Marcia and thank you again Sam for another very informative reply.

After looking at you web page Marcia I will just have to try soluble salts and terra sigillata in the saggar. :)

Sam,  thanks, good idea about raising the chimney wall above the lid of the saggar. 

re: Glazes. I've got a couple of off the shelf Raku glazes, White Crackle and Transparent to be getting on with. Then we'll see what comes up. 

Yes, I have an electric kiln (about 350 litre) so I can fire the saggar first. A couple of questions about this. If the saggar is castable should it still be biscuit fired first?

If I make the saggar from a Raku/Crank mixture should I fire it to vitrification? Would that make it stronger?

Cheers Roderick

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8 hours ago, roderick said:

Yes, I have an electric kiln (about 350 litre) so I can fire the saggar first. A couple of questions about this. If the saggar is castable should it still be biscuit fired first?

If I make the saggar from a Raku/Crank mixture should I fire it to vitrification? Would that make it stronger

Yes, many castable recipes will benefit from being fired to 04. I would avoid any recipes that call for portland cement in them; not likely many do, but the cement at hotter temps will degrade; as a cold face to a wood kiln it wouldnt normally see temps that would be an issue.

You dont want to fire your saggar to total vitrification; a vitrified clay body has essentially formed a crystalline matrix which creates a glass like nature to the clay body. This crystalline matrix (mullite and cristobalite) are more prone to damage from thermal shock. Most raku and sculpture clay bodies are meant to be fired at low fire ranges (06-04) and not vitrify, which allows them to withstand the thermal shock they incur during most raku processes; other materials in the clay body recipe help with the thermal shock, but the lack of vitrification is a big benefit. When you "rap" it with a knuckle, it should "ding" but not "ting/ring" like a bell... it should sound like bisque ware.

If you've got no experience with soluble salts, be aware that the fumes during firing are more toxic, and the handling of them during application requires more precautions. We've actually been discussing soluble salts on another thread here. 

There is a good book, which if I havent mentioned it yet, might be of great use to you. Published by Lark, its called "Alternative Kilns and Firing Techniques". It covers different barrel/saggar style kilns similar to what you are doing, and a section devoted to soluble salts.

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