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

[Forsythe]

...And there is a lot to be said for using insulating refractories. I hope my posting of this info about refractory durability does not dissuade people from using them. I fear that that may be the misperceived takeaway here for a lot of folks — whereas the intention is to give ideas for preserving the integrity of your refractories, including the insulating ones,  not to turn you off from using insulative firebricks.  Quite the opposite.

Indeed, insulating refractory is integral to rocketstove combustion efficiency. That's what makes burning a small amount of wood at extremely high temperatures —for clean combustion— even possible to begin with.


Low thermal conductivity in secondary combustion zones means the heat is being recycled right back into the combustion zone, keeping the burn hot, and thus clean:



So one key takeaway, rather, is mitigating the weak-point problem of *surface* porosity in insulating refractories. Virtually every type of insulating refractory will have a porous interior — that porous interior is what makes them insulative — but it's the surface porosity which can make the refractory unit more susceptible to woodash slag attack.

That's one of the main reasons why refractory hard-coating is so useful on brick and castable surfaces. (It's super-duper useful on ceramic fiber surfaces, too, but that's a topic for the other, ceramic fiberboard thread.)


Another key takeaway is that not all insulative firebricks are created the same.

Not only do insulative firebricks vary in (a) level of porosity [and its corollary: relative density], they also vary in (b) chemical composition, and (c) method of manufacture.



For example, here are two separate grades of insulating refractory firebrick from the same manufacturer. At first glance, the only advertised difference between them is their temperature rating, (and of course, their price.):






...but there are multiple very good, completely justifiable reasons for their price difference.

First is their relative density [and level of porosity,] shown here in comparison to dense firebrick on the far right:



and here by comparison with each other:



The cheaper brick has a whole lot more open-pore structure on its surface, and those are all weak point crevices for woodash slag to coalesce.


...but that's not the only difference.


Second is their chemical composition, shown here. First image is the cheaper, lower-rated 2300ºF brick, followed by the 3000ºF brick:





It's the same Amazon item listing for the same brand of firebrick, with only the drop-down selection options for temperature rating having been changed. The temperature rating in this case means a far more durable brick, both in terms of structural AND chemical durability.

The 3000ºF brick not only has lower surface porosity, it has nearly TWICE the alumina content and HALF the silica content of the cheaper 2300ºF brick — and its "slingering" method of manufacture concentrates the porosity at the brick interior while concentrating relative density at the surface— far more than the cheaper brick's cast or moulded method of manufacture. ...So it's not simply a matter of the cheaper brick being more full-of-holes.

All of those things together will make the 3000ºF brick last more-or-less indefinitely in a rocketstove's secondary combustion chamber / heat riser / afterburner, particularly when you go the extra mile of adding a slag-resistant refractory coating to that higher-rated brick surface.

...and when you consider the durability aspect of not having to replace those 2300ºF bricks every few months or years, it actually comes out cheaper in the long-run to buy the 3000ºF at twice the price... which you will only have to pay for once.

On top of all that, when your slag-resistant refractory coating is also InfraRed-reflective [zirconia-based] — then your combustion efficiency is even higher, too... which means less wood burned, less emissions, and less ash clean-up.


So, insulating firebrick is a highly-necessary refractory to use, when you put it in the right location.

[Forsythe]

Edit: Here's an analogous way to think about the above:

Keeping your home warm in the winter and cool in the summer —efficiently— is only possible when you insulate your building structure.

However — no one in their right mind would build the walls and roof themselves out of entirely-exposed rockwool or fiberglass insulation.

The exterior face of an insulation-only wall or roof would never be durable in exposure to rain, wind, snow, hail, etc. which would weather away in no time. Rockwool and fiberglass insulation were never intended to be exposed to exterior wear in the elements.

Likewise, it would be foolish to omit the interior wall-covering over the rockwool or fiberglass insulation. Inside the home, you would be exposed to inhaling those fibers, they would accumulate dust and grime, and they would be worn away by incidental contact with both the building's inhabitants and the circulating indoor air from fans, HVAC unit blowers, etc.

These same basic principles of home insulation are just as true for rocketstove insulation. You really do need to insulate, but you should put a hard, durable cover over that insulating layer to protect both it and you.

[fuegos]

Ok a lot to digest here, I'll give it a proper read later but yes insulation is fundamental to rocket stove performance

[fuegos]

I think you've made a pretty good case for using insulating bricks generally but in this case i think that dense bricks have an advantage & the trade off between ultimate performance vs better characteristics for an oven.Our own experience with a dense brick core is that it stores heat for a long time - firing at 08.00, by 10.00 it's coaling & then at 16.00 or 17.00 it's still 100c in the oven.I'm thinking of using hollow extruded clay brick next to the core - the ones with holes running vertically - and then vermiculite & finally an outer skin.
Cost isn't a big problem here but ultimately the end users will decide on the final details.

[fiedia]

fuegos,

What I understood is that Forsythe was speaking about insulating the heat riser in order to reach second combustion temp ASAP. You are speaking about insulating the "core", do you mean the fireplace ?

Insulating the fireplace seems to me not optimal if you want to avoid thermal runaway and balance heat distribution between top and bottom of your rocket.

What do you mean by "ultimately the end users will decide on the final details" ? Sounds like we are taking products from the shelf and returning them if not "satisfied".

[fuegos]

""core", do you mean the fireplace ?
the firebox and "riser" - in Matt Walkers riserless core they are alongside each other , the "riser" is horizontal. Insulation will be placed underneath & alongside the core.The photos in this thread help explain although Mannys build is with insulated brick.
The end users are the folks who i'm building the oven for.

[Forsythe]

Apr 26, 2022 2:03:20 GMT -8 fuegos said:

In this case i think that dense bricks have an advantage & the trade off between ultimate performance vs better characteristics for an oven.Our own experience with a dense brick core is that it stores heat for a long time - firing at 08.00, by 10.00 it's coaling & then at 16.00 or 17.00 it's still 100c in the oven.I'm thinking of using hollow extruded clay brick next to the core - the ones with holes running vertically - and then vermiculite & finally an outer skin.
Cost isn't a big problem here but ultimately the end users will decide on the final details.

Agreed that with an oven as large as you're building and intended to be used for 3 families' baking needs, I think the dense brick lining —backed-up by an insulating layer behind it— will probably work well. There might be a thermal flywheel advantage to retaining all that heat between frequent uses... the interior may never actually cool down to ambient temps with daily, multifamily use.


I'm curious, though, what function the hollow extruded clay brick is intended to serve... Is that to be heat-storing thermal mass? ...Or insulative lining? ...Or structural support for the oven situated above the core?

The reason I ask is that I would think the insulative layer (the vermiculite in your current materials list) should go directly behind (and immediately against) the dense firebrick of the core, helping reach clean combustion faster —and also helping to retain heat in those dense firebricks. Then, a course of structural supporting brick would encase the insulating layer. (The images below come from that Self Reliant Potter book I linked earlier)

...in this first image, the "insulative brick" internal lining shown in these 2 wall cross-section diagrams would be substituted for your dense firebrick hot-face in the core, and the "loose filling" would be the insulative layer between interior and exterior facing:



...If the role of the hollow extruded clay brick is intended to be insulative, I'm afraid those hollow spaces are much too large to serve an insulating role...

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.

...unless you filled the hollow cavities with something insulative like a sawdust-clay or [washed] ash-clay mix. 



... or... if you filled them with clay mortar and grog or sand, they could potentially be used as thermal mass... but... if you're laying the dense brick as "Stretchers", that's 4.5 inches of wall thickness which will retain a lot of heat. Adding more thermal mass after that 4.5" thick, dense-firebrick hot-face may take a lonnnnng time to come up to working temp and thermal equilibrium. ...Or are you intending to lay the whole core on edge as "Shiners"?



...and... leaving those hollow, extruded bricks hollow without any insulation between them and your hot-face lining... that might work more like a radiative heat sink, wicking away heat from the interior and losing it to convection in the same way the finned heat sink on a computer CPU draws heat off and puts it into the air. (Having air pockets that big right next to the un-insulated hot face will probably cause enough heated-air expansion to force open cracks between mortar joints for the expanded, rising air to escape... further adding to the heat-sink effect)



from a story about Aprovecho Research institute, the successful failure of the Lorena cookstove, and the continued quest for ideal cookstove design: www.newyorker.com/magazine/2009/12/21/hearth-surgery

[fuegos]

"I'm curious, though, what function the hollow extruded clay brick is intended to serve... Is that to be heat-storing thermal mass? ...Or insulative lining? ...Or structural support for the oven situated above the core?2

I was thinking about structural support for the core and the oven chamber although the oven floor would help to tie it in to the second wall even with a cavity.We didn't leave any cavity in our own build apart from the back - we were in Covid lockdown & couldn't get vermiculite - but yes insulation is needed ! Yesterday we fired at 08.00 & 10.00 we had 550 F & by 18.00 we still had 200 F .
"Or are you intending to lay the whole core on edge as "Shiners"?
yes as seen here & here .

as for the flywheel effect i think they intend to bake maybe twice a week ,making a big batch of sourdough which keeps fresh for days. Even with a rocket core 6 to 8 KG of wood per firing is a substantial amount per year if fired on a daily basis ,plus the wood needed to heat 3 homes. The build is going to be in a communal ,semi open building for year round use (we're in Spain).
I've seen traditional communal village dome ovens here that have literally tons of earth & rock in then & yes they can store the heat for a long time but have all the problems associated with basically building a fire in a cave.

[hammer]

regarding coating the insulated Bricks, thanks so much Forsythe, I am using them for my fire box and riser, I am glad I was given this thread to read before I built it,I have ordered a pint of the 100 HT

[fuegos]

Apr 26, 2022 2:03:20 GMT -8 fuegos said:

I think you've made a pretty good case for using insulating bricks generally but in this case i think that dense bricks have an advantage & the trade off between ultimate performance vs better characteristics for an oven.Our own experience with a dense brick core is that it stores heat for a long time - firing at 08.00, by 10.00 it's coaling & then at 16.00 or 17.00 it's still 100c in the oven.I'm thinking of using hollow extruded clay brick next to the core - the ones with holes running vertically - and then vermiculite & finally an outer skin.
Cost isn't a big problem here but ultimately the end users will decide on the final details.

UPDATE : the community members have decided to go for IFBs in the core .It will be interesting to see hoe this works in comparison to my own build with dense firebricks in the core.

[Forsythe]

Jul 19, 2022 1:54:44 GMT -8 fuegos said:

UPDATE : the community members have decided to go for IFBs in the core .It will be interesting to see hoe this works in comparison to my own build with dense firebricks in the core.

Nice— I’m really curious to hear what you find on the differences between the two — Keep us posted!

Matt’s riserless core has always intrigued me, —especially with the possibility of an amazingly low vertical height for a cooktop / hob — but I’ve been hesitant to build one due to his earlier statements that insulating firebrick or ceramic fiber board had to be used for the firebox hot-face for combustion efficiency and a clean, smokeless burn [in that particular Walker Riserless design.] I find myself pretty solidly in agreement with the school of thought that: the firebox interior should be dense in order to be mechanically robust — and thus resistant to the abrasion of fuel loading, ash removal, etc.

I just dislike the idea of having to be ginger with firebox walls when also having to be careful about the intense heat…like when reloading wood into a white-hot firebox, or trying to adjust actively burning wood with tongs without burning myself, or accidentally knocking flaming wood out of the firebox as I try to oh-so-delicately adjust a bit of fuel that has shifted and blocked the port, for example.

It probably (best guess) boils down to how many wood loads you fire with. …It would stand to reason that dense brick may only smoke on the first loading, and once the firebox walls have heat-saturated, they’re no longer robbing the heat needed for secondary combustion — and thus any subsequent, consecutive fuel loads would burn just as cleanly as they would with insulating materials in the firebox.

And I’m sure an element of the stove operator’s overall finesse plays into philosophy of use, too. I tend to be not-so-great at using tools delicately, myself. 😂

[Forsythe]

Jul 18, 2022 6:37:57 GMT -8 hammer said:

regarding coating the insulated Bricks, thanks so much Forsythe, I am using them for my fire box and riser, I am glad I was given this thread to read before I built it,I have ordered a pint of the 100 HT

Hey, that’s great — Glad you found the info useful!

That ITC-100 is amazing stuff for retaining heat where you need it in combustion zones.

I should probably reiterate, too, that ITC-100 (or any other zircon-based IR-reflectant refractory coating) should only be used in combustion zones. As in: if one were to use it on brick faces that you intend to be thermal storage mass, it would entirely defeat that thermal mass purpose… because it’s that good at blocking heat transfer.

Example: this is a small-batch ceramic kiln built a couple decades ago by the renowned ceramicist Lou Nils — it’s constructed of 3/4inch plywood, simply coated on the interior with ITC-100 — being demonstrated during one of 2 consecutive ceramic firings at the National Council on Education for the Ceramic Arts conference. In this photo, (from Nils’ book The Art of Firing) a participant is placing their hand on the outside of the plywood to feel how cool it is, while the clay wares inside are fired to cone 10 (~1330°C)



ITC-100 had been promoted within the furnace / forge / foundry industry as an emergency heat patch — like if a furnace door had a chunk fall off, ITC-100 could be painted onto a plywood board and used to block the opening until repairs could be made. Nils came across that info, tried it in studio kilns, and started advocating for its use by clay artists / potters…who were obviously skeptical of the claims.

This is NOT to say anyone should attempt to make a permanent installation with plywood and ITC-100… but is meant to be an example of just how much heat it is capable of blocking.

Since that time, there have been a number of very similar products brought to market — all featuring a zirconia content of about 60-65%. ITC’s formulation is still proprietary trade-secret, but this whole class of zircon-based refractory coatings all boast the same characteristics.

In addition to the Victas brand “Zircon” mentioned earlier in the thread, I’ve also come across this one from Simond Refractories on Amazon US —a relatively new company in india that exports to the US (and elsewhere, if I’m not mistaken.) So, if ITC-100 is insanely expensive where you live and you can’t find Victas’s product, this may be another option to look for or compare pricing. (Haven’t tried this one, myself, but I can vouch for the quality of their 2600°F-rated zirconia-aluminosilicate ceramic fiber blanket, which I found to be excellent.)

Simond Store “Heat-Guard” runs about $56 USD for a pint, at least where I live.
(Amazon link) a.co/d/f5wTTnu




…and…

For anyone using metal P-channels / floor channels for secondary air tubing through the firebox, ITC-213 is a product is meant for application to metal and graphite surfaces to prevent oxidation and spalling in high temperatures. Might be something to consider to prevent those metal air supplies from burning out:

ITC-213 is specifically formulated to adhere to metal, whereas ITC-100 is not


ITC-213 (metal coating) Data Sheet


(Amazon link) a.co/d/2kswNtb
it runs ~$60 USD per pint here, but appears to vary a lot in asking price, so DO shop around if you’re in the market for it.

[fuegos]

Jul 26, 2022 4:50:21 GMT -8 Forsythe said:

Jul 19, 2022 1:54:44 GMT -8 fuegos said:

UPDATE : the community members have decided to go for IFBs in the core .It will be interesting to see hoe this works in comparison to my own build with dense firebricks in the core.

Nice— I’m really curious to hear what you find on the differences between the two — Keep us posted!

Matt’s riserless core has always intrigued me, —especially with the possibility of an amazingly low vertical height for a cooktop / hob — but I’ve been hesitant to build one due to his earlier statements that insulating firebrick or ceramic fiber board had to be used for the firebox hot-face for combustion efficiency and a clean, smokeless burn [in that particular Walker Riserless design.] I find myself pretty solidly in agreement with the school of thought that: the firebox interior should be dense in order to be mechanically robust — and thus resistant to the abrasion of fuel loading, ash removal, etc.

I just dislike the idea of having to be ginger with firebox walls when also having to be careful about the intense heat…like when reloading wood into a white-hot firebox, or trying to adjust actively burning wood with tongs without burning myself, or accidentally knocking flaming wood out of the firebox as I try to oh-so-delicately adjust a bit of fuel that has shifted and blocked the port, for example.

It probably (best guess) boils down to how many wood loads you fire with. …It would stand to reason that dense brick may only smoke on the first loading, and once the firebox walls have heat-saturated, they’re no longer robbing the heat needed for secondary combustion — and thus any subsequent, consecutive fuel loads would burn just as cleanly as they would with insulating materials in the firebox.

And I’m sure an element of the stove operator’s overall finesse plays into philosophy of use, too. I tend to be not-so-great at using tools delicately, myself. 😂
some great points Forsythe but im at the end of a tiring day so im off to drink a well earned glass of wine & give your ideas some thought.I'll get back to you later.

[fuegos]

The idea of having mass in the firebox , for an oven makes sense ,to me at least , the idea is that it forms part of the overall mass.This next build is going to have a much bigger oven chamber , 67 x 78 cm and with the addition of a bell any heat robbed by the core could limit the whole system.For practical reasons my first build had little or no insulation around the core but this next one will allow a cavity of between 7 & 10 cm which i'm going to fill with vermiculite.
Other oven builds using Matts riserless core - Tom from Dragon tec & Manny (build thread on here) have simply made the oven more or less the same size as the core , ours as well but building for a community space demanded a bigger oven.
The low height of the riserless core is a great advantage for cooking applications and being almost square in form it can be rotated in the build to place the firebox at the front or on the side & making a left hand or right hand version alters the position of the core exit into the oven , very useful for keeping this away from the bell entrance , chimney or oven door.
"the firebox interior should be dense in order to be mechanically robust — and thus resistant to the abrasion of fuel loading, ash removal, etc"
With a dry stacked core you have to be careful regardless of the density of the bricks.Cleanout can be done when cold with a stick. i tend to leave 2 or 3 cm of ash in the bottom as insulation , something taught to me years ago with box stoves.
" It probably (best guess) boils down to how many wood loads you fire with. …It would stand to reason that dense brick may only smoke on the first loading, and once the firebox walls have heat-saturated, they’re no longer robbing the heat needed for secondary combustion — and thus any subsequent, consecutive fuel loads would burn just as cleanly as they would with insulating materials in the firebox"
from what I've read on here a core with IFB is a lot cooler & easier to reload. My own build stays hot & this in it's self can provoke over fueling.I use a pair of welders gauntlets that reach almost to the elbow for reloading , highly recommended. Secondary air in the form of the floor channel acts as a end stop so the wood can't block the port.
Apart from the first load i tend to add wood in half or part loads during the burn -this gives some control over oven temperature - & with the welders gauntlets its easy to place the wood precisely ,also getting away from the idea of getting as much wood as possible in , important if you're doing just one or two firings a day for a heater ,not so much with an oven.
I think there's a lot of scope for development of wood fired ovens with this or other cores as traditionally the amount of wood needed has been excessive. We have a bakery in the nearby village with a wood fired oven but even with the price of natural gas & electricity they are fast disappearing in Spain being labour intensive which tends to negate the savings by using wood - currently selling for between €100.00 & €150.00 a metric ton here.Thanks for all the input so far Forsythe any other feedback will be most welcome.

[hammer]

I ordered 100Ht online as I could not locate any in Western Australia, I have been wafting nearly a month so obviously lost. I have however been informed of another product called RTZ washcoat so I will be picking some up when I go into the city. Sines reading Forsythes excellent posts I am trying everything to make sure the core which is made from IFBicks is as good as it can be. It frightens me to think about dismantling my build to get to re do the core should it fail.