Disintegration of Superwool ceramic fibre board

[fiedia]

Thanks Forsythe for all these accurate data. Very useful.

[Orange]

great info Forsythe - so the best would be to simply avoid ceramic fibre and insulative firebrick.

I didn't have much knowledge so I just coated CFB with waterglass and then wild clay paint:

i.postimg.cc/NFFh6ZMm/DSC-0282.jpg
donkey32.proboards.com/post/36568/thread

So that is not a good solution - I'll repaint it with fumed silica or just throw it out. (coating with aluminum powder to me sounds like a recipe for alzheimer)

And I have a question - what do you think about air-concrete (Multipor) for insulation after refractory bricks, it's cheaper and more available than calcium silicate boards that are normally used:

i.postimg.cc/V6F0HGnq/rmh-6.jpg

Or perlite:

i.postimg.cc/WbYgvHKS/rmh-8.jpg

[Forsythe]

Apr 2, 2022 5:28:04 GMT -8 Orange said:

- so the best would be to simply avoid ceramic fibre and insulative firebrick.

I really, really hope that isn't seen as the takeaway, here. The riser must be insulated in order to combust completely and cleanly — and the most consistently reliable and durable refractory materials for insulation at these temperatures and in the conditions of these combustion locations are ceramic fiber board/blanket and insulating firebrick.

The point here is that we just need to handle them responsibly and employ them safely in order to utilize them properly.

The point is not that they are "bad."

Dangerous when misused or handled carelessly? Sure. Of course. So are kitchen knives. But concluding that we shouldn't have kitchen knives where there are people who can get accidentally cut or intentionally stabbed by those kitchen knives is black-and-white "good/bad" thinking, which doesn't ever actually solve any real-world problems. It in fact creates a whole bunch of new, larger problems. Like the issue of food prep and eating. Which is... ...important. And it often requires kitchen knives. ...Which in turn requires that kitchen knives be in your home — you just need to know how to handle the kitchen knives safely, and store and employ them appropriately.

So it is with these refractories.

As one quote I recently saw stated:

I didn't have much knowledge so I just coated CFB with waterglass and then wild clay paint [...]So that is not a good solution - I'll repaint it with fumed silica[...] or just throw it out.

I guess I did a poor job of explaining the purpose of the fumed silica in this thread. ...I think the confusion was introduced when we strayed from the topic of rigidizing ceramic fiber products to mitigate the inhalation risk of decomposing fibers and swerved into the topic of improving refractoriness and slag resistance of all types of insulating refractories to prevent them from decomposing.

Just to re-re-re-state this again: these are two separate problems:

1) [protecting YOU from decomposing fiber inhalation risks]
— vs —
2) [protecting THE REFRACTORY from slag-induced decomposition.]

...and even though these two problems are indeed related, they need to be thought-of and addressed separately — in order to adequately mitigate each of them appropriately. That was one of the motivating reasons for starting the other thread: "Why FireBricks / Refractories Fail: Silica+Flux, Not Heat."

I'll go back and edit my post in this thread for clarity, but in the meantime, some of this discussion spilled over into the discussion of using *just* fumed silica, alone and nothing else (***which is not advisable***) on Insulating firebrick — in that thread, linked here: donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux

Mar 28, 2022 14:05:16 GMT -8 Forsythe said:

To further clarify the purpose of fumed silica [as applied to ceramic fiber board and ceramic fiber wool] — and the role it can potentially play in sealing highly porous insulating firebrick:

With ceramic fiber board and ceramic fiber wool, it's important to prevent the ceramic fibers from becoming an airborne inhalation risk, and the fumed silica — once fired hot enough to soften the silica and make it bond with the ceramic fibers — does an excellent job of this.

Fumed Silica also "rigidizes" the ceramic fiber into a strong [read: structurally-stable] substrate, such that a slag-resistant refractory coating can *then* be applied to it — without that hard-coating causing sagging in an otherwise loose-fiber substrate, and/or tearing away ...taking chunks of the ceramic fiber with it... which would expose the underlying fibers to decomposition and potential airborne contamination, anew.

If your insulating firebrick is extremely soft and extremely porous (refer to the scratch-it-with-a-thumbnail-test mentioned above) , it will likely accumulate a great deal of ash in the pores — *AND* it will lack structural stability under heat... and that structural stability decreases with the influx of ash/slag.

In that instance, giving extremely soft firebrick a hard, fumed-silica surface upon which you can *then* apply a refractory hard-coating may offer the best results in terms of 1) sealing the surface from ash/flux penetration, 2) applying a slag-resistant IR reflective coating and 3) having a structurally-strong bond between the two for the best durability through thermal cycling.


fiedia 's question does raise the point that fumed silica *itself* is not slag-resistant, so if you're trying to accomplish both goals A and B from the original post (that is: to prevent ash accumulation in the pores of your refractory, and also provide a slag-resistant "hot-face" coating on lower-alumina, soft, easily-eroded refractory components) Then the recommendation is:

1) coat the ceramic fiber (or brick) with fumed silica,
then
2) fire it very hot to bond the silica to the ceramic refractory
then
3) apply a high-IR-reflectant, slag resistant refractory coating to the now-hardened silica surface,
and finally,
4) fire it very hot again to set the refractory coating (by vitrifying the kaolin and silica in the refractory coating.)

[...]

I will coat with just fumed silica.

My fear with JUST fumed silica on a low-alumina and highly porous brick is that it may cause some melting. At least throw some clay into the fumed silica solution before you spray it on... like... even if you can't get your hands on kaolin or true fireclay... at least use some of the same clay you use for clay mortar between the bricks when laying them. ...Dig it out of the local earth if you need to, and levigate it with the process outlined here: (how to levigate your local clay for purity and plasticity.) donkey32.proboards.com/post/36645

[...]

There is a point which is still unclear for me :

The cheapest route is to use fumed silica (AKA "Colloidal silica") and be sure to fire it good and hot as soon as it's dry.

What I meant there was that fumed silica was the cheapest route to 1) rigidize the ceramic fiber and mitigate the fiber inhalation risk, [much cheaper than paying for a name-brand ceramic fiber “rigidizer” from a refractory company] and 2) to create a substrate layer for a slag-resistant refractory coating — and I recommended following that with Infrared-reflectant, high-temp refractory coating over the top to boost performance and longevity, even if that coating is just simply fireclay. If you *are* just going to use plain, ordinary clay instead of the IR-reflectant, slag-resistant coating, I would add it to the fumed silica and spray them on, together, at the same time. Particularly if your refractory brick is very soft, light, and porous.

(coating with aluminum powder to me sounds like a recipe for alzheimer)

While handling aluminum powder is dangerous —no doubt, and not to mince words— the risk is an explosively exothermic oxidative flash burn which is extraordinarily hot and is nigh impossible to extinguish once established... It's not really possible to ingest the powder into your system if you're wearing a respirator while working with ball milled materials.... which you should be wearing a respirator when handling ball-milled materials. And even if you did inhale some accidentally, it would still be "biosoluble alumina" —aluminum oxide— by the time your immune system responded to the wetted/reacted particle(s). ... FWIW, I think the aluminum toxicity risk is much more of an issue from consuming the acid-reacted aluminum -citrate -sulfate -carbonate -chlorate -fluorate etc. byproducts of eating acidic and salty foods cooked under heat in aluminum pots/pans and stored in aluminum foil or beverage cans...which then has to proceed through more metabolic pathways in your bloodstream before getting filtered out (or deposited in white-matter plaques in a dementia patient's brain for still-frustratingly-unknown-and-poorly-predictable reasons, which may be caused more by genetic protein-coding mutations resulting in prions that bioaccumulate dietary aluminum — than it is caused by the exposure to the dietary aluminum, itself.) ...For example, canned beer's expiration date is tied to the aluminum leaching over time from storage in the can via dissolution by carbonic acid, not from the spoilage of the beer or UV degradation of the hops.

But I digress.

And I have a question - what do you think about air-concrete (Multipor) for insulation after refractory bricks, it's cheaper and more available than calcium silicate boards that are normally used

I searched for that product but couldn't find what you were referring to... If it's like air-crete, and uses lime mortar to bind the cement together, then I would advise against it.

Lime-based mortars and concretes have practically zero flexural/tensile strength; the re-hydrated calcium decomposes under relatively low heat, and the bond is not very chemically stable, either, in the presence of water (condensation) and mineral salts (ash, etc.) The lack of flexural strength would probably be the first thing to cause crumbling, though, since the whole heater needs to be able to expand and contract with thermal cycling... and encasing it in lime-based concreted which...well... it simply doesn't expand or contract without cracking apart... would cause either the expanding refractories inside to break — or cause the concrete "shell" around those refractories to crumble.

Or perlite

I know a lot of permaculture folks are big on perlite, but I don't trust it, myself. It's a natural volcanic glass that has a fairly murky, imprecise chemical composition depending on where it was sourced, and thus has an unreliably wide softening-point temperature range. The minerals of which it is generally-and-vaguely comprised, and which make its high silica content a volcanic "glass" instead of a crystalline phase, are within the class of aluminosilicate fluxing slags, too, which cause that lowered softening point, despite perlite's incidental 12-15% alumina content [if-and-when it's even that amount... which it often isn't.]



Some people report perlite-encased risers "melting" as low as 800º-850ºC. Even if it's only softening enough to contract and leave air pockets in a clay body (in which it had been entrained), that can be a problem for the whole mass's insulative value. The densified, contracted perlite globules will conduct heat much more rapidly, and the pores they leave behind in the clay body become larger...and the process of insulative loss is then self-propogating and it worsens exponentially rapidly as the mass increasingly densifies in some areas while pockets open wider in others, causing more heat conductance and absorption. Air pockets over about 1.5mm in diameter (approx. 16 mesh) begin to lose their insulative properties, because the larger air pocket starts to allow for convection and air rotation within the pore, transferring the heat from hot-side to cold-side more readily as the pocket of air circulates around and around.



Erica Wisener (co-author of the Rocket Mass Heater Builder's Guide with her husband Ernie) has changed her position on perlite over the years, too... 'cause they've seen a lot of RMH builds and some of those builds are not always super successful at staying assembled and thermally- and structurally- sound.









I've got some other stuff to share in a bit on refractory types, but I think we should continue this convo in the other thread (mentioned/linked above) regarding firebricks and aluminosilicate refractories — and leave this thread more for it's original topic of ceramic fiber products... That'll probably help folks keep the concepts separate in their minds so they're better-empowered to make informed decisions and mitigate risks. (and, a lot of convo around these very topics have kinda already started rolling over there in that thread.)

[Forsythe]

Thinking back on this, I realized I only indirectly responded to what Orange said in this part, and I should have been more direct. That wasn't as helpful as I could have been.

(I need to get better at recognizing where other people aren't connecting the same dots that I am —after having had my nose buried in a bajillion research papers on the topic— and I need to remember how much of this info is completely new to a lot of people, and how difficult it may be to follow for those who aren't as nerdy about ceramics as I am. It also must be extra hard to follow where there are language barriers.)

Apr 2, 2022 19:24:52 GMT -8 Forsythe said:

Apr 2, 2022 5:28:04 GMT -8 Orange said:

[I didn't have much knowledge so I just coated CFB with waterglass and then wild clay paint [...]So that is not a good solution - I'll repaint it with fumed silica[...] or just throw it out.

I guess I did a poor job of explaining the purpose of the fumed silica in this thread. ...I think the confusion was introduced when we strayed from the topic of rigidizing ceramic fiber products to mitigate the inhalation risk of decomposing fibers and swerved into the topic of improving refractoriness and slag resistance of all types of insulating refractories to prevent them from decomposing.

Just to re-re-re-state this again: these are two separate problems:

1) [protecting YOU from decomposing fiber inhalation risks]
— vs —
2) [protecting THE REFRACTORY from slag-induced decomposition.]

...and even though these two problems are indeed related, they need to be thought-of and addressed separately — in order to adequately mitigate each of them appropriately.

So, to clarify here:

Now that Orange's ceramic fiber board is coated with waterglass, (sodium silicate) and wild clay paint, there would be no purpose in painting it with fumed silica — because he has already solved the first of the two separate problems above, which is [protecting HIM from the decomposing fiber inhalation risks]


...Why is that?

Because the silica in the sodium silicate [as well as the silica in the wild clay, which contains aluminosilicate of some variety or another] with which he rigidized the fiberboard will definitely hold the ceramic fibers together and keep them from becoming airborne. Problem 1: solved.


Why are sodium silicate (and potassium silicate) waterglasses not recommended if sodium silicate works for the first problem?

Because Sodium and Potassium, remember, *are fluxing slags* which lower the softening temperature of the original ceramic fiber board, and will shorten its working life, and could potentially cause it to actively melt or bloat and swell —and then crack upon cooling— if the stove is run hot enough. *That* is where using waterglass not only fails to solve for problem number 2, —which is: [protecting THE REFRACTORY from slag-induced decomposition.]— it actively works *against* solving problem number 2.


Why does sodium silicate work for the first problem if it makes the second problem worse?

Because with the silica fused in place in the pores between all those ceramic fibers, those fibers are now bonded together, and they're not ever going to become the cristobalite fiber inhalation risk that untreated ceramic fiber poses. That remains true even if the source of that silica also added a lot of fluxing sodium

Even if that waterglass-treated board crumbles after being fired beyond its now-lowered temperature threshold, those particles are not the light-weight, airborne, "high-aspect-ratio" crystalline silica risk that we need to worry about. Even if those larger and heavier crumbles *do* contain cristobalite, they're FAR less likely to be inhaled, AND they're not of the high-aspect-ratio shape which causes the high risk of cancer.


Why is fumed silica recommended instead of sodium silcate?

Fumed silica is just plain, very fine-grained, unreacted silica which will fuse to the ceramic fiber at the lowest temperature possible for a silica which doesn't have an added flux.

Adding Sodium with the silica causes aluminosilicate (IE: the ceramic fibers) to "melt" together at *vastly* lower temperatures, because sodium silicate contains a whole lot of sodium, which has a powerful fluxing effect upon aluminosilicate ceramics. Fluxes lower the refractory’s melting temperature by vastly more than silica *alone* will. And sodium silicate will most definitely lower the working temperature of the refractory ever-more in the future IF that sodium silicate surface is installed directly in the flame path without an IR-reflective and slag-resistant coating.


Why is the IR-reflective, slag resistant coating recommended after treatment with fumed silica, if fumed silica doesn’t flux the refractory like sodium silicate does?



Because adding silica *does* cause the SURFACE of the aluminosilicate (IE: the ceramic fibers) to "melt" together at *slightly* lower temperatures. (It has to soften the ceramic fiber *at least a little* because that slight surface softening is what creates the bond which holds the ceramic fibers together, preventing them from becoming airborne. And that slight softening could further lower the softening temperature in the future *IF* that fumed silica surface is installed directly in the flame path without an IR-reflective and slag-resistant coating.


Why would the plain-[fumed]-silica-sealed surface lower the softening temperature in the future, even more than it is now? Doesn't sealing the pores keep ash from accumulating in them?



• Yes, a smooth surface will accumulate a lot less ash than a porous one
• A smooth, solid surface will also diffuse the heat *a slight bit* better than a porous one, thus lowering the surface's thermal reactivity to the ash *slightly*
...however:
• Ash which does stick to the exposed hot-face surface —even though it's a smaller volume of ash accumulation— will have increased reactivity with a surface of higher % silica than it did with higher % alumina.
• Silica AND Sodium on the hot-face surface will be much, much, MUCH more reactive to any ash that sticks to it.


What does the IR-reflective, slag-resistant coating do that the fumed silica doesn’t?

The coating is “slag-resistant,” meaning that it acts as a barrier preventing the ash (which is a mixture of multiple fluxing slags) from melting and absorbing into the silica-sealed surface.



And because the coating is "IR-(InfraRed)-Reflective," it reflects heat back inward toward the burning fuel, improving the combustion performance inside the burn chamber(s) which simultaneously reduces the amount of heat absorbed by the ceramic fiber refractory.

Both of those attributes are unique to these [zirconia-based] IR-reflective, slag-resistant refractory coatings, and both of those attributes will keep the silica-sealed ceramic fiber from decomposing, extending the life of the refractory. That’s what solves problem #2: [protecting THE REFRACTORY from slag-induced decomposition.]


...whether the added sodium from the sodium silicate causes Orange's sodium-silicate-treated ceramic fiber board to decompose much sooner or later will depend on A) What % of alumina his ceramic fiber board originally started out with, B) how hot he operates his stove, and C) how much ash his particular, individual stove design will expose those fiber board surfaces to in the course of its operation. ...because ash, remember, is nothing but fluxing slags, including even more Sodium.


...But it must be said here: There would be no reason at this point for Orange to throw out his already-waterglass-treated fiber board unless it is showing signs of decomposition. And the reason for throwing it out would be because of the increasing insulative- and structural- failure of the refractory — NOT because a cristobalite fiber inhalation risk. He has already mitigated that.

...and it should also be said that he could probably greatly extend the life of that current, already-waterglass-treated fiber board by simply coating it with an IR-reflective, slag-resistant refractory coating.

[fiedia]

Thanks for this summary.

Did I understand well : it is better to coat with fumed silica (biding fibers) before spraying IR coating on top than just use IR coating ?

[Forsythe]

Apr 5, 2022 6:35:09 GMT -8 fiedia said:

Did I understand well : it is better to coat with fumed silica (biding fibers) before spraying IR coating on top than just use IR coating ?

Yes, you want to ensure those fibers are bound together first.


The only exception would be where the IR coating itself is sold specifically as fulfilling both roles as a ceramic fiber rigidizer AND an IR reflective coating.

There was one such zirconia-based product from Australia [which I can't seem to relocate now, nor recall the name of, unfortunately] which specifically stated that it works best —and binds to the fiber best— without a separate rigidizer applied beforehand.

[Forsythe]

I'm cross-posting this info here as it was removed from a permies thread (https://permies.com/t/153507/Morgan-Superwool-ceramic-fiber-blanket) about an hour after I shared it, without any acknowledgment of the misinformation regarding AES wool...

...to be clear: AES wool can be used safely as a back-up liner... but not when you don't allow people to know the unsafe aspects of using it irresponsibly as the hot-face.

Unfortunately, this culture of denial about —and obfuscation of— health hazard issues is a large reason why so many completely unsafe myths regarding refractory material safety persist within the permaculture rocketstove community. Blatantly censoring this info denies people the right to make informed decisions for themselves and their families, I fear. And that doesn't sit well with me.



I guess its a good thing I wrote the draft for the original message in a separate text editor... which I thought had been respectful yet succinct:

Morgan superwool plus is a completely safe alternative to highly hazardous ceramic blanket. Low bio-persistence makes this safe to handle.

To preface this:

I've got a lot of respect for what Dragon Heaters are working to provide — especially the use of 330 stainless riser stubs and turnkey batchbox doors, which are laudable accomplishments that practically no-one else is taking the lead on, and they are forward-thinking accomplishments for which Dragon Heaters should be very rightfully commended and respected— but there's a lot of misinformation floating around the web about AES (Alkaline Earth Silicate) wool, of which this Morgan Superwool Plus is an example.

AES wool may not be a "ceramic" because of its lack of alumina content — but that also lowers its useful temperature range. It may be biosoluble up to its continuous working temperature limit —but— it converts to cristobalite above that limit. Cristobalite is not biosoluble, and it is a known carcinogen. (‡)

The working temperature of this AES wool is also lower than its maximum temperature threshold —cited erroneously higher than its true working limit in the advert above— and that maximum working threshold is *far below* rocketstove riser temps.... meaning: using it in your riser, as the hot-face WILL cause it to lose its biosolubility and convert to cristobalite.


[(‡) Part of this misinformation is a byproduct of the limited research available on so-called "in-vitro" cristobalite exposure; research which does not actually take place in-vivo — experiments wherein the cristobalite is only exposed to cultured, isolated macrophage cells in a laboratory setting, outside of the whole, living organism. ...But, unfortunately, fiber toxicology is notoriously dangerous for delivering its carcinogenic dose to the parietal pleura, in a combined-effect fibrosis between lodged fibers in the lung substrate, and the macrophage response to that invasive, high-aspect-ratio silicate fiber. The research in this field is hamstrung by the ethics of experimenting on live animals... but the evidence we have from live humans suggests extremely strongly that the cultured-macrophage research is insufficient to account for cristobalite fiber pathogenicity.]


This product does not require a MSDS safety sheet.

As much respect as I have for Dragon Heaters, this is an irresponsible statement to make.

In Europe, for example, all such material by law must be provided with an SDS — and for very good reason, as we'll see below. ("MSDS" is a no-longer used term in the material science industries.)

...Reviewing the SDS (and its cited Technical Product Data Sheet) will reveal where this product's limitations do not align with a misinformed retailer's claims —whether those claims are misleading unintentionally or otherwise— and will thus reveal the potential health hazards resulting from encouraging people to use this product wantonly in the direct, unshielded flame path of a rocketstove's riser.

First, the temperature rating, which is cited erroneously in the above advert as 2100ºF:





The SDS clearly shows this temperature limit is only 1920ºF … not 2100ºF, nor 2190ºF.

And this lower threshold is important for two reasons:







(1) The AES fiber only stays soft and biosoluble, and safe to handle without a respirator up until the working limit — NOT it’s highest, short-term, peak “classification” limit. That peak classification limit is the highest it can go and still remain insulative without transferring heat through its face and burning your structure down. It is *not* the safe working limit.



(2) AES and ceramic fiber were never intended to function as the “hot-face” lining of a wood-burning combustion zone, at high temp, high ash exposure, and high gas velocity. The constituent mineral salts of woodash are all alkali fluxing slags (like potassium, sodium, magnesium) which will further lower silica's cristobalite formation temperature AND the melting temperature of the directly-exposed silicate wool. The SDS classifies this fibrous insulator as a **back-up** lining for very specific reason; meant to be placed BEHIND the hot-face. It is not meant to be exposed directly to woodash impregnation and direct flame impingement.


To claim that this product is "completely safe" and "biosoluble" is like saying that clay is always water-permeable, wettable, re-shapable, and even edible.

...It sure is...

until you fire it hot enough to crystalize the particles into a solidified, “fired” clay ware..

...And a 6" or 8" system-sized rocketstove will definitely fire this AES wool hot enough to crystalize the amorphous silica particles into cristobalite. Those rocketstove cores —as Erica Wisener has mentioned in multiple posts— can easily exceed 2300-2400ºF.



To be frank, it breaks my heart when I see permaculturists parroting dangerously inaccurate info about these types of refractory materials and referring to the ceramic material research as "eggheaded"... I want all of you lovely people to stay healthy and safe… and ignoring the Safety Data which is publicly available is tantamount to shooting ourselves in the lung.

If you'd like to explore some more details on how to employ fibrous refractories safely, and the pathogenicty-risk context of employing them un-safely, we've been discussing a lot of that in depth recently over on the Donkey32 pro boards forum for rocketstove experimentation.



(Disintegration of Superwool ceramic fibre board

: donkey32.proboards.com/thread/3676/disintegration-superwool-ceramic-fibre-board)


"We're here to help each other through this thing, whatever it is" — Kurt Vonnegut.

Be safe, and be well,
-Forsythe

[jules]

Feb 21, 2022 18:45:53 GMT -8 Forsythe said:

The cheapest route is to use fumed silica (AKA "Colloidal silica") and be sure to fire it good and hot as soon as it's dry. Fumed silica is cheap if you buy it as sold for use in fiberglass and resin casting...

Forsythe  Thank you for the helpful and important information and explanations to help me a lot. It is not easy to find relevant information and suitable materials, I am not an expert in this regard. Please just keep going, waiting for the promised information, thanks again!

I have a question: I finally found a place where I could order suitable material. What do you think about that? Will it be good as a rigidizer? What coating can I use for that?
Fumed Silica Thixotropic Powder     www.easycomposites.eu/filler-powders-and-additives

Or do you have other recommendations? 
Thanks,
Julius

[Forsythe]

May 1, 2022 4:18:51 GMT -8 jules said:

Feb 21, 2022 18:45:53 GMT -8 Forsythe said:

The cheapest route is to use fumed silica (AKA "Colloidal silica") and be sure to fire it good and hot as soon as it's dry. Fumed silica is cheap if you buy it as sold for use in fiberglass and resin casting...

Forsythe Thank you for the helpful and important information and explanations to help me a lot. It is not easy to find relevant information and suitable materials, I am not an expert in this regard. Please just keep going, waiting for the promised information, thanks again!

I have a question: I finally found a place where I could order suitable material. What do you think about that? Will it be good as a rigidizer? What coating can I use for that?
Fumed Silica Thixotropic Powder www.easycomposites.eu/filler-powders-and-additives

...So, seeing as that earlier post has generated a lot of confusion for people, I went back and edited that snippet itself to now read:

The cheapest route [**SEE EDIT NOTE BELOW**] is to use fumed silica (AKA "Colloidal silica") and be sure to fire it good and hot as soon as it's dry. Fumed silica is cheap if you buy it as sold for use in fiberglass and resin casting... but they charge a much higher premium if you buy it from a refractory company. It's literally the same product, though; amorphous, unreacted silica. When fired, it will incorporate and crystalize with the aluminosilicate. An added bonus of using fumed silica is that it creates a tougher substrate to bond an IR reflective coating like ITC-100, HTC-100, bubble alumina, satanite, etc... which will actually increase the insulative value of the refractory, making the rocketstove burn cleaner, sooner in the burn-cycle. Here's a short set of 60-second vids on using fumed silica to seal ceramic fiber blanket (had to split this into <1 min blocks, as that's the max imgur allows)

[**EDIT NOTE** — "the cheapest route" here means the cheapest route to rigidize a ceramic fiber blanket — instead of buying a ceramic fiber rigidizer from a refractory company which is usually nothing more than fumed silica, itself. If you are installing the ceramic fiber blanket **AS THE HOT-FACE** of a rocketstove core, it is strongly recommended to follow the fumed silica rigidizing treatment with a slag-resistant refractory coating. The fumed silica will ONLY "rigidize" — thereby keeping the ceramic fibers from becoming airborne. Fumed silica WILL NOT increase the ceramic fibers' "slag resistance" to wood ash, and will not help a low-temp-rated ceramic fiber from melting if your stove design exceeds its working temperature, and that working temperature decreases with direct exposure to woodash. Considering the two-step process involved in firing each coating, it may be easiest to rigidize the ceramic fiber with a slag-resistant refractory coating specifically sold as fulfilling both roles: as a rigidizer, AND as a slag-resistant refractory coating. The zirconia-based products for this purpose are also Infrared-Reflectant, and thus will also improve combustion efficiency...and that class of slag-resistant, infrared-reflectant, ceramic fiber rigidizer coatings is what I would personally recommend for ease of application. ...Further detail on this can be found later in this thread, starting here: donkey32.proboards.com/post/37535]

Or do you have other recommendations?

My best recommendation is to rigidize the ceramic fiber with a slag-resistant, infrared-reflectant, refractory hard-coating — one which contains zirconia for the slag resistance and infrared-reflectance, and one which is ALSO specifically marketed as a ceramic fiber rigidizer. That would allow you to accomplish both goals:

Just to re-re-re-state this again: these are two separate problems:

1) [protecting YOU from decomposing fiber inhalation risks]
— vs —
2) [protecting THE REFRACTORY from slag-induced decomposition.]

fumed silica solves for problem #1, but it may not solve for problem #2 (like, for example, if your ceramic fiber blanket is rated under 2400ºF, and you're building a 6" or 8" J-tube rocketstove, which can easily exceed 2300-2400ºF)

fiedia found this zirconia-based product from Vitcas available in the EU, and it will indeed serve both roles as a rigidizer AND slag-resistant IR-reflective refractory coating.

shop.vitcas.com/vitcas-zircon-paint-coating.html#tab-label-description



If you scroll down on their product page, you'll see these important notes pertinent to that application:



With a product like this one, you wouldn't need to use the fumed silica before adding a separate zirconia coating. It's all in one, applied in one single step.

...and do be sure you are wearing a respirator while handling the ceramic fiber blanket for installation.

[Forsythe]

oh, and for anybody in Australia, I found that product I referred to earlier... it's called "Fibrecoat" from Consolidated Refractories.

...and it appears that their product "Furnascote" can also be used as a dual rigidizer / IR-reflectant, slag-resistant coating:

consolidatedrefractories.com.au/products/coatings/


(Their product is one of those which specifically states that it doesn't adhere well to surfaces which have already been rigidized with a silica sealant.)

Fibrecoat is a Zircon based refractory composition specially developed for use as a coating and binding agent for ceramic fibre and other fibrous refractory insulating materials. Fibrecoat will enhance the performance of Refractory Ceramic Fibres (RFC) in applications, whilst at the same time solving many of the problems associated with Refractory Ceramic Fibres. Fibrecoat binds the surface fibres forming a skin over the ceramic fibre lining. Fibrecoat strengthens the surface of an otherwise weak material greatly improving its resistance to erosion in areas subject to high gas velocities, flames, corrosive gasses, splashes from molten metal and many industrial chemical processes.

The ceramic fibre is rendered harmless under normal conditions of use and also serves closer to its classification temperature.
Fibrecoat is supplied as a dry powder which is mixed to a slurry. The amount of water will vary depending on the method of application, weather you are going to brush or spray the Fibrecoat to apply it to the surface. A coating of 1mm to 3mm produces a good firm skin over the ceramic fibre. We have found spraying is the best method of application especially over larger areas. When compared to some of the other zircon based coating we find that the Fibrecoat material produces a hard protective layer over the fibre rather than a soft layer that is easily broken.

Fibrecoat can be applied to both new and fired ceramic fibre provided the surface is in reasonable condition, however coating rigidized surfaces is not recommended. We supply Furnascote Non Vit to do the work of Fibrecoat as it as able to be used in both applications

Also appears to be available in AU's southeast-Asia trading partner countries:

www.rictec.com.sg/fibrecoat/

Fibrecoat is a zircon based refractory composition specially developed for use as a surface coating and binding agent for ceramic fibre and other fibrous refractory insulation materials.

Fibrecoat will enhance the performance of Refractory Ceramic Fibre (RCF) in all applications, whilst at the same time solving many problems associated with reclassfication of RCF as a Class 2 Carcinogen.

By binding the surface fibres and forming a thin skin over the ceramic fibre linings, Fibrecoat strengthens the surface of an otherwise weak material greatly improving its resistance to erosion in areas subject to high gas velocities, flames, splashes from molten metals and many industrial chemical processes.

The ceramic fibre is rendered harmless under normal conditions of use and is also able to serve closer to its general classification temperature. Improved insulation is also achieved as the layer of Fibrecoat stops the passage of hot gases into the fibre lining reducing heat loss from energy radiated into the fibre.

Fibrecoat is supplied as a dry powder which is mixed with cold water to form a slurry. A typical application will use a 1 - 3mm spray applied coating to encapsulate and seal the ceramic fibre with these resulting benefits:
• Increased erosion resistance to flames and high velocity gases from approxiamately 12m/s to 30-50 m/s
• Reduced surface dusting and broken fibre breakdown.
• Fibre is protected against chemical attack from combustion gases and harsh process environments.
• The fibre works closer to its specified maximum working temperature.
• Reduced penetration of hot gases into the fibre.
• Reduced heat loss from energy radiated into the fibre.
• Improved insulating quality of the fibre.
• Increased lifespan of ceramic fibre linings.
• Fibrecoat is suitable for use on both new and old fibre linings.

APPLICATION NOTES

Fibrecoat should only be applied to clean, sound surfaces. Any loose material, powders or contamination should be removed.

Fibrecoat should not be applied to RCF which has been treated with silica-sol rigidisers, neither will it adhere properly to modules containing processing oils. Any mineral oil should be burnt off prior to application of Fibrecoat.

The product is designed for spray application using textured paint or cement spraying equipment. It may also be brushed onto hard surfaces.

A coating of approximately 1.5mm will provide economical protection to most ceramic fibre linings. A gas tight coating is attained at >2.5mm. Several thin coatings each allowed to dry out are the best way of achieving thick coatings (>3mm).

[jules]

I found a suitable Vitcas distributor for Central Europe, free shipping for some products:
sklep.vitcas.pl/produkty/materialy-ognioodporne.html

Maybe someone is using a material consumption calculator:
www.vitcas.pl/kalkulator/

[Orange]

thanks Forsythe for the detailed response, although I cant process all that information

so to sum up:

- yes my primary concern was the air particles, I'm glad that's not an issue

- I'm not concerned about durability of IFB, seems to work well under stress after coating, check this test: donkey32.proboards.com/post/36285/thread

- I mentioned light air-concrete (ytong multipor) and perlite only as an insulation after firebrick. Yes, air-concrete will crack but air pockets remain (an insulative properties). Perlite was fine in my case. Both are just fillers and don't hold any structural load.

- I am a bit concerned about off-gassing. I didn't measure an increase in VOCs but if I smell right above my stove I can smell something unusual and can't tell if it's IFB, air-concrete, perlite, mill-scale, old cast-iron coating or something else. Any ideas?

[Forsythe]

May 13, 2022 9:32:54 GMT -8 Orange said:

I am a bit concerned about off-gassing. I didn't measure an increase in VOCs but if I smell right above my stove I can smell something unusual and can't tell if it's IFB, air-concrete, perlite, mill-scale, old cast-iron coating or something else. Any ideas?

My money is on the bet that it's the organic binder used to form ceramic fiber into board-shape — most likely a phenolic- or urethane resin.

“For insulation board, a substance that hardens by a chemical or a physical process is employed to shape the insulation board – this substance is called a binder. In the current industrial production of ceramic wool insulation board, organic binders such as phenolic resin or urethane resin are widely used [9,10]. These organic resins provide good mechanical strength, low deformation temperature (~100°C), and good heat resistance than other organic binders. However, the limitation of these organic binders is indisputable. Most of all, they release toxic gases during fabrication and curing processes, and when burned [10]. Besides, these organic binders have their weakness under high-temperature conditions, as they disintegrated above 400°C. Furthermore, under high humidity conditions, the loss of compressive strength have been reported [11,12], as well as the degradation of thermal resistance [12]. More recently, it was shown that phenol-formaldehyde binder and other hydrophobic compounds that protect fibers against moisture in MW and give compressive resistance degraded under high humidity and aging.[13]."

(source: www.tandfonline.com/doi/pdf/10.1080/21870764.2020.1842118)


Side note: Those organic binders often have to be burnt off before some types of zirconia coatings can be applied. That "hydrophobic" property of the organic binder can prevent the zirconium's silicate-colloid carrier from adhering, and the zirconia coating just flakes right off.

The organic binder resin should burn off fairly easily if you get it hot enough, and then be done with the off-gassing, though, once you've gotten through that thorough heat-treatment stage.