Why FireBricks / Refractories Fail: Silica+Flux, Not Heat

[martyn]

Vermiculite is very absorbent and just sucks in water glass, you would think it would be a good key for Zircon paint?
I have had lots of dealings with Vitcas over the years, there staff are normally polite and helpful but I am not sure about their practical advice or experience?
You would think they would be able to give a better reply than a copout!

[Forsythe]

Nov 3, 2022 11:23:27 GMT -8 foxtatic said:

Regarding Zircon application to vermiculite board: I am looking to build my next core out of it. After reading this excellent thread, I contacted Vitcas to ask if Zircon coating could be applied to vermiculite board. The response was “We wouldn't advice this as we haven't tested the application ourselves.”

Clearly covering their butts, so I’m wondering if anyone has any direct experience in trying this application?

Personally, I would not build an entire core out of vermiculite coated in zircon. The vermiculite board itself is not rated for those types of temperatures, abrasion, and condensation... so you'd basically be relying on the less-than-3mm-thick zirconia coating to maintain the whole of the firebox's structural integrity. Might be okay for a temporary experimental build but probably would not make a good long-term installation.

For safety and stove-build longevity, I personally would advise selecting only materials that can hold up to firebox conditions on their own, and then use the zirconia coating to A) greatly extend their service life, and B) improve combustion efficiency.

For the afterburner or riser, there's more wiggle-room for using things like vermiculite or ceramic fiber (provided that you do coat them in a durable refractory coating) since there is less ash exposure in the secondary combustion zone [with a fixed-bed burning firebox like a rocketstove batchbox] and none of the fuelwood abrasion. (although there will be flame impingement and high gas velocity around the port / throat area.)

Also, I built a DSR2 core entirely out of very soft CFB. Not knowing any better at the time, I sprayed all the pieces down with sodium silicate to contain the fibers and firm up the edges. Otherwise my test pieces virtually disintegrated under flame. So if I understand correctly from this info, sodium silicate alone is not great and will cause fluxing and eventually weakening of the structure.

If you used sodium silicate and then didn't give it a slag-resistant top-coat, then yes, the sodium from your sodium silicate will add up along with the fluxing alkalis depositing from the wood ash, and the combined accumulation of those sodium/potassium/calcium deposits will eventually destroy the ceramic fiber.

The key thing to note with the effect of slags like sodium, potassium, calcium, iron, etc. is that they are cumulative. A tiny amount which doesn't increase over time will only have a negligably tiny effect. A large amount —or an amount which is allowed to gradually increase over time— will have a large and ultimately detrimental effect.

I see earlier in this thread where you recommend applying colloidal silica, then a refractory coating.

Yes, that recommendation is based on which zirconia coating you purchase. The better-quality coatings should be applied in THIN layers. 1mm is the target, ideal thickness. Glopping it on in layers more than 3mm thick can actually make it crack away from the substrate... so you don't want to lay it on thick. Because of this, if the product is not specifically labeled as a ceramic fiber rigidizer (being already thinned to a watery consistency and/or you don't have a pressurized sprayer to drive a thin layer of the zirconia coating into and throughout the fiber structure to bind the fibers together) then your best result will likely be from rigidizing first with colloidal silica, then applying the zirconia coating in a thin surface layer over the top.

I have only fired the core maybe 8 to 10 times. Do you believe I will eventually see issues develop,

If left uncoated and exposed to ash and flame impingement — Yes.
The causes of deterioration will accumulate over time. Exactly how much time is a guessing game, hinging on the type of wood you burn (both the tree species and whether you burn split cordwood or small diameter sticks each surrounded by their own bark layer,)  the large variations in wood ash composition, the operating temp of the stove, how each stove design's shape accumulates / funnels away / or blows around and distributes the residual ash, etc.

and should I apply a refractory coating now before much further use?

Yes.

In fact, I'd be interested to find out how that all-CFB-firebox fares with a zirconia topcoat. It would be valuable to know how well a zircon layer might contribute to resisting abrasion from wood fuel loading.
Apart from that finding, the zircon would definitely help preserve the secondary combustion chamber, regardless.

If so, would that be to the entire core (inside and out $$$!) or just to the hot surfaces in the firebox and afterburner?

Only the inside. Interior hot-face surfaces. You would see no appreciable benefit in coating the exterior, and it would be a waste of the product and your money.

[foxtatic]

Thanks for the detailed reply. I examined my ceramic fiber DSR2 core yesterday and do indeed see signs of degradation. How much of that comes from poor quality material vs the reactions with sodium silicate, I can’t be sure. Perhaps this is a decent place to warn against the CFB from Skyline Components. Here is how it reacts to flame untreated: www.youtube.com/shorts/lwmUGAVHQtk



I will plan on painting the hot faces with zircon and see where that gets me. Hopefully I’m not just “polishing a turd.” I’m pretty good at following up on results, I’m just not entirely sure when I’ll get to doing that project. 


This next build I mentioned with the vermiculite board DSR3 will be different. I plan on only using the board as the outer housing. The firebox will be lined with fire brick. The floor layer, shelf, and top layer will be cordierite. The top box walls will be lined with ½ inch zircon treated CFB (floor and ceiling are the cordierite previously mentioned.) Zero fuel abrasion or flame/gas contact with the vermiculite board itself, it’s just there for insulation and structure. Granted, it will be exposed on the outer surfaces to the gas within the bell. I welcome your feedback on that plan, as well as that of martyn, the king of vermiculite cores 😊. Overall structure roughly 14"wide, 23" tall, 19" deep. 

[martyn]

I will get around to buying some zircon, I see you can but it through Amazon Prime £37.
I would expect it to adhere to vermiculite board but it would be nice to have some conformation before I buy it.
My firehouse stove core is built from ceramic fibre treated with water glass, it is around 5 years old and in very poor shape!
It still works but is very brittle and dry, just touching it makes lots of dust!
I would like to replace it with treated vermiculite next time around.

[Forsythe]

Nov 4, 2022 4:30:22 GMT -8 foxtatic said:

This next build I mentioned with the vermiculite board DSR3 will be different. I plan on only using the board as the outer housing. The firebox will be lined with fire brick. The floor layer, shelf, and top layer will be cordierite. The top box walls will be lined with ½ inch zircon treated CFB (floor and ceiling are the cordierite previously mentioned.) Zero fuel abrasion or flame/gas contact with the vermiculite board itself, it’s just there for insulation and structure.

Okay, so this is a different use than what we had discussed. What you're describing here is using the vermiculite as a backup lining. That's very, very different from "building a core out of vermiculite." What we're now discussing is "building a core out of firebrick and cordierite, insulated with a vermiculite backup liner."

In this new description, the firebrick and cordierite will be your hot-face and THEY should be coated with zircon.

(cordierite itself normally cracks in use as the hot-face, and I've only seen it used successfully with the dual ITC coating of ITC 100 and ITC 296A. Might work with the Vitcas Zircon, but you'll be relying on that coating to maintain the firebox structural integrity.)

These zirconia coatings re-radiate heat back inward toward the burn chamber, so if you're coating a surface, but then putting another material in front of that surface you may end up subjecting that inner-most material to more heat than it would have initially been tasked to withstand.

When I say "coat the hot-face" I mean coat the interior-most surface facing the fire. That's what the hot-face is. If you're putting another material between the coated surface and the fire, then the coated surface is no longer the hot-face — that other material facing the fire is your hot-face.

Granted, it will be exposed on the outer surfaces to the gas within the bell.

The outer surfaces of the backup liner within the bell are of no concern because the temperature will be below the melting point of any negligible amount of fly ash that might be present, and the amount of ash exposure itself in that area is so low as to be virtually nil, regardless.

Think of the zircon more as a refractory coating and hot-face liner — and not as a general sealant for things like ceramic fiber and vermiculite. Its proper placement is directly facing and in direct, unobstructed exposure to the burn chambers.

The hot face of the combustion zones is where you need to be concerned about the combination of high heat + ash. Not in backup liners already shielded by firebrick, and not downstream in the cooler gasses in a bell.

[Forsythe]

Nov 4, 2022 4:59:27 GMT -8 martyn said:

I will get around to buying some zircon, I see you can but it through Amazon Prime £37.
I would expect it to adhere to vermiculite board but it would be nice to have some conformation before I buy it.
My firehouse stove core is built from ceramic fibre treated with water glass, it is around 5 years old and in very poor shape!
It still works but is very brittle and dry, just touching it makes lots of dust!
I would like to replace it with treated vermiculite next time around.

I can't see any reason why the Vitcas Zircon wouldn't adhere to vermiculite. Vermiculite is literally just silica, alumina, and magnesia. Provided that your vermiculite isn't coated in some kind of hydrophobic polymeric resin (like many ceramic fiber boards are) there's no reason for the zircon coating to have any issues adhering with a good, strong bond.

[martyn]

I have never been able to find out exactly how vermiculite board is made, I have been led to believe it is made from powdered vermiculite, water glass and fire clay but only because that is what other folk have told me?
It definitely seems to have a powdered clay but I dont actually know? 
When super heated the surface becomes hard and sintered, you can almost make it ring if you tap it, so I think that is the clay aspect? 

[Forsythe]

Vermiculite is normally mined, and then heat-treated to evolve the absorbed water out of it, and puff the particles into expanded, insulative form, much the same way as perlite:
www.vitcas.com/prd/cty/vermiculite-board-cement .

Vitcas uses a hydrophobic polymeric coating to make their boards into board-shape, but they can be formed by hot-pressing and firing to sinter the particles together.

Vermiculite itself is a clay-like mineral... so firing it hot enough will vitrify or sinter it into uniform shapes without a secondary binder needed.

[martyn]

There seems to be several different materials that make up the board, the dust from cutting is two parts in the form of soft vermiculite and dense powder.
It could just be two forms of vermiculite but it is quite soft before firing, you can dig your finger nail into the surface but much harder once heated.
I would love to a ‘how it is made video’…

[Forsythe]

Its certainly possible. Vermiculite board is far less common in the US, so there's very little info here on its application and usage characteristics. Doesn't even appear in common stove or fireplace installation guidelines.

Kinda weird how different regions treat different substances as "common-use" construction materials...

[martyn]

Yes indeed especially as virtually all the modern metal box stoves in the UK and France are lined with vermiculite!
The reason my local store sells the vermiculite is for replacement stove inserts although it is also used as fire block on walls behind stoves or to build fire place surrounds that will be tiled.

[Forsythe]

Here in the US, vermiculite is normally only encountered as a chemical absorbent. Fire crews normally have some on board fire engines in loose form for pouring over oil, gasoline, and coolant spills at vehicle crash sites.

Vermiculite is in that same general family of expanding / swelling clays with bentonite and smectite that are capable of up-taking liquids in volumes many dozens of times their own particle size/weight.

I think the tendency for vermiculite to absorb and expand is why it's not used much (or at all) here in the US for refractory products. Condensation could really mess up the vermiculite product depending on how it's used...and if it happened to absorb something like leaked liquid fuel or accumulated creosote, it could be really, really bad news...so there's apprehension about using it for fireproofing.

[foxtatic]

Nov 4, 2022 16:01:20 GMT -8 Forsythe said:

Okay, so this is a different use than what we had discussed. What you're describing here is using the vermiculite as a backup lining. That's very, very different from "building a core out of vermiculite." What we're now discussing is "building a core out of firebrick and cordierite, insulated with a vermiculite backup liner."

In this new description, the firebrick and cordierite will be your hot-face and THEY should be coated with zircon.

(cordierite itself normally cracks in use as the hot-face, and I've only seen it used successfully with the dual ITC coating of ITC 100 and ITC 296A. Might work with the Vitcas Zircon, but you'll be relying on that coating to maintain the firebox structural integrity.)

To clarify, I referred to it as a core "made of vermiculite board" because it's both outer insulation AND the structural box which contains the dry stacked refractory materials. At one point I wanted to do just vermiculite board (and hence asked Vitcas about painting Zircon directly on it to get away with such a scheme) so I likely mis-spoke as my design was still developing and changing in my head. Definitely settled on hot-face zircon coated CFB in the afterburner area now. 

I'm glad this came up though because of your comments on cordierite. I had never heard of it cracking. In fact, I was very impressed reading about it's thermal shock resistance (cherry red to quenching in water) and 2350 F temperature rating (per the manufacturer of the kiln shelf I'm looking at). I was emulating Peters use of it in his DSR3 core development. That was the one component I was not worried about! Can you tell me more about the conditions under which it cracks? 

[Forsythe]

Nov 7, 2022 6:50:14 GMT -8 foxtatic said:

Nov 4, 2022 16:01:20 GMT -8 Forsythe said:

Okay, so this is a different use than what we had discussed. What you're describing here is using the vermiculite as a backup lining. That's very, very different from "building a core out of vermiculite." What we're now discussing is "building a core out of firebrick and cordierite, insulated with a vermiculite backup liner."

In this new description, the firebrick and cordierite will be your hot-face and THEY should be coated with zircon.

(cordierite itself normally cracks in use as the hot-face, and I've only seen it used successfully with the dual ITC coating of ITC 100 and ITC 296A. Might work with the Vitcas Zircon, but you'll be relying on that coating to maintain the firebox structural integrity.)

To clarify, I referred to it as a core "made of vermiculite board" because it's both outer insulation AND the structural box which contains the dry stacked refractory materials. At one point I wanted to do just vermiculite board (and hence asked Vitcas about painting Zircon directly on it to get away with such a scheme) so I likely mis-spoke as my design was still developing and changing in my head. Definitely settled on hot-face zircon coated CFB in the afterburner area now. 

I'm glad this came up though because of your comments on cordierite. I had never heard of it cracking. In fact, I was very impressed reading about it's thermal shock resistance (cherry red to quenching in water) and 2350 F temperature rating (per the manufacturer of the kiln shelf I'm looking at). I was emulating Peters use of it in his DSR3 core development. That was the one component I was not worried about! Can you tell me more about the conditions under which it cracks? 

From another post on kiln shelves:

Oct 25, 2022 15:49:43 GMT -8 Forsythe said:

Oops I forgot too address the second part of your question:

Oct 24, 2022 11:36:16 GMT -8 chrisholtslag said:

A local ceramic shop sells the kiln shelves but they are not as thick, does it matter for the performance of the DSR2 if this middle plate is not as thick?

As a general rule of thumb: thinner refractory ceramics will be more susceptible to thermal shock cracking than thicker ones. They don't distribute heat as evenly across their breadth, and there's less material between the faces to take advantage of microcrack deflection around grain boundaries.

Kiln shelves typically come in one of three formulations: Mullite, Cordierite-Mullite, or nitride ceramic.

Nitride shelves are hellaciously expensive, and they may slowly burn out / devitrify the nitriding compounds in a rocketstove use. They also don't handle any level of moisture (read: condensation) well at all without cracking.

Cordierite is formed at relatively low ceramic temps in an extremely narrow firing range, and its crystal structure can decompose between 1200-1375ºC (depending on the precursor materials from which it was originally formed) ...so even though cordierite has practically zero thermal expansion across its entire [crystallographically-stable] temperature range (under 1300ishºC, usually,) Cordierite-Mullite shelves aren't rated for (nor capable of) holding up to the temperatures we see in rocketstove batch boxes. (‡)exception, see below)

Mullite kiln shelves may work in some areas where the heat load is distributed evenly across the whole face of the shelf, but I would be hesitant to use one by itself as a DSR roof. I would certainly advise against cutting holes in them for port slots.



(‡) I *have* successfully used 0.6in.-thick cordierite pizza baking stones in some firebox and afterburner/secondary burn chambers without cracking, but only when those cordierite slabs were dual-coated; first with ITC-100 and then the superinsulative ITC-296A top-coat over the ITC-100 layer, (which is about $80-$100 USD total for both pint containers of those coatings...granted that I only used a very tiny fraction of each pint's contents.)

2350F is 1260ºC, which is probably the upper end of its stability range. The thing about cordierite is that its very stable within its entire range — but the cordierite crystalline phase quickly decomposes to mullite and spinel crystal species when its usage range is exceeded, (if it goes much above 1325ishºC,) and that's when cracking can occur. (this also varies slightly by the staring materials' composition)

I'm not 100% sure what the max temp is in a DSR3, but trev has mentioned his Vortex afterburner has gone above 1400ºC, and he's experienced kiln shelf cracking.in his stove as well.

[Vortex]

Nov 7, 2022 10:34:55 GMT -8 Forsythe said:

I'm not 100% sure what the max temp is in a DSR3, but trev has mentioned his Vortex afterburner has gone above 1400ºC, and he's experienced kiln shelf cracking.in his stove as well.

1286ºC was the hottest afterburner temp IIRC, and that was an exception as it was in a force 12 gale, my previous hottest was 986ºC. My Vortex stove is normally a slow and steady burn though, and only ever one load.

I did have a kiln shelf in the afterburner crack on the first firing. I have no idea what type of material they are though as they were just sold as 'kiln shelves'.