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cabako

Soda Firing Questions

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Hi All,

I'd like to do some cone 10 soda firings in my small propane/oil fired  kiln and I have some questions.

  • When adding the soda into the horizontal flame/ports, in terms of spreading soda thru the kiln,  is it best to use saturated soda liquid sprayed in or dry powder?
  • Is there any visual glaze difference between baking soda(bicrabonate) vs soda ash (carbonate)?  
  • If using baking soda /bi carbonate only, would I have to use twice the amount to that of soda ash to provide similar effects (1 sodium for bicarb compared to 2 for carbonate)?
  • I have daves porcelain from laguna, can I use this without a flashing slip and still have some glaze effects? 
  • If i soda fire unglazed porcelain, could I do one single fire and not have to bisque?
  • What if I did a salt (NaCl) firing, would results/color be similar?
  • How much soda to use? Is it determined visually by looking at buildup on the pieces during firing?

Thanks

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Soft brick will get very severely eaten by any sodium compounds. It isn't recommended to use soft brick on the hot face of a soda or a salt kiln, and definitely not for the firebox.  You can get coatings that will protect the kiln, but they're expensive, and might not be worth it, depending on the state of your used brick.

As to the rest of your questions 

1) Soda is "lazy" in the kiln, and doesn't volatize as explosively (and therefore disperse as evenly) as salt does. This is the reason many people who soda fire use a sprayer, and introduce their soda soloutions into a number of spy holes, not just the burner ports. That said, it's really easy to melt sprayer tips.  I prefer to use Gail Nichols' method of mixing calcined soda ash and bicarb with equal amounts of calcium carbonate, and water. Previous to this method, I'd dissolve the amount of soda ash I wanted to use in hot water, and use sawdust to soak it up. I'd then wrap the wet sawdust into a newspaper burrito, and throw those in the fire box in 20 minute intervals. Any which way, you need some kind of water introduced with your soda to make it volatize better in the kiln.

2) Not that I've observed. But I haven't seen everything yet.

3) molecularly, yes. The amount of soda that gets deposited on the pots is also dependant on the pack of the kiln, and any number of other things that will affect kiln draft. The first few times, it's nice to place some draw rings so that you can judge buildup.

4) Yes, but in my experience, porcelain is very pale in soda. If you like whites and greys, then use porcelain.  Stoneware, ball clays, and less-pure kaolins (eg helmer) will give toastier flashing. It all depends on what aesthetic you're after. 

5) yes, just go very slowly through quartz inversion, and observe all cautions that you would in any other once-firing scenario.

6) No. There are differences. Soda is much better for flashing, and tends to be more subtle. Salt gets glossy and that orange-peel effect much more readily.

7) until you're familiar with your kiln, it's a good idea to use draw rings, just to give you  an idea of the buildup on your pots. Soda and salt kilns are a bit like cast iron frying pans: as you use them, they get kind of "seasoned" for the lack of a better word. Residual sodium will build up in the kiln over time, and you'll need less and less to get similar results with more and more firings. Towards the end of a kiln's life, you almost dont need to add any soda if you're after flashing effects. 

Edited by Callie Beller Diesel
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Here’s a shot of the inside of the Medalta soda kiln I took last January. It’s 16 cu ft. Note the texture of the soda eating into the silica of hard brick on the arch. This is a well-seasoned kiln, and I think Terri and I used about 5 kg of our soda/calcium carbonate mix (about 10 pounds). It was quite generous, and in the “wetter” spots of the kiln in the front, you can see we got lots of buildup. The dry spots in the bottom in front of the flue got some very light flashing. 

35F83738-00BC-40C8-AF04-7602AEF82729.jpeg

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+1 to everything Callie said, plus: You need a liner glaze. The soda will not get inside of the pots enough to glaze them over well.

You should only use a soft brick kiln if you're willing to destroy it via the process. Once you salt/soda it, it will forever be that way, and the salt/soda will eat it away.

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Callie really great info thank you for the pic! Yes this kiln is expendable. But maybe i'll put some thin lining of ceramic wool to cover the bricks from the salt and try to extend the life.  

This first firing only has some small shot glasses and a bottle. maybe if I tip them over on their sides and face the inside towards the flame they will accept the glaze better.

Another question: do i HAVE to use wadding? Or can i use the kiln stilts with needle tips to place work on? Could I then just pluck off the stilts? I don't mind having marks left on the pieces.

Thanks for the feedback all!

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You're welcome!

Ceramic fibre will get eaten even more quickly than soft brick. If you're looking at it as waster, then go ahead. It'll stand up to a few firings, anyways.  Don't mess with the fibre.

If you wish your pieces to hold liquid, you should use a liner glaze. Neither soda nor salt will  give you an even enough coating to even if angled to catch the best possible application. 

Kiln stilts don't work at all at cone ten. The wire melts. Use wadding. There are lots of recipes, but the basic one we usually start out with is 50/50 alumina and some kind of clay, usually EPK because it's cheap. The alumina doesn't react with sodium, and doesn't flux out by itself, so it keeps your pieces from getting welded to the kiln furniture. The clay makes it plastic enough to form. (If you want to get fancy, use a clay like helmer to induce flashing around the wadding marks.) You also need to use it between the posts and the shelves, or they'll get welded to each other too.  You can see a squished post wadding at the bottom of the middle shelf in the picture. Kiln wash is also non-optional.

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I would echo all the points above. There's really no point in trying ceramic fibre - it will dissolve like candy-floss the first time you salt or soda the kiln.

The commercially available protective coatings are rather expensive, but you can make a substitute. I used the following mix on my salt kiln (which was made from a home brewed castable), and it performed relatively well. It needed patching/repainting every second firing or so, but that's a ten minute job. When it came to demolishing the kiln, the castable was in remarkably good condition underneath the ever-thickening skin of kiln wash, considering the abuse it received. The firebox was heavily punished, and needed continual running repairs, but molten salt will do that. All salt/soda kilns are works in progress, and all will eventually succumb to that process, whatever you do.

  • Alumina Hydrate     - 500g
  • China Clay                  - 250g
  • Zirconium Silicate   - 250g
  • Silicon Carbide         - 100g
  • + 1 Litre Water

...painted on a few mm thick.

This is not my recipe - I think (although I can't be sure) that it came from Peter Meanley.

Edited by Sputty
Just because.

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The danger of using a wash to protect the bricks is that it will flake off during the firing, and you'll end up with pieces in your pots. It's best to use a thin layer. It's a matter of finding that balance between being thick enough to protect the bricks, but thin enough to not flake. Most coatings don't stick well to old bricks, so that may be an issue, too.

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I would leave out the silicon carbide from that formula as salt sticks to any silica.

The Zirconium Silicate  is not a liquid as I was thinking of something else  . You should dampen your soft bricks with a spray bottle and only apply a thin coating-spraying is best.

I have posted some details on this process and you can search for them.

I have a salt kiln with soft brick in it and a fiber door so I know a bit about it.

Callies post covers most items

I also use daves porcelain in my reduction work but in salt I prefer stoneware clays -you can cover the daves in slips.

Daves alone will be very bland.

I'll post a photo in a few days-I'm setting up my xmas pottery booth now and doing some ocean crabbing and will have more time later .

Edited by Mark C.

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@Mark C.  Um... Zirconium Silicate is not a liquid. It's a powder. Or at least it is this side of the Atlantic. The Silicon Carbide - well, who knows what it's doing there? I just slavishly followed Peter Meanley's recipe - I'm sure he had his reasons for inclusion, and it worked for me. (If you don't know Peter Meanley, he was the Head of Ceramics (University of Ulster) for over 30 years, with a DPhil in salt glazing - that was good enough for me, knowing less than nothing, to try what he suggested!)

@neilestrick  Indeed. However, I slapped it on with little regard for such niceties, and never had a problem. By the time my salt kiln came to be demolished, the coating was pretty thick in places, and mostly not flaky. I don't remember ever losing more than a couple of pots to bits of random kiln material in many, many, many firings. I am aware that others have had different experiences, including having the commercially available materials peel off in sheets during a firing.

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@Mark C.  you may be thinking of sodium silicate.

Salt will not affect silica itself. It only melt when there are both silica and alumina present. Silica alone will repel the effects of salt. In grad school we used silica sifted onto the shelves instead of wash, because it was much easier to maintain. Not as healthy to deal with, though.

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Ok no fiber.  I probably won't make a kiln wash. But I have some activated alumina beads I can spread out under the ware to act as wadding and to let it move around a bit during the fire this may protect from cracks as well?  I was also thinking about adding these into the clay itself and see what happens...

https://www.deltaadsorbents.com/activated-alumina-desiccant-f200

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OK - so with respect to Silicon Carbide in the coating, Peter Meanley says:

Quote

It was noted that of the original applications of materials to the brick surfaces, those containing a high percent of silicon carbide often tended to be among the best in resisting the breakdown of the brick surface. What seemed to happen in such cases was that initially the silicon carbide mixture formed a dark glass.

Using Insulating Materials for the Construction of Salt Kilns - Peter Meanley - (PDF)

(This paper is the result of a four-year investigation to try to find combinations of insulating materials which, when allied with different hot facings, might retard or even prohibit their deterioration so that potters might be encouraged to build insulated kilns for salt and soda firing.)

I imagine that the whole of this paper will be of interest to those looking to build a salt or soda kiln from insulating bricks.

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While I have no specific recommendations for the choice of refractory for cabako's kiln,  I do want to comment on where to look for insights.
The interior of a black liquor furnace (a industrial device used in the manufacture of paper) is exposed to similar refractory corrodents as found in a salt/soda pottery kiln.  Anyone seriously studying deterioration of a soda/salt kiln should not ignore the understandings of refractory corrosion that have been published in the black liquor furnace literature.  I recall reading a late 1990's paper on refractory corrosion in black liquor systems. The European electric power industry has for several decades sponsored  studies on resistance to alkali corrosion from biomass combustion (again an industrial environment analogous to a salt/soda kiln environment). 
 
The answers to many of a studio potter's questions on kiln materials for salt/soda kilns are already published in the refractory and other industrial & scientific literature, but these studies will not be found using 'salt firing' or 'soda firing' as internet search terms. 
LT

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if you are using the burner ports for the point of introducing the soda or soda ash , the burners will need some protection too. You could use the alumina/kaolin kiln wash mix on them as well. I always used the 50 alumina/50  kaolin  recipe , same as the wadding.  Plus dipped the posts ends in it as well. and the peep hole bricks. I prefer dissolving soda ash in boiling water and spraying it into the kiln through higher peep holes. and damper shut with burners on. As Callie says, the salt is lazy. It needs to be introduced slowly to volatilize. Otherwiise you get a molten corrosive lava flow on the ground of the kiln. When I was at the Mary Anderson center in Indiana I used an antique fire extinguisher with the hot solution of soda ash to spray into the kiln. Ruthanne  used an orchard sprayer. both had brass nozzles. I have used orchard sprayers in the States but you need to get a brass nozzle so it doesn't melt.

At La Meridiana, Pietro Maddelena built a soft brick kiln with high alumina insulation bricks. It lasted a few years. Ruthanne Tudball fired the first load during a workshop there in 2000. 

You'll need silicon carbide shelves too. 

Marcia

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wow again more great info to explore. The meanley article was very good and thorough thanks sputty!

Marcia your post inspired me, I've been debating on what to use to spray the soda in. I have a  brass  compressed air venturi nozzle that I  used to fire with liquid biodiesel.  I think I'll fire with propane and use that nozzle to spray in the soda solution. I think that would really help distribute the soda through the kiln. It may plug off but should be easily cleaned  with water...

I don't know if the shelves I have are carbide or not but they are too big to fit into this small space. A half moon section is just slightly too wide to fit.  I think I'll cover the floor with the alumina beads and the pots can just sit on top of it...lets see what happens :) 

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(Um... Zirconium Silicate is not a liquid. It's a powder. )

I was thinking of something else (colloidal silica-which is liquid)I use it along with G milled Zircon on hot areas in reduction kilns(not salt or soda) although soda may be fine -salt will attract to silica products.

 

I use alumina and G milled Zircon (from Laguna )-or in the old days sprayed everything with ITC

High in  alumina coatings is what you want in a salt kiln.

Edited by Mark C.

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The mug in bottom of  salt kiln on lower right is porcelain-without much slip-its very plain as you can see.This is a fire from some years back-this kiln is hard nick soft brick and fiber.

 

 

salt pots.jpg

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One of the problems with soft bricks is that if any salt vapor gets past the face of the brick (between the cracks), it will eat away the bricks from there. The front face will look great, but there will be a massive hole behind it that you can't see, eventually breaking through. So you have to be very, very diligent about maintaining whatever sort of coating you're using. If you do that, there's no reason you can't get a good number of firings from your kiln.

Another question- where is the kiln located? If indoors, you need to have a really good powered vent hood, and even then it'll make for a lot of fumes in the kiln room. If outdoors, be prepared for questions/complaints from your neighbors. Salt/soda makes a lot of fumes. It's primarily water vapor, but it looks like smoke and could cause alarm.

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We spray our soft bricks almost every fire with the thinest coat possible .You will Kill your kiln doing this-just so you can plan on that and be aware of this -once a salt kiln always a salt kiln.

I myself like salt pots more than soda as they seem more dynamic surfaces. I like soda pots but they are more subtle surfaces  to me.I think the salt will eat your kiln sooner than soda as well.

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What if I spray on some iron oxide stain or dust some iron oxide powder onto the raw ware, would that give give the sodium something to play with?  I don't mind bland color ...these are just test pieces to see if I can get a good coating of glaze from the sodium.  

Could I do both Salt and Soda in the same fire?  Or would the salt  mask the soda effects?

The "kiln" is outdoors, its tiny. I'd like to get 10 firings out of it before upgrading.

photo 3.JPG

photo 1.JPG

photo 2.JPG

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41 minutes ago, cabako said:

What if I spray on some iron oxide stain or dust some iron oxide powder onto the raw ware, would that give give the sodium something to play with?  I don't mind bland color ...these are just test pieces to see if I can get a good coating of glaze from the sodium.  

Could I do both Salt and Soda in the same fire?  Or would the salt  mask the soda effects?

The "kiln" is outdoors, its tiny. I'd like to get 10 firings out of it before upgrading.

 

Spray what you like on. It'll all be a learning experience. There's a million and one slip recipes for use in salt and soda - you just have to try them all!

I'd stick with either soda or salt. Salt will certainly obliterate anything that soda might do. It's good to have a narrow focus rather than a scatter-gun approach, I think - you'll learn far more in the long run.

gallery_76831_1203_191817.jpg

This little salt-glazed jug had a slip with added iron spangles (iron dust). I rather liked the speckled effect the spangles gave.

Beware: salt/soda is more addictive than any other form of pottery, IMHO. It's probably worth getting one of the several good books available on salt or soda - loads of slip recipes, techniques and info to get your head around.

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If I had that kiln in photos I would salt it.

Porcelain is just to white -make up some colored slips-You can apply hen pots are wet or we apply on bisque ware a lot with zero issues.

consider fuming them as well. Nothing looks as good as fumed porcelain.You will need to read up on all the safety stuff involved with fuming.

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