kiln wash and woodfiring clay kiln

[yinjuehua]

I'm planning to make a woodfiring kiln out of clay. I've always had a question:

Why can't we apply kiln wash to the interior walls of the kiln to protect them, especially when using wood firing? It should help prevent the accumulation of flux from the wood ash from damaging the kiln itself, right?

Also, why is kiln wash only applied to one side of the kiln shelf? I saw a statement that keeping one side without kiln wash helps maintain a flat, stable surface, but I think applying it to both sides would be more stable. This way, the expansion and contraction rates would be the same on both sides.

I am thinking about using refractory satanite as a protective coating on the interior surfaces of the kiln...

[Kelly in AK]

Kiln wash has a tendency to flake off, better put, kiln wash WILL flake off sooner or later. In an atmospheric kiln there’s lots of airflow so loose bits of wash on shelves can easily blow into pots and ruin them. Add fluxes flying through the kiln (ash, sodium) and it’s more complicated. Every firing changes the coefficient of expansion of the surface, a little flux gets absorbed, a little more heat work gets done, the coating shrinks a tiny bit more than it did before.  Placed on walls, roofs, or the underside of shelves is a guarantee of having flakes of kiln wash stuck to pots. Coating walls is less problematic, but still carries some risk. In that case a coating that’s formulated to stick very well is necessary and that’s no mean feat. Hard brick wood, soda, and salt kilns are often coated with a spray of shino glaze which creates a protective barrier for the bricks. 

[Bill Kielb]

3 hours ago, yinjuehua said:

I am thinking about using refractory satanite as a protective coating on the interior surfaces of the kiln...

I cannot speak to satanite as protective or whether it will last and not drop off on the wares, my understanding is it is grout or grout consistence. I did have the opportunity to try a commercial refractory coating by ITC (ITC 100) and it seemed to perform well and stay put. How much did it extend the life and thermally what did it save? I don’t know -  only anecdotal results here. My best thought after 50 + firings in three different kilns, It seemed to help and not hurt in soda firings and stayed put on the soft brick. 
It is a commercial product intended for large kilns (Not necessarily ceramics) and rated to 5000f I believe. Not sure what Satanite is rated, I think I would test before committing to large scale coating.

[yinjuehua]

2 hours ago, Kelly in AK said:

Kiln wash has a tendency to flake off, better put, kiln wash WILL flake off sooner or later. In an atmospheric kiln there’s lots of airflow so loose bits of wash on shelves can easily blow into pots and ruin them. Add fluxes flying through the kiln (ash, sodium) and it’s more complicated. Every firing changes the coefficient of expansion of the surface, a little flux gets absorbed, a little more heat work gets done, the coating shrinks a tiny bit more than it did before.  Placed on walls, roofs, or the underside of shelves is a guarantee of having flakes of kiln wash stuck to pots. Coating walls is less problematic, but still carries some risk. In that case a coating that’s formulated to stick very well is necessary and that’s no mean feat. Hard brick wood, soda, and salt kilns are often coated with a spray of shino glaze which creates a protective barrier for the bricks. 

This is so helpful, thank you!

I'll consider applying a layer of Shino glaze in my kiln. I'm not too worried about the yield since I already have an electric kiln. But the electric kiln is too stable (boring), I want to experiment with various effects (wood firing, reduction, soda firing, etc.).

[yinjuehua]

2 hours ago, Bill Kielb said:

I cannot speak to satanite as protective or whether it will last and not drop off on the wares, my understanding is it is grout or grout consistence. I did have the opportunity to try a commercial refractory coating by ITC (ITC 100) and it seemed to perform well and stay put. How much did it extend the life and thermally what did it save? I don’t know -  only anecdotal results here. My best thought after 50 + firings in three different kilns, It seemed to help and not hurt in soda firings and stayed put on the soft brick. 
It is a commercial product intended for large kilns (Not necessarily ceramics) and rated to 5000f I believe. Not sure what Satanite is rated, I think I would test before committing to large scale coating.

Thanks！ I know satanite is a commonly used coating material for furnaces and can withstand approximately 3500°F. 

I google ITC 100, and it is also a common coating material for furnaces, and it seems more stable and professional. However, it is a bit expensive for me, costing $72 per pint. I will look into it further and might end up using ITC 100 to be safe. 

[PeterH]

1 hour ago, yinjuehua said:

I will look into it further and might end up using ITC 100 to be safe. 

You may be interested in this thread & its references

 

[Russ]

I coat the ENTIRE inside of my kiln with the zircon/glomax/cmc gum wash. AND both sides of my shelves and stilts. I have not had ONE flake of this kiln wash come off period.  Flyash will stick to it a bit but it protects the brick and shelves.

 

[Kelly in AK]

@Russ, Oh damn! You just blew my mind.

I scrape my shelves every firing not because I’m fastidious or a paragon of ceramic virtues. I have to. Soda makes a foamy glass on them, it’s soft, scrapes off easily enough. Perhaps you’re familiar with this. If I skip that step only once it’ll build up enough to drip and ruin something. The thing is, it doesn’t form on the kiln washed areas, only the sides and bottoms. The other thing is…I can’t imagine putting kiln wash on the bottom of a shelf. That’s crazy talk. And yet, here you are. Picture to prove it. 

I know fly ash isn’t the same as injecting soda into a kiln, and you’re firing to cone 10, and a significant part of my kiln is soft brick (Now that’s crazy), so there are some differences. Similarities too though. Corrosive atmosphere, unfriendly to most everything but the pots  

So, do you ever have to scrape these shelves? Are these silicon carbide? Do you have to re-coat things? How often? I’m in quite a state, imagining not having to scrape my shelves every d@#& firing. 

[Russ]

Most of my shelves are pressed alumina. The zircon wash protects then from the corrosive effects of the ash. I also have the nitrite bonded sio2 in the very hottest part of the kiln (because I've "melted" the pressed alumina ) and yes they are kiln washed also. If mixed and applied correctly this wont flake like the alumina/kaolin stuff. I recoat every glaze firing. 

I would imagine with soda you'll have to scrape or grind the wads off that stick depending on how much soda you use. I dont think there's any way of avoiding that.