Protecting a fiber board from flame & ash

[hairypotter]

Hello everyone, after reading through the site, I think I have an idea on what to do next, but would like your feedback. The short of it is, I'm building a pottery kiln, don't have enough bricks for a catenary arch so a flat top will do. My options are
1) high alumina kiln shelves (over $300 +shipping),
2. Ceramic fiber board, 2x3 ft, 2", K26 rating,
3. Two layers of 2" rigidized fiber blanket.

To the point, though rated for 2600deg, I'm not sure how the board will fare long term in a wood fired kiln, hence why I'd protect it with a thin coat of zircon based ceramic coating to protect against the ash and provide IR benefits. Also, thinking of doing this for the entire ware chamber for the IR heat gains.

I'd really appreciate any feedback regarding the fiber board. For those who have used this product as a hot face, how long has it lasted, what's worked and not worked, any feedback would be greatly appreciated. Thank you.

[martyn]

You should also consider your health when using ceramic fibre products!

[masonryrocketstove]

In addition to silica-based rigidizer, there is also now colloidal alumina rigidizer made from nano aluminum hydroxide that dries hard and cures at 300°C. That will have much more resistance to the sodium and potassium vapor of wood ash than colloidal silica.

The most important thing to consider with wood fuel and refractory deterioration is surface porosity. Coating an insulative surface with a product that is non-reactive to alkali in order to seal the insulative pores is the primary goal of making the refractory last in the presence of wood ash vapor.

[hairypotter]

If you don't mind a follow up; I'd imagine the alumina based coating would be less expensive than zircon based? If so, might you suggest such a product?

But, thinking about all this, in simple terms, this is just a high performance kiln wash, only it's protecting the fiber board vs kiln shelves. Hansen from digitalfire has a fantastic recipe for a zircon kiln wash, I might just have to try.

Finally, should the board be prepped in some way prior to applying the wash, sort of like how fiber blanket should be moistened prior to applying rigidizer? Hope this makes sense and thank you for your time and knowledge.

[masonryrocketstove]

Just to get a sense of the conditions your refractory will be exposed to, can I ask whether this fiber board will be lining the firebox, or will it be lining the ware chamber? Reason I ask: the firebox conditions will be a lot harsher than the walls of the ware chamber.

Also: what pyrometric cone are you pushing to reach? Cone 12 will need less refractoriness than cone 14, for example.

Apr 30, 2024 7:22:01 GMT -8 hairypotter said:

If you don't mind a follow up; I'd imagine the alumina based coating would be less expensive than zircon based? If so, might you suggest such a product?

Probably depends on where you’re located, but I’m seeing colloidal alumina for about $112 per gallon. CeraMaterials and Jyoti Zircoat are two brands of alumina rigidizer I’m aware of at the moment. I’m sure there are probably others as well.

But, thinking about all this, in simple terms, this is just a high performance kiln wash, only it's protecting the fiber board vs kiln shelves. Hansen from digitalfire has a fantastic recipe for a zircon kiln wash, I might just have to try.

…well, almost but not quite. A kiln wash is generally formulated to avoid permanent bonding to your kiln shelves.. that allows any glaze drips to flake off of the kiln shelf with some abrasion or chipping. The kiln wash basically acts as a non-adherent and sacrificial dust-coat. ..Kinda like how flour or cornmeal keeps a pizza from sticking to a pizza oven floor. Might be a poor analogy but hopefully you get the idea.

With a refractory coating, you definitely want the coating to adhere to the substrate.. preferably with something that is a mild glass-former, but is also resistant to the flame and alkali ash atmosphere (rare earth elements are pretty ideal for that, like a blend of 1.5% lanthanum oxide and 1.5% cerium oxide. It forms an adherent glassy phase that’s also alkali resistant.. and is also a little IR emissive to improve fuel efficiency.)

Tony Hansen is a great resource. Do you have a link to the kiln wash recipe you mentioned? We might be able to modify it so it binds to your refractory substrate a little better than a straight kiln wash will.

Finally, should the board be prepped in some way prior to applying the wash, sort of like how fiber blanket should be moistened prior to applying rigidizer? Hope this makes sense and thank you for your time and knowledge.

I would burn the board first: you’ll need to remove the polymer coating that’s used to make the board into board shape. That will then allow you to bond coatings to it. Better yet: buy 2600°F ceramic fiber rolls that don’t have any polymer rigidizer in them. (Simond Store makes a good 8lb density 2600°F fiber blanket, for example.)

Generally, 2600°F fiber blanket has a micron-thick coating of zirconia on each ceramic fiber.. that’s what boosts its temperature rating up to 2600.

Without that zirconia coating, ceramic fiber generally maxes out at 2400°F.

Pure alumina fiber is normally rated for 3000°F, but it’s really insanely expensive.

Pretty sure we can DIY a good coating for your needs.

[hairypotter]

Not gonna lie, I'm new to this forum and not sure to add links or even quote in my replies but thanks for your generosity. I've been doing pottery since 2005 and I'm following everything your saying. The board would be the roof of the ware chamber and I wouldn't even think of using a non-dense refractory as a hot face in the harsh conditions of a firebox. I have enough firebricks for a small train kiln, minus the ware chamber roof, hence where I'm at and why I'm here.

Hansen's link:https://digitalfire.com/picture/ivQa9qniRf
and recipe: insight-live.com/insight/share.php?z=K5hHzDs2Kr

Doing all this from my phone, so hope that worked. only thing I can think of that'd be different for me is that Tony applied that to a solid/dense material and I'd be applying it to a fiber board. Finally regarding cone temps, I'm shooting for 10 max. I don't mind hitting C8/9 and firing a bit longer for the heat work.

You mentioned K26 fiber blanket, but I don't want to deal with fiber blankets, not so much the health aspects, I can mitigate those issues when prepping the blankets, but rather, I don't know if the would sag across the chamber from repeated use. If rigidizing and coating a 2" K26 blanket is more economical and as effective & long lasting as the fiber board ($150 for 2", K26), I'd be open to hearing it, but I think the board will work just fine.

I hope this makes sense and thanks again for your time and knowledge. I think and feel like I'm really close to the finish line with this project. Fire bricks were not easy to get so I'm excited to be nearing completion. I'd also like to share how this goes with other on here in case it might help someone else.

[masonryrocketstove]

Apr 30, 2024 21:04:13 GMT -8 @hairypotter said:

only thing I can think of that'd be different for me is that Tony applied that to a solid/dense material and I'd be applying it to a fiber board.

Pretty sure that kiln wash would flake off the ceramic fiber without a bond-setting agent (and especially with the calcium chloride included) ..but I’ll circle back to that in a sec just in case you want to give it a shot making it into an adherent refractory coating.

You mentioned K26 fiber blanket, but I don't want to deal with fiber blankets, not so much the health aspects, I can mitigate those issues when prepping the blankets, but rather, I don't know if the would sag across the chamber from repeated use. If rigidizing and coating a 2" K26 blanket is more economical and as effective & long lasting as the fiber board ($150 for 2", K26), I'd be open to hearing it, but I think the board will work just fine.

Well, fiber board will sag, too, unless you use ceramic buttons to pin it to a roof superstructure. All ceramic fiber tends to shrink in use, and the middle will bow inward toward the ware chamber without fastners holding it up in the center.

The other thing about most ceramic fiber board is that it’s usually rigidized with phenolic formaldehyde resin.

Normally the boards are intended as a backup liner around things like ovens, autoclaves, boilers, etc. where there’s a decent chance of moisture.. that polymeric resin makes the board mildew resistant because it repels water. (And that resin’s hydrophobic property will prevent coatings from sticking to it until the resin is burnt out.) The ceramic fiber board’s temperature rating doesn’t cover the burnout of that polymer resin: it only indicates what temperature the ceramic fiber itself will withstand in the case of a catastrophic appliance failure and fire. Basically meant to keep a whole building from catching fire. Not meant to repeatedly withstand that temperature in normal use and hold its shape.

With ceramic fiber blanket, you’re basically getting the same thing, just without the phenolic formaldehyde resin. And since it’s flexible, you can accordion-fold it and hang it from a roof superstructure .. or you can form it with a slight upward arch and coat the inside with a strong inner liner, which is what I think I’d recommend for your situation. That’ll help prevent sagging even if the fiber shrinks.

At cone 10, you’re not going to be abusing the roof with a huge amount of heat, and so an arch shape (running the length of your train kiln ware chamber) lined with a superduty refractory mortar and patching compound will probably be all you need.

The best one I can recommend is Greenpatch 421 from Harbison Walker. It’s kyanite-based, 53% alumina, 3200°F rated mortar and refractory patching compound that sets hard, fires into a solid mullite-bonded coating, and has very good thermal shock resistance. They recommend applying it in 1/8” coats (you can build up a thicker layer, but they advise layering on one 1/8” coat at a time and firing each one on, successively building up to the thickness you want.)

It runs about $105-$130 per 55lb pail. Cheapest I’ve found in my location is $155 after shipping.

What I’d do is make a form in the shape of the arched roof, invert it on the ground and layer the ceramic fiber blankets into it. Then rigidize them, and after the rigidizer dries, coat the inner surface with Greenpatch. Once that’s dry, you should be able to flip the form over and set in place as the arched roof, and fire the coating in. Then maybe add a second or third layer and fire each of those in.

With that setup, you shouldn’t need another type of coating on top of it, and you can always add more of that patching compound later to fix any potential cracks, etc. over the life of the kiln. (The patching compound will adhere a whole lot better to ceramic fiber and to itself than something like a castable refractory or satanite coating will. And definitely a whole lot better than a kiln wash. Makes future repairs a heck of a lot easier)

This method is what a lot of knife makers use to make the inner lining on their ceramic fiber forges.

The other accordion fold method was mentioned in a post about roof construction a while back here:
donkey32.proboards.com/post/37710/thread

Pretty sure that post has links to some YouTube videos on accordion folding method, too

The pics shared look like they’re from Frederick Olsen’s The Kiln Book. You can find it as a PDF online many places. It’s also a great resource if you haven’t already read it. (It also has info on the ceramic button fastener method, but those hardware bits are pretty expensive by comparison, and they usually require some welding to the roof superstructure.)

Hansen's link:
digitalfire.com/picture/ivQa9qniRf
and recipe:
insight-live.com/insight/share.php?z=K5hHzDs2Kr

If you were to convert Tony’s kiln wash recipe to a refractory coating, I’d leave out the calcium chloride, and add that blend of 1.5% lanthanum oxide and 1.5% cerium oxide, then add 3-4% Veegum T (magnesium bentonite) to make the slurry sticky enough to adhere deep into to the ceramic fiber when it’s wet. You might need to add some colloidal silica rigidizer too, just to make it stick well enough. (If the coating doesn’t penetrate deeply enough into the fiber and adhere into the fiber structure, then weight and gravity will simply tear it off the surface-most layer of fiber and collapse it into your ware chamber.) . ..but.. I honestly think that zircon / calcined kaolin coating would probably be overkill for your ware chamber roof, and might make it harder to patch, add layers, or make repairs later.

I'm excited to be nearing completion. I'd also like to share how this goes with other on here in case it might help someone else.

I’d love to see your results. Potters building kilns and stove makers often have to solve many of the same problems ..and I wish we shared more of the same community spaces so we don’t each have to keep reinventing the wheel in our own little siloed fields. We could learn an awful lot from each other’s experiences. ..And if it weren’t for potters, there wouldn’t be tile stoves.

[masonryrocketstove]

Oh, and I forgot to mention about that DIY refractory coating modified from Tony Hansen’s recipe: you’d probably need to fire it to cone 13-14 to get it to bond well.. which probably also isn’t ideal for the interior of a cone 10 kiln.

[hairypotter]

Well damn...this is all still making sense! Thanks for the product FYI. You've answered/ addressed my questions and concerns thoroughly, one of them being the coating not adhering deeply and properly to the board. Would the green patch adhere effectively after burning off the resin? If so, that's an option. But, you also laid out the case for fiber blankets. And yeah, I would secure them with buttons to a roof structure. Rigidizing and applying the coating is simple work... Just an extra few steps compared to the board option. Damn, seems little fiber blankets will work just as well and be less expensive!

Question regarding adhesion of greenpatch to surface; minus the resin, how different is a rigidized blanket surface vs the resin free board surface? Again, thinking about adhesion of greenpatch to both surfaces. I'm leaning towards the blankets seeing as how adhesion could be an issue w/ the board and I already have enough blanket for at least one 2" layer. I've never rigidized blanket before, so not sure what that looks and feels like. I got some sodium silicate and I'm actually a believer in tinkering and learning, so I'll try this out next 24 hrs... should probably get a small board piece while I'm at it.

Learning more about fiber boards and actually changed my plan of using it as a hot face. Alumina rigidizer + greenpatch is just as doable and more cost effective... just getting a small quantity of the greenpatch though... large volume of it ain't cheap.

Wood firing potters are a friendly bunch once you're in their circle ( most folks are I guess), but they are/can be territorial at first. Understandably, they're usually firing months worth of work, trying to make a living, and can't afford to stray from the tried and true. I haven't seen/heard of many potters experimenting with kiln materials the way knife/metalsmiths & stove builders have. The use of insulating fire bricks, carbide shelves, and the new advancer shelves is as experimental as most potters get with respect to refractory materials...Excluding Hansen and other pottery techs in the pottery industry. I've actually learned more about refractory materials from my own research/ curiosity, and from non pottery folks. Because of Hansen, I've thought about buying a small test kiln just to test materials... Fascinating stuff! I also have a background in soil sci and materials testing (construction)... For sure this plays a role in my fascination w/ this stuff.

Sincerely, thank you for your time and generosity. You wouldn't believe how high and loud naysayers are regarding this topic in the wood firing community. Looking forward to finally getting some pieces fired in the coming months and hopefully inspiring other nonproduction potters to give it a go with a small wood kiln. I myself was inspired by a kid in suburbia Chicago and the rocket stove community. Thanks again.

[masonryrocketstove]

May 1, 2024 17:15:00 GMT -8 hairypotter said:

Would the green patch adhere effectively after burning off the resin?

Assuming you’re able to burn out all the resin completely, without leaving carbon char behind, coating some of the fibers, yes.

But.. The thicker the board is, the harder it will be heat it all the way through to the resin’s complete/clean combustion temp, though. ..because of all that insulating thickness from exterior face to the center. Not too hard to do with 1/2” board because the heat only has to penetrate 0.25” from each face.. pretty tough to do with 2” board, because the heat has to reach a full 1” depth from each face.

And yeah, I would secure them with buttons to a roof structure. Rigidizing and applying the coating is simple work... Just an extra few steps compared to the board option.

I guess I kinda didn’t clearly make this point earlier and I should have: You’d still need to re-rigidize the fiber board after resin burn-out, before applying the coating. Otherwise you’d risk it tearing out from the surface, same way as the blanket. ..And you’ll need to re-rigidize all those board fibers again after burnout anyway, just to keep them from going airborne and risking inhalation during the build and over the life of the kiln.

Damn, seems little fiber blankets will work just as well and be less expensive!

Would honestly be much easier too, not just less expensive. There’s no benefit (only extra steps, more fuel expenditure, more effort, and stinky mess from the burning formaldehyde resin) to buying fiber board for this purpose if you haven’t already got the board on hand. I’d only consider doing the resin burnout if I had the board already purchased. (I did in fact make this very mistake myself, years ago. lol)

Question regarding adhesion of greenpatch to surface; minus the resin, how different is a rigidized blanket surface vs the resin free board surface? Again, thinking about adhesion of greenpatch to both surfaces. I'm leaning towards the blankets seeing as how adhesion could be an issue w/ the board and I already have enough blanket for at least one 2" layer.

Depends on the quality of the blanket. Cheap blanket that falls apart as you try to unroll and work it into shape would be about like fiber board after burnout. With good quality blanket that has a sort of woven or “needled” structure (like the Simond Store fiber rolls I mentioned earlier do) you’ll get a stronger bond between coating and blanket. The weave of the structure will inherently hold together a little better while wet, and be stronger after the rigidizer dries and cures.

I've never rigidized blanket before, so not sure what that looks and feels like. I got some sodium silicate and I'm actually a believer in tinkering and learning, so I'll try this out next 24 hrs... should probably get a small board piece while I'm at it.

For the best structural strength, it’s recommended to soak the fiber completely, all the way through.

Before rigidizing, the fibers are basically a loose clump of agglomerated strands, where each fiber can kinda slide on the fibers next to it and potentially allow tear-out or airborne dusting. The dried/cured rigidizer will cement the microscopic junctions where each fiber touches the fibers next to it, making them into a single, porous, locked-together matrix.. (kinda like how the inside of a hollow bird bone looks, but on micron scale.)

I’d definitely recommend soaking it all the way through for at least the innermost blanket layer that will have the Greenpatch put on it later. For the other (outermost) blanket layers, you could probably get away with only rigidizing to keep the fibers from going airborne. That only requires the rigidizer to penetrate to 3mm depth from each face and every edge. Not as structurally strong, but those outer layers don’t need to be if you’re using ceramic buttons to pin the whole thing to the roof superstructure. And only soaking to 3mm depth will use less rigidizer, so it’s slightly less cost.

Alumina rigidizer + greenpatch is just as doable and more cost effective... just getting a small quantity of the greenpatch though... large volume of it ain't cheap.

If you’re using a 1/4” - 3/8” or more layer of Greenpatch for the interior hotface coating, you really don’t need to buy the more expensive colloidal alumina rigidizer. Alumina is more refractory and more alkali resistant, but also more costly per square ft rigidized. Colloidal silica is a little cheaper and will work perfectly well for the outermost layers where there isn’t any vaporized ash potassium / sodium penetration through the Greenpatch.. ..If you were building a cone 14 kiln, or were using a super thin zircon coating on the fiber, that’s when I’d recommend the colloidal alumina to resist potential future infiltration by the ash alkali vapor. (Like if the zircon coating were to crack or flake.) ..but with Greenpatch, you can always slap on a new coat of it over the top of any future cracks or chips, so you don’t have to worry about ash vapor ingress.



Always glad to help when I can. Sorry it took me a few days to get the reply and respond back. Hopefully I’m not too late to help you economize the build.

Please do keep me posted on how this is going for you — I’m very interested in seeing how it comes together, and how the setup works for your needs, what adaptations you’ll be likely to make as you break the kiln in and make it fire the way you want it to.

Maybe shoot me a DM with some of your pottery work? I learned a lot from an olllllld school potter who was in the Michael Cardew Pioneer Pottery camp of thought. (That’s another great book for the from-scratch DIYer, too. It’s got a bunch of ideas for building all the equipment for a studio from the ground up in the literal middle of nowhere, and goes into depth with the geology of clays and glazes for sourcing your own raw materials from the local landscape. Might be right up your alley with your background in soil science.)

and, who knows.. maybe with the crossover between pottery and tile stoves, and if there’s enough interest from other forum members, we could start a new thread somewhere or request a sub-category for heat/thermal-shock-resistant pottery, clays, and glazes, like is needed for making stove-tiles. Maybe also for stuff like flameware pottery used on stovetops and cooking directly over fire. It’s very hard to find good info on those spodumene / lithium aluminum silicate clay body recipes that have near-zero thermal expansion or contraction.

Welcome to the stove community here, eager to see your progress and process!