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

[Forsythe]

Aug 17, 2022 5:05:09 GMT -8 hammer said:

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.

Aw, that sucks you've been waiting for so long on the 100-HT... but it's awesome that you've found yet another zircon refractory coating option! I hadn't heard of RTZ yet, but it appears to be another great alternative for folks in AU and NZ. Thank you for posting about it!

Here's a link to their website for anyone else looking for info on RTZ Washcoat: 
mathews.com.au/company-profile/products/rtz-washcoat/


...And... just in case you can't find RTZ Washcoat in stock when you visit the city: another option in Australia is "Furnascote" from Consolidated Refractories:
consolidatedrefractories.com.au/products/coatings/

Furnascote was originally posted about in the ceramic fiberboard thread here: donkey32.proboards.com/post/37570/thread.

...and if you do come across products from Consolidated Refractories, take note of those caveats mentioned in the above-linked ceramic fiberboard thread, re: their "fibrecote" formulation...and how fibrecote may not adhere to rigidized surfaces.... whereas furnascote is said to serve both functions:
A) as a ceramic fiber rigidizer [read: serving a structural role *within/among/between* the refractory fibers]
—and— 
B) as a refractory coating on hard surfaces... like brick, castable, and pre-rigidized ceramic fiberboard. [read: serving a surface-bonding role as an *outer layer* upon the refractory face.]

[fiedia]

Hello Forsythe

I tried to order the Vitcas Zirconium coating but the price is again higher than the bricks. The product being shipped from UK, adding fret + VAT + import taxes doubles the price.

Once again, I thought fumed silicate would be the only reasonable solution. Then I looked for Zirconium silicate at ceramic material suppliers I found that my local supplier for bricks apparently offers this powder at a very reasonable price (less than 10 Euros/kg).

Is it worth coating my soft HR bricks with an aqueous solution of this powder ?

Thanks for your help

[Forsythe]

Aug 21, 2022 5:44:03 GMT -8 fiedia said:

Hello Forsythe

I tried to order the Vitcas Zirconium coating but the price is again higher than the bricks. The product being shipped from UK, adding fret + VAT + import taxes doubles the price.

Man, that sucks. Have you looked for any of the other alternative options? Like perhaps the Simond Refractories brand of "Heatguard"?

Then I looked for Zirconium silicate at ceramic material suppliers I found that my local supplier for bricks apparently offers this powder at a very reasonable price (less than 10 Euros/kg).

Is it worth coating my soft HR bricks with an aqueous solution of this powder ?

The short answers is: no, I highly doubt you could get zirconium silicate to work the same way, based on what I've been able to learn about how these formulations are concocted.


The longer answer is...

[Disclaimer: this goes into a bit of detail which a lot of folks may not be interested in, but it's the relevant ceramic thermochemistry behind zircon / zirconia's use as a slag-resistant and highly refractory substance —vs.— its use as a glaze opacifier.]

I suspect that the zirconium silicate would not adhere to the brick — not under the firing temperature of the rocket stove (because the rocketstove can't achieve temperatures above the 1600ºC range required to dissociate the Silica (SiO2) from the Zirconium Silicate (ZrSiO4) crystallites— and any coating you made from it and plain water would crack away very quickly with use. (‡)

The cracking would be due to the relatively large particle sizes of the Zirconia in crystalline bond with the Silica grains, which would not have been able to form bonding crystallites with the refractory surface upon drying. Even using isolated zirconia grains in a water suspension would crack away as they phase-shifted between tetragonal and monoclinic structure, since the particles (1) aren't evenly distributed as ultafine, nano-sized grains, (2) aren't already dissociated from the silica as a singe oxide — to then (3) be carried in suspension of colloidal silica that can be applied before re-crystalizing, and (4) aren't stabilized in the tetragonal phase with a rare-earth oxide dopant like Ceria or Yttria. (I suspect some of these companies might be using Ceria as a tetragonal Zirconia stabilizer... but that's only a guess. None of them will reveal their trade secret formulations. Believe me, I've tried to find the recipes!)

The cracking due to zircon temperature-dependent phase shifting was briefly discussed here: donkey32.proboards.com/post/37657/thread

Some clarification of terms might be helpful here, because "zircon" and "zirconia" are very often used interchangeably in common speech, but they are not technically the same thing from a chemical / ceramic / refractory perspective.

"Zircon" *technically* refers to Zirconium-Silicate (ZrSiO4) — which is the naturally-occurring form of the element — you always find Zircon in nature as a crystalline combination with Silica, most frequently as "zircon sand" which is then processed and purified to varying degrees. The Zirconium-silicate your ceramic supplier sells is only purified insofar as to remove the very small traces of radioactive hafnium that often occurs in zircon sand deposits, and then milled into [relatively large] particle sizes to be used as a glaze opacifier. In *that* glaze-additive use, mutiple-micron sized Zirconium-Silicate doesn't crack away because of the fluxes used to make the zircon stay dispersed in a glaze melt ... but those glaze fluxes pretty much defeat any "refractory" characteristics of Zirconia... since it's suspended in a glass specifically designed to melt at relatively low ceramic temperatures, (thus acting as a "glaze") and something that melts at low temperature is, by definition, not "refractory." (Zirconium-Silicate has become a very common glaze additive used on things like commercial porcelain floor tiles, porcelain toilet bowls, and food preparation pottery, etc. and that's why it's in stock at the ceramic supplier.)

"Zirconium" (Zr) is the un-oxidized, metallic, pure form of the element ...much the same way as Aluminum (Al) is the pure, un-oxidixed, metallic form of "Alumina" (Al2O3). [This Zirconium metal is not commonly used, except in very specific metalurgical applications, as it oxidizes on its surface extremely rapidly, just like aluminum does.]

"Zirconia" (ZrO2) is the purified oxide of Zirconium, **which has been dissociated from Silica (SiO2).** (it's sometimes also referred to as "Zirconium Oxide.") It is what is used in technical ceramics, dental implants, and (I think) is what is used in these refractory coatings.


From what I can tell, these refractory products use sub-micron sized Zirconium Oxide in a suspension with colloidal Silica — which I learned is *very slightly* different than fumed silica (in that the silica stays in solute suspension as a "colloid" (unless / until it dries out or freezes, at which point the suspended silica precipitates out of solution.) Fumed silica very nearly "dissolves" into water, but doesn't technically stay in the same type of colloidal suspension indefinitely, because its particle size is ever-so-slightly larger. This tiny difference is why the product "Sairset" (a colloidal silica) is an air-setting rigidizer, while fumed silica has to be fired hot enough to cure and set the very tiny silica particles.

The reason this is my guess about these Zircon coatings:
(A) These coatings — as well as Sairset and other "colloidal" silica products — all carry warnings not to let the unused product freeze... because freezing knocks the colloidal silica out of colloid solution and makes it form silicate crystallites before it has been applied to your refractory surface... which render the product unusable, because it can't be brushed on and made to stay in place as a coating without cracking and falling away.
(B) These coatings — as well as Sairset and other "colloidal" silica products — can all be thinned with water and reconstituted so long as the product hasn't thoroughly dried out, because upon completely drying, the same thing happens (as when freezing) to the silica colloid: it forms crystallites. IE: it "cures."

(‡) There *are* other ways to dissociate the Silica (SiO2) from Zirconia (ZrO2) of the Zirconium-Silicate (ZrSiO4) and form a sol-gel ceramic, having started from the cheaper Zirconium-Silicate used as a glaze opacifier — and thus make it into a DIY refractory coating— And I've been playing with a few of the methods — but they involve a lot time, specific heat inputs, and varying levels of acids to dissociate the Silica, precipitate the Zirconia, then disperse and suspend a ZrO2-rich mixture in an aluminum carboxylate sol-gel that can act as a carrier for applying it. ...It's a very lengthy and involved process that I haven't quite nailed myself... and so I don't recommend that route.

... The vast majority of folks would be much better-off just buying one of the pre-prepared refractory coatings. That's what I would recommend.

Once again, I thought fumed silicate would be the only reasonable solution.

And again, I do NOT recommend using *only* fumed silica —by itself— as the surface coating for those insulating bricks. It might seal brick pores, but it won't be resistant to alkali woodash slag attack upon that sealed surface.

[jorgeenelpueblo]

Jul 26, 2022 5:47:31 GMT -8 Forsythe said:

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

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

Reading this thread has been very interesting at right the exact time, when I was about to order the bricks for my first build.
I am asking now prices of higher AL content bricks.I am wondering now if with this kind of covering, ITC-213, we can build the structure in any material we want, even metal, although I would expect it to be not suitable for the fire camber as it could be teared out by the wood or whatever used to remove the ash.Sorry if this is off-topic, no problems if it is moved elsewhere.

[Forsythe]

Sept 11, 2022 6:49:39 GMT -8 jorgeenelpueblo said:

I am wondering now if with this kind of covering, ITC-213, we can build the structure in any material we want, even metal, although I would expect it to be not suitable for the fire camber as it could be teared out by the wood or whatever used to remove the ash.Sorry if this is off-topic, no problems if it is moved elsewhere.

I would not advise it, for that reason you mention. These coatings are hard and fairly durable, but they could potentially risk being worn away by abrasion from fuel loading and ash removal... Relying solely on such a thin surface coating for maintaining the structural integrity of an entirely metallic firebox would be risky. If a chip in the floor's or sidewall's coating were to occur and go unnoticed, it could spell disaster during peak thermal load on the now-unprotected/exposed metal substrate.

Also, If you look at the technical data sheet posted, under the torch test section, you'll see that the ITC-213 coating only lowers the cold-face temp by 300ºF. If the ITC-213 is used as an intermediate layer between the steel and an upper coating of ITC-296A, then the temp reduction on the cold-face side is 1050ºF. ...So the takeaway there is that you'd need the ITC-213 as a bond-coat for a more insulative layer of 296A over the top of it.... but again: I wouldn't rely on it for physical abrasion resistance in metal fireboxes.

What ITC-213 can do for secondary air tubes is prevent them from oxidative spalling. P-channel and floor-channel tubes self-cool fairly well with the intake of combustion air... so they don't "melt" within the stove... they instead "burn away" — that is: they oxidize at high temp, embrittle, and crumble away in delaminating layers of weakly-magnetic flakes (IE: metallic "spalling"). What the ITC-213 coating can do is adhere to that steel tube surface, forming a thin, ceramicized layer of refractory which is oxygen-impenetrable — thus preventing the O2 from attacking the steel, and thus preventing spalling of the metal.

[jorgeenelpueblo]

Thanks for the aswer, I had not read the 300 degrees difference.

Another question for using this ITC-213 for the P-Chanel: I guess it is only required in the outside of the pipe, the part exposed to fire, because the inside of the pipe will not have the carbon that destroys the metal.

Is that right?

(really good to be looking to ways to make everything more durable)

[Forsythe]

Sept 12, 2022 10:48:16 GMT -8 jorgeenelpueblo said:

Another question for using this ITC-213 for the P-Chanel: I guess it is only required in the outside of the pipe, the part exposed to fire, because the inside of the pipe will not have the carbon that destroys the metal.

Is that right?

I’d recommend coating both the inside and outside of the steel pipe / tube / P-channel / floor-channel. It’s the exposure to oxygen at high temp which is the primary cause of the spalling of the steel.


Alkali ash probably accelerates steel’s oxidative spalling on the outside of secondary air tubes, but if the steel is hot enough, oxygen will attack the metal from both inside and out. The ITC-213 should seal the metal surface to protect from both oxygen and ash exposure.

To get good adhesion to the metal, scuff all the surfaces with some course steel wool, a wire brush, or rough the surface up with a file. Then clean it with some rubbing alcohol to remove any machine oil, grease, etc. Having scratches on the [clean] metal substrate will give you a nice, strong bond— much stronger than if the surface were smooth and scratch-free.

[stinwell]

What is the potential for maintenance and repair of surfaces with these refractory coatings when chips and abrasive wear do happen? Can it effectively be patched up with new coating, or would there be bonding issues?

[Forsythe]

Sept 13, 2022 8:54:25 GMT -8 stinwell said:

What is the potential for maintenance and repair of surfaces with these refractory coatings when chips and abrasive wear do happen? Can it effectively be patched up with new coating, or would there be bonding issues?

Yessir, it can be *very* easily and effectively given a second coat later-on, with no bonding issues (provided that the surface you're re-coating is either A) refractory substrate , or B) newly-exposed steel which hasn't yet oxidized.)

Chipping of the ITC is not common, though. The only places I've seen it happen are on major expansion joints (where the substrate physically moves about 5mm or more during thermal cycling. That amount of movement will obviously crack the hard-coat layer of ITC across the expansion joint.)

The surface which ITC makes after the first firing is essentially like that of a ceramic glaze. Hard, durable, and adherent (within reason commensurate to the substrate's inherent ceramic brittleness and thermal expansion.) ITC is used to rigidize ceramic fiber, after all. It's quite tough stuff.

The surface prep procedure for refractory brick re-coat is stupidly simple: wipe down with a damp cloth until the cloth comes away clean — having cleared away any loose ash particulate resting on the surface. Usually just takes one or two passes with a wet rag on previously-coated ITC-100 surfaces. If you've got a heavily slagged & clinkered firebox which has never been coated with ITC, then you'll want to chip that clinker slag away and get back to the brick surface before coating. Ash clinker and slagged brick surfaces are very subject to softening-expansion at stove temp... so that doesn't make a stable substrate for bonding the ITC coating.


If the surface of the 213 compound chips away from the steel, wipe it down with mineral spirits or rubbing alcohol to clean the metal surface. Let dry, then re-coat.

*Helpful Tip:* If the steel has been left for many, many stove cycles without re-coating the 213-chipped area, there will likely be some steel oxidation. The Solution? grab a sheet of aluminum foil, double it over, lay it flat against the steel that's rusted/ oxidized, and rub it briskly in short strokes along that FeO-chemically-altered steel surface. (You'll want to be pressing firmly & rubbing abrasively enough to wear away the microscopic surface-layer of Al2O3 on the aluminum foil, keeping it in direct surface contact with the oxidized steel such that there's no air gap between the freshly-abraded Al foil and the oxidized iron [steel alloy] surface.)

The chemistry which takes place is rather remarkable. It's essentially a microscopic-scale, cold-temp, "thermite" (reduction) reaction between the freshly-exposed metallic Aluminum atoms [the reducing agent] — and the Iron Oxide molecules [the oxidizing agent to be reduced]. Aluminum has a MUCH greater affinity for oxygen than Iron does. Normally, when you tear, cut, or abrade Aluminum in ambient (oxygen-rich) atmosphere, it forms a microscopic (1-2 nanometers thick) surface layer of Aluminum Oxide. That oxidation reaction occurs within nanoseconds — much faster than you could perceive. (The rapidity of Aluminum oxidation is what makes powdered Aluminum so violently reactive in self-sustaining exothermic Al-FeO "true" thermite reactions.)

The instantaneously-formed Al2O3 surface "skin" then prevents further oxygen from contacting the Al metal beyond that 1-2nm thick passivation layer ...until/unless that Al2O3 is worn away and the metallic Al beneath it is freshly exposed.

And therein lies the rub: The freshly-abraded Aluminum foil (with no air gap between it and the FeO molecules) immediately rips the Oxygen atoms off of the rust molecules, thereby turning the rust *back* to metallic Iron, in situ.

Once you've satisfactorily de-oxidized the steel surface, just wipe it down with mineral spirits to clean; allow to dry; then re-coat. Done-and-done.


Oh, and a note on the shelf-life of unused ITC:

You'll very likely have a good bit of the pint container left after coating air tubes (with 213) or after coating firebox & secondary burn chamber (with ITC-100.) That material will stay usable so long as it doesn't dry out (or freeze) Once ITC dries, it will no longer adhere to desired surfaces. It's toast. ...The containers they ship the stuff in are *very* poorly air-tight... and I suspect the container faultiness is not entirely unintentional. (Most users will never go through a whole pint in one application...but if the company can make the surplus leftover in your container dry-out and "expire" before you have another surface to coat... well... then they get to sell you *two* pints that you'll never fully use-up before they each dry out. ...Evil genius, that.) ...So just portion-out and thin small amounts at a time, and re-seal the pint container inside a ziplock freezer bag at room temp. I've gotten into the habit of ziplock-bagging ITC as soon as I buy it, before I even open it. leaky —even new, unopened— ITC containers are not terribly uncommon. ...Those dastardly evil geniuses.

[stinwell]

Seems like a no brainer for something that increases combustion efficiency, increases the lifespan and maintainability of a core all at once. Currently a dense high alumina (70%) brick from my local supplier is over 60$ each, I consider it a small investment to protect those bricks for the $ and the time spent building with them.

While we are in the business of optimizing things can we do the mortar and sand as well?


lancasterlimeworks.com/basics-of-sand/

www.limeworks.us/lets-talk-sand/#:~:text=Our%20Ecologic%E2%84%A2%20Mortar%20uses,hard%20and%20will%20not%20crumble.

It seems like if we were going to purchase something like Aluminum Oxide sandblasting grit to replace mortar sand it would be wise to mix some percentage of fine, medium and coarse grits, or tailor the mix towards the particular goal. The aluminum oxide seems like a good choice as well because it is very angular, which lots of sandblasting medias are not. 

Is it possible we want at least some silica in the mix for the same reasons we want at least a tiny portion in our high alumina firebrick, or would the naturally occurring silica content of most fireclays/binders be providing that already? 

Forsythe perhaps I can also prod you into talking about potential reasons to select one clay over another? Fireclay vs. pure kaolin? The only fireclay I can source without extra effort is Hawthorn 50 or 30 mesh. I am far from informed on the subtle differences of Kaolins (and clays in general), but there are a ton out there! Maybe a few that are commonly available from ceramics suppliers are worth noting for our uses here.

[Forsythe]

Sept 20, 2022 6:20:17 GMT -8 stinwell said:

Currently a dense high alumina (70%) brick from my local supplier is over 60$ each, I consider it a small investment to protect those bricks for the $ and the time spent building with them.

Ooof, that seems awfully high for mullite brick. (~70% Al2O3 : 28% Silica is approximately the ratio for stoichiometric mullite, and is an outstanding choice for firebox material) … but $60 USD per brick is at least double or more the price I would expect, all shipping/handling costs included.

Sheffield Pottery normally carries them for ~$9-$10 USD per brick, (prior to shipping cost) but they’re currently out of stock. Might be worth a call to see about backordering or expected resupply timeframes.

www.sheffield-pottery.com/LVA70-70-ALUMINA-STRAIGHTS-2-5-p/lva70.htm


Here’s an online seller of 70% Al2O3 dense brick for $231 USD per carton of 25 bricks. (Orders less than 1 pallet incur a $50 broken pallet fee…so a single carton of 25 bricks would be about $11.25 per brick — prior to the cost of freight shipping, of course, which is going to be the biggest cost factor for folks like you and I who live wayyy back in the woods / mountains / off-grid.) Ordering more than >1 carton will lower the price-per-brick slightly, as that $50 “broken pallet” fee only applies once per order.

www.ceramaterials.com/product/high-alumina-hard-brick/


If you haven’t tried the pottery supply route yet, I’d consider searching for the closest pottery supply center to your locale. You’ll be looking for a place that supplies firebrick for potters building "wood-fired salt kilns” AKA "soda kilns” —which require those 70% alumina brick (to withstand the sodium bicarbonate or sodium chloride briefly blown into the kiln for the flash-glazing effect on pot surfaces toward the end of the firing schedule.) Driving to a pottery supply store to pick up an order of firebricks is spectacularly cheaper than trying to order online and have them shipped via LTL freight to your home. (The freight cost — if you live in a place like I do — will be as much or more than the cost of the bricks themselves. Which is infuriating.)

An alternative to locating a pottery supply dealer which has those bricks already in stock — is to negotiate with a local hardware / building supply retailer or warehouse —one which doesn’t have the bricks in stock, themselves, but has a freight dock that can receive your order. For a small fee paid to your local folks, you can usually arrange to have your order received by that local business, and they’ll contact you when the shipment arrives so you can bring it home. MUCH cheaper than trying to negotiate with an LTL freight company for home delivery on a tractor trailer with a lift gate... if they're even willing to go to your off-grid house.

…Or you might see if one of your local masonry supply houses — who, again, doesn’t carry that 70% Alumina brick in stock — but might have an account with a brick vendor who distributes 70% brick. That would make it possible for them to place your order along with a larger order they make, themselves — which would further lower your shipping costs, as your order would be palletized along with the masonry supply house’s other order items. (The downside there is a likely longer lead-time on delivery… since you’d have to wait for the masonry supply house’s regular restocking cycle & eventual order placement via their brick distributor.)


Another 70% Alumina route, which I’ll only mention for other forum members reading this, is to purchase 70% Alumina castable refractory. That’s usually cheaper than buying (and shipping) the pre-fired bricks. However, that option requires you to precisely hydrate, then sufficiently vibrate the mix to fill evenly throughout the mold forms (but not over-vibrate, causing the aggregate to settle out in a bottom layer, with fine particles and cement accumulation on top) …then carefully dry, and finally heat the casts in a very controlled slow-ramping of the temperature — up to & above the stove operating temp in order to properly cure and sinter the cast.

Proper casting and drying, followed by the controlled cure-&-sinter firing schedule are needed to (1) avoid the all-too-common issue of freshly-cast slab cracking — and (2) to sufficiently sinter-bond the refractory particles/grains, which is necessary to impart the refractory’s intended corrosion resistance. 

Many, many people — even including the “professional” operators of industrial fuel-burning power plants and incinerators — fail to properly heat-treat their castable refractory installations, and that failure results in decreased refractory service life. Most of these folks don’t realize that the temp-, strength-, and slag-resistance- info supplied on the castable refractories’ technical data sheets is only true if the heat-treat firing schedule is followed diligently. (I know stinwell mentioned the inability to do a controlled-firing schedule at his off-grid location, so using castable refractory in his case would be iffy at best.)


...There's a lot to cover here, so I'm breaking my response into segments and will reply to your other questions in follow-up posts shortly.

[Forsythe]

Sept 20, 2022 6:20:17 GMT -8 stinwell said:

While we are in the business of optimizing things can we do the mortar and sand as well?
It seems like if we were going to purchase something like Aluminum Oxide sandblasting grit to replace mortar sand it would be wise to mix some percentage of fine, medium and coarse grits, or tailor the mix towards the particular goal. The aluminum oxide seems like a good choice as well because it is very angular, which lots of sandblasting medias are not.

You are 100% correct on the distribution of particle sizes and sharp-edged grains, good instinct there. A mix of sizes is preferable to having just one single size of grain, for a handful of reasons.

Lars Helbro (creator of the “Gymse” and its “Centrifugal Carburetor” secondary combustion chamber design) put it this way in an email to Norbert Senf of the Masonry Heater Association of North America:
www.mha-net.org/docs/temp/080817lars.htm

I read with big interest the debate at MHA´s site about cracks. Here in Denmark, we almost only build single-skin heaters, mainly because of the weather conditions. Of course we experience cracks too, but through the years (about 20) we have found ways to minimize them, that are not mentioned in the discussion, and maybe it could be useful to some.
 
Why they come: As one (I think Alex [Chernov]) said in the discussion, the brickwork is not heated up equally. On a contraflow heater, the sidewalls will be heated up first, and from the inside. On a single skin heater, this will happen quite rapidly. When the inside is heated up faster than the outside, the walls will try to bend, since the inside will expand faster than the outside. The forces are away from the middle, and since there very often is less brickwork in the face than the back, the cracks will occur here. (This is only a theory, but it corresponds with natural laws)


What we have experienced that helps us:
We only use clay mortar, since the cracks will be very easy to repair for the customers, and we teach them how to do that. I very much agree with Alex, that it is important to teach the customers what might happen, and how to maintain their stove. (A little water and a thumb will often be enough, when it is done in time).

 Most important here though - the quality of the sand in the mortar has a great effect on how big, for how long the cracks will be. With very good quality sand, they might not come at all! The importance of the quality is, how sharp the sand is and the mix of different sizes. We use 0-4mm in a (if possible) reasonable equal amount of every size.
 If the small sizes are missing (0-2mm) we have to use more clay, and be aware of the risk for drying cracks.
If the bigger sizes (2-4mm) are missing, the risk of [thermal expansion] cracking is bigger, and will come back more often after repair.
 Round sand particles (e.g. from the beach), will make troubles for the customer for the life of the stove.
 
Many years ago I built a masonry heater for my grandaughter in Norway. The sand was sharp as broken glass, and my fingers were bleeding before I finished. This stove has been used very heavily ever since, but has never shown any signs of cracks. 
I think that sharp sand has about the same effect as metal reinforcement, but it will stay there forever.

About the castable quality:
 As you maybe can see at the pictures of Bujnas, we have given up upon this product. From our experience, it only makes trouble. After we quit that (about 8 years ago) we could fire up every single stove we built immediately after it had been closed. [meaning: the brickwork encasement of the flue path had been completed; the stove “enclosed”] This has given us much more experience with every single stove, and we can keep warm when we make the finish if it is in the winter time. (In the summer it can get quite hot, but we do it anyway - of course). The customers are very pleased with that, since there is no bigger trouble, than new toys you are not allowed to play with immediately.
 
I think that this also has some importance for the decreasing of cracks. When we fire up a new stove, the mortar is still very wet, so the stresses can much better be absorbed in this period, that often last for a couple of days with a reasonable fire each day. Like children, they can learn more easily what its all about, when they are soft and easy to bend :-)


—> An important note here: Lars was referring to the sand used in the clay mortar for the outer walls of the heater. 
For the firebox and secondary burn chamber walls (the internal-most combustion zones exposed to the highest heat), he says elsewhere that he uses crushed firebrick grog instead of silica sand for the clay mortar aggregate... which leads into your next question:

Is it possible we want at least some silica in the mix for the same reasons we want at least a tiny portion in our high alumina firebrick, or would the naturally occurring silica content of most fireclays/binders be providing that already?


The silica content of clay and refractories actually gets incorporated into either mullite crystals or glassy amorphous silica in between alumina grain boundaries — so it doesn't exist as crystalline silica after firebrick / refractory sintering or vitrification the way that silica sand grains do. It's kinda like how —in cooking and baking— egg interacts with bread batter, and can either be incorporated into the cooked bread, or be a cooked egg encased in breading... all depending on whether you break the white and the yoke and beat the egg into the batter. In the former case, the egg disappears into and becomes part of the structure of the breading — whereas in the latter case, the egg cooks inside of a bready encasement, never becoming part of the breading itself.

Silica sand grains added to clay would be analogous to cracking a whole egg into a pocket of bread batter, and allowing its yolk and white to cook separately from the bread pocket. In both egg/bread and silica/clay cases, the cooking [firing] will elicit different sets of chemical reactions, based on how finely pulverized and well-blended the starting components are.

Silica sand is in the form of quartz grains — a silica crystalline phase. Quartz, when rapidly thermally cycled through the temps produced by our stove’s firebox and secondary burn chambers, will go through polymorphic crystalline phase shifts called “quartz-cristobalite inversions.” Those rapid shifts in the silica’s crystalline structural shape coincide with abrupt .05-1% linear expansion & contraction of the silica sand crystals — which can be destructive to any ceramic bodies and/or mortar beds in which those quartz/cristobalite sand grains reside. …So while using sharp silica sand on the outer walls of the heater is perfectly acceptable — it could be harmful [by promoting cracking] in the firebox / burn chamber walls. The corollary is: you don't have to use white alumina sandblasting grit (or crushed firebrick grog) for the entirety of the flue path — the post-combustion areas don't need it.)

I feel I should also reiterate that —just like the recommendation for white fused alumina sandblasting grit or high-alumina grog in the clay mortar for the firebox and burn chambers— the entire heater doesn’t need to be made of these more-expensive 70% Alumina refractory brick. Downstream from the actual combustion zones, the temp drops off pretty rapidly, and in all those downstream heater walls —anywhere the temp doesn't exceed the 525-575ºC range (or about 1000ºF)— it’s perfectly acceptable and durable to use a lower-alumina dense firebrick, clay-mortared with sharp silica sand as the aggregate. The only advantage to using high-alumina brick downstream would be the very small boost in heat storage capacity attendant to the high alumina content… but that tiny improvement to thermal storage capacity doesn’t justify a greatly higher build cost.

So, unless there’s a possibility to actually end up saving money by buying 70% brick in bulk —perhaps, for example, you may be able to obtain a substantial discount and reduce your total shipping costs by purchasing an entire pallet of all just that one type of 70% alumina brick— then downstream mass and outside heater walls should be made with more standard (and usually more affordable) dense firebrick. …But each person is going to have to run their own numbers for where they live, the shipping costs, what’s available, and/or the feasibility of various DIY routes for their individual, unique circumstances.

Likewise, buying white fused alumina sandblasting grit isn't exactly cheap... but you won't need vast quantities of it for the combustion zones' clay mortar.

...and I do recommend buying the white alumina grit vs. the brown alumina, even though it's slightly more expensive. The brown variety usually has substantial iron content giving it the brown coloration, and that iron impurity isn't great for refractory durability. Iron oxides lower the softening point of refractory clays, and iron can also catalyze destructive carbon reactions within refractory bodies, even at relatively low temps, which can weaken and compromise the refractory... so the lower the iron content, the better the refractory properties — that goes for any clay or aggregate used in the firebox, flame path, and downstream flue path.


... I'll follow up with a response to your other Qs on clay types shortly...

[Forsythe]

Sept 20, 2022 6:20:17 GMT -8 stinwell said:

Forsythe perhaps I can also prod you into talking about potential reasons to select one clay over another? Fireclay vs. pure kaolin? The only fireclay I can source without extra effort is Hawthorn 50 or 30 mesh. I am far from informed on the subtle differences of Kaolins (and clays in general), but there are a ton out there! Maybe a few that are commonly available from ceramics suppliers are worth noting for our uses here.

This PDF document is a USGS report that gives a really good overview of kaolin/halloysites, fireclays, ball clays (the common, high-firing, plastic, stoneware-type clay used for pottery and sculpture,) and refractory clays — what they are, how they’re typically formed, and the geologic features where they’re generally found— plus the locations of economically significant kaolinic-type deposits in the title areas.

Kaolin, Refractory Clay, Ball Clay, and Halloysite in North America, Hawaii, and the Caribbean Region: Geologic descriptions of kaolin deposits occurring at 46 localities (1984)

pubs.usgs.gov/pp/1306/report.pdf


When viewing the above document, it’s important to note that USGS has (for the most part) only highlighted here the largest deposits — and thus the deposits which are the most economically feasible for industrial mining. Smaller kaolinic deposits are widely distributed —nearly anywhere there are / have been granite or gneiss intrusions into the continental surface. (and often just downstream of those granitoid formations— in the surface watershed— frequently concentrated at the drainage mouth of glacially-formed ponds, lakes, and river deltas… )




Broadly speaking, Kaolins (and Halloysites) are weathered from igneous rock (most often granite) via hydrolysis. The weathering caused by water and oxidative acids (both inorganic mineral acids and organic carboxylate acids) selectively leaches out the soluble mineral salts native to granitoid lithology (potassium, sodium, magnesium, iron, calcium). These salts, being soluble, are much more mobile in the watershed than the less-soluble silica and alumina fractions in granite’s crystallites, so they’re percolated out and washed away from the remaining aluminosilicates, carried downstream by rivers to their eventual accumulation in the ocean, where they’re responsible for our oceans' salinity. 





Kaolinization of granite happens all around us, all the time… even to quarried granite in urban construction. Here’s an 18th century church in Spain being weathered by sulphuric acid and moisture (fossil-fuel polluted rain), microbial nutrient-waste ion exchange, and annual cold / hot-humid cycles. The stone structure is slowly having the K, Na, Mg, Fe and Ca soluble salts washed out of its granite and lime mortar— turning to kaolin with calcium sulfate (gypsum) scaling, right where it stands:
www.researchgate.net/profile/Nick-Schiavon/publication/227320174_Kaolinisation_of_granite_in_an_urban_environment/links/58596dc708aeffd7c4fd0a3d/Kaolinisation-of-granite-in-an-urban-environment.pdf?origin=publication_detail




Native Clays


When the hydrolytic weathering is allowed to proceed in situ without fast-flowing water currents or high winds washing away the alumina and silica along with the mineral salts, the weathered granite rock progressively degrades to feldspar sand and ultimately to kaolin (or halloysite. …Halloysite is just hydrated kaolin with tubular —rather than platy— particle morphology.)

Since these types of kaolinic clay still remain deposited at the site native to the parent feldspar-granite rock from which they were weathered, they’re called “native clays” — AKA “residual clays” AKA “primary clays.”


Native granite-derived kaolin clay is generally considered the purest, whitest-burning form of kaolin, because it hasn’t been completely washed away and co-mingled with impurities in downstream sediments… even though you often have to screen relict feldspar sand out of it, and there may be some residual iron streaking within the clay seam (because iron has so damn many different forms of oxides and oxyhydroxides —17 of them known to date— which it loves to shift between, and so cling stubbornly to otherwise-purified, water-washed aluminosilicates.

 Native clays are also usually close to the surface, with minimal overburden soil covering them. Native-clay kaolin is typically fairly low in plasticity compared to ball clay (because of the large, stacked-book-like, hexagonally platy shape of the clay particles.) But if there's a presence of rounded halloysite tubules in the kaolin, it can be surprisingly plastic and easier to work with. Once a native clay is washed away and tumbled with other sediments, those large platelet particles of kaolin become smaller and more rounded — which also increases its plasticity.



Kaolin-like clays can also be formed from other igneous rocks like basalt… but… because basalts & other extrusive igneous volcanic rocks (and ash) cool & harden from magma much more rapidly than granites do, their crystal structure is poorly developed, disordered, and super-finely grained… (with the soluble salts, silica, and alumina homogeneously mixed throughout.) The lack of large, ordered crystallinity —dividing the rock’s mineral components along cleavage plains which would allow water to penetrate along ionizable crystal faces— makes selective hydrolysis of soluble minerals from basalts and volcanics a lot more rare. Hydrolysis tends to weather away all of basalt’s minerals uniformly, spheroidally, from the outer-rind inward, resulting in little or incomplete in situ purification. The clay thus produced from volcanic rocks is the superfinely-particled, ionically-charged, water-absorbing (and “swelling” or “expanding”) clays of the smectite / bentonite group. (These swelling clays are the ones most common in the Northern and Central Great Plains… where they’ve coalesced mostly from volcanic ash depositing on the flat, featureless surface over the millennia.)




Bentonite/Smectite “Swelling” Clays

The swelling “bentonite” clays are only useful for refractory bricks/ceramics in extremely small amounts as additives — their content of fluxing mineral salts make them melt at low temps (therefore non-refractory); their superfine particles and ionic charges make them capture and hold onto impurities as well as huge volumes of water, causing enormous volumetric expansion when hydrated. All of which makes these bentonites/smectites nigh impossible to shape and then dry uniformly without cracking. (Their enormous capacity for water absorption makes them useful for kitty litter, drilling mud, and cleaning up chemical spills, but terrible for forming ceramic bodies and mortar joints. The in-ground bentonite/smectite clays are also responsible for the soil swelling that’s destructive to building foundations and walls.)








However, because of these properties, bentonites are hyperplastic, and by adding very small amounts (usually <3%) to very-high-alumina minerals— which are so extremely non-plastic that they’re difficult to form into cohesive shapes— the entire non-plastic alumina mix is suddenly given enough cohesive plasticity to mold. …Also related to their hyperplasticity / ionic charge / fine particle size: bentonites added at 1-3% to slip-casting mixtures can deflocculate all the non-clay heavy particles which would otherwise settle out of the slip too rapidly — suspending them in an evenly-dispersed bentonite colloid, capable of being poured into slip-casting plaster molds.




Transported Clays



Kaolinization of basalt and volcanic tuff *can* happen, but the process is a bit more complicated… and results in the prodigious varieties of “fireclays” you’ve heard about. These kaolin-like fireclays are almost always formed far away from the parent rock, and only achieve concentrated accumulation of the aluminosilicate fraction after being transported to- (and deposited in) a continental basin where they can be purified by other mechanisms. Thus they’re called “transported clays.”




Transported clays have an infinite variety of composition — everything from ball clay and flint clay, to diaspore, gibbsite goethite and bauxite. They can have all number of impurities, or almost none. They can range from magnesium-rich illite bearing very little alumina — to alumina contents as high as 80%. They can be plastic or highly non-plastic, but most (aside from montmorillonite) are non-swelling. Transported clays are rarely as white-burning as native clay kaolins, but there are exceptions.

• 

(In fact, the two largest and best-quality kaolin deposits currently being mined in the US are *technically* transported clays…They’re located in Washington County, Georgia, and Latah County, Idaho, respectively. They’re unique hybrids in that they both originated from feldspar-granite, and were accumulated by combined geologic and climactic changes to the parent granites’ drainage sheds. In both cases, the kaolins outflowed from their native deposits, but were captured and accumulated just a handful of miles’ distance from the parent rock by newly-formed ridge lines, preventing them from washing away entirely. This type of semi-transported accumulation makes for larger, deeper concentrations of kaolin which are favorable to economically-profitable industrial exploitation… but such large deposits are highly non-representative of kaolins’ typical, widely-distributed occurrences.) 





• Related: The kaolin I personally have been working with is from the Latah formation. The current commercial name for that clay is “Helmer kaolin.” Latah’s kaolin was also the very-white “fireclay” used by the now-defunct AP Green Firebrick Company, which used to dry-press one variety of their high-duty “Empire DP” firebrick with clay sourced directly from the ground at their old facility in Troy, Idaho. (An older, alternate name for the kaolin is "Troy Clay" — both Helmer and Troy being towns atop the Latah formation with exceptionally high-quality beds of kaolin at/near the surface.) Helmer Kaolin is remarkably plastic for a kaolin, especially for how pure it is — due to its fraction of halloysite tubules. It’s fantastic for refractory bodies, and cheaper than the slightly-whiter, yet less-plastic Georgia kaolin prized by many potters for pure-white porcelains. Helmer kaolin also has about 38% alumina, slightly less than Georgia kaolin’s 43-44%. A small fraction of Titanium oxide is the principle impurity, followed by trace iron, which have minimal impact upon refractoriness, and are useful as sintering aids at the low percentages present in Latah/Helmer/Troy kaolin.)




Fireclays

The class of transported clay generally known as “fireclay” is most commonly found as the “underseat” of a coal seam… (which is —or rather, was— the clay pan underlying a marshy swamp, preventing basin drainage.) The stagnant water underlain by an impermeable clay pan gradually accumulates organic acids (carbon- and hydrogen- based acids) from the organisms that live atop the nutrient-dense marsh, secrete waste into it, eventually die and submerge into it, where they never fully decay, due to the stagnation of the water held in place by the clay pan underseat. This accumulation of organic acid hydrocarbons increasingly acidifies the marsh, forming a peat bog. The abundance of organic acids in peat progressively leaches the soluble mineral salts (potassium, sodium, magnesium, iron, calcium) from the clay underseat, while the off-gassing of methane and sulphur compounds which bubble upward brings those dissolved nutrients to the surface, where it encourages further plant and bacterial growth on top, perpetuating the accumulation of successive peat layers.




On average, most of these “fireclays” will have slightly less alumina than native, granite-derived kaolins, and will contain higher amounts of fluxing metallic impurities like iron.






Iron- and magnesium- digesting bacteria coalesce in the bog water (being one of the few anaerobes which can survive submerged there) and consume those metal salts via an aerobic metabolic process which generates waste byproducts of (1) reduced iron (This is the origin of “bog iron” ore, and of the high-iron “red mud” content in bauxite alumina — and (2) dissolved hydrogen sulphide gas…which is highly corrosive. The hydrogen sulphide bog-acid solution can, under the right conditions, leach even the silica and some of the alumina out of the clay pan underseat, leaving highly-concentrated alumina minerals like diaspore, bauxite, gibbsite, goethite, etc… but the bacteria which create this environment also die and re-deposit their bio-accumulated iron into the peat/coal/fireclay somewhere in the coal seam strata…somewhere in time….meaning somewhere in the layers. If that happens to be above a few accumulated, successive layers of coal and clay sheets which keep it from redepositing to the clay pan — well, then you have a semi-purified underseat of high-alumina, kaolin-like fireclay… which is mineralogically unique to that one single layer of whichever one particular coal seam created it.




This is why there are an endless variety of named clays —whose names are often a nod to the coal seams from which they’re sourced— and is why trying to learn about them all is …. not useful. (‡unless you’re a potter trying to replace a specific clay that has run out of supply in order to match its firing shrinkage to a specific glaze in order to prevent glaze cracking…which isn’t quite relevant to the field of refractories)



Since these fireclays are so widely varied in purity/contaminant composition, each unique to themselves, and each of a finite and exhaustible supply… it’s instead more useful to talk about these various clays in terms of:


• 1) their alumina-to-silica ratios, (On average, most of these “fireclays,” —being transported clays— will usually have slightly less alumina than native, granitic kaolins, along with more impurities. The highest-alumina “clay-like” mineral layers (e.g. Bauxite) in these deposits don’t have enough plasticity to be considered “clay” in the traditional sense of the term, and thus won’t stick together for forming bricks/shapes or mortar beds without a plasticizing additive.
)

• 2) their impurities of significance to refractoriness (chiefly iron, but occasionally also calcium, sometimes magnesium. Also on average, most of these “fireclays” will contain higher amounts of fluxing metallic, metalloid, and rare-earth-mineral impurities than native, granite-derived Kaolins. ...The presence of higher impurity content within a clay body *prior* to ash/flux/slag exposure will often cause that clay body to be more reactive to slag. Said another way: between 2 clay bodies having the same starting PCE [refractoriness] — the one with fewer starting impurities will often be the one more resistant to slag penetration between the two.)



• 3) their Pyrometric Cone Equivalent ["PCE"] (which is a measurement of a clay’s “refractoriness” — expressed in the units of heat-work a clay body can tolerate before experiencing deformation or destructive alteration to the clay body’s molded shape. Pyrometric Cones are sets of tall, skinny, triangular pyramids of precisely blended clays & fluxes, designed to bend or “slump” when each respective cone number’s heat range is surpassed in the kiln, indicating the “heat-work” (temperature + time) thus-far applied. Pyrometric cones are expressed as numbers like cone 06-01 for low-fire ranges of temperatures, and cone 32-35 for purified fireclays, kaolins, and the high-alumina bricks made from them. Some of the highest alumina-content clay minerals (which are hardly “clays” by the conventional standards of clay plasticity) can reach PCE of 37. Below is a chart I’ve compiled of PCEs from the major pyrometric cone manufacturers’ numbers, which extends into the refractory temperature range. [Most PCE charts only show values up to a maximum of cone 14 — the upper end of the range useful to potters, who typically don’t make refractory bodies, themselves …Other charts only show the higher PCE numbers for industrial refractories, without the lower plastic clay PCEs. I’ve been unable to find one spanning both ranges with figures in ºC and ºF, so I compiled this one. Image & link to Google Sheets spreadsheet below.)






docs.google.com/spreadsheets/d/1B9fnSFdmmFp2wk-CbTmHgcoA0pr1N3WCMf4kqlTMqmE/edit?usp=sharing”



• 4) their plasticity (a “plastic” clay can be easily formed/deformed with water for pottery shaping, and they're “sticky” — plastic clays are often needed as the binder portion in high-alumina brick compositions (or those containing a high percentage of grog) for the green shape to hold its form between the molding, drying, and loading into the kiln— before firing creates sintered bonds that permanently hold it together. They also contribute to the glassy phase during firing, which precipitates the evolution of mullite crystals. And plasticity is also an important characteristic needed for clay mortar beds.)


• 5) their “Loss On Ignition” [“LOI”] (which is a measurement of their firing shrinkage — due to mass loss resulting from the off-gassing of molecularly-bound water and of the more-volatile contaminants which “burn out” of the clay during firing or calcination. ...Side note: if you're making your own bricks, there will be additional dimensional shrinkage *without mass loss* from particle packing and increasing densification during the higher temperature stages used in sintering the clay. This second type of firing shrinkage is more apparent with native-clay kaolins which have larger particle sizes, and can be mitigated by things like pre-calcination, mullite formation from the glassy phase, and/or grog additions like kyanite, andalusite, etc. which expand slightly as their crystalline structure dissociates into its alumina and silica fractions and precipitate upon cooling to form mullite.)





Also worth noting: 

The transported alumina and silica leachate —which percolates upward into coal seams from the underseat fireclay layers— is why coal fly ash is useful as a pozzolanic / geopolymer concrete material; as the main constitutent of coal-slag sandblasting grit; and is even sometimes used as a raw material for low- to medium-duty firebrick….whereas wood fly ash, (lacking those alumina and silica leachates) is not useful for such things.


It’s also been recently researched/documented that the alumina and silica residues in coal slag —which had normally deposited in the fireclay refractory walls of previously-coal-burning power plants— actually helped to preserve the refractory walls from the alkali attack of potassium, sodium, and calcium slags … and now that many of those plants have been converted to burning biomass waste instead of coal… their fireclay refractory linings are wearing out approximately 4-6 times as fast as when they were burning coal. (this isn’t a defense of coal burning whatsoever, just an observation in the importance of —and increased need for— mitigating refractory corrosion in biomass burners. The “common wisdom” regarding fireclay’s refractoriness specifically under the conditions of burning woody biomass has been a bit slow to wake up to the differences in corrosion kinetics as compared to the long-held industry practices / assumptions when burning coal, oil, or gas.

[josephcrawley]

I have used Hawthorne 30 for quite a few stoves over the years and have no complaints. For sand I use the fine bagged play sand available at the various box stores. This is mixed in a 2/3(clay to sand) ratio by volume. All my heaters are single skin and I haven't had any significant issues with cracking.

For the firebox and various risers I use sairset which was recommended by another heater builder. It is rather hard on skin so wear rubber gloves. I have also used heat stop 2 but it is rather pricey and comes in a bag so not so good for storage between jobs.

In the past I experimented with fireclay sand mortar with sodium silicate as a setting agent with mixed results.

[Forsythe]

Sept 25, 2022 8:54:03 GMT -8 josephcrawley said:

For the firebox and various risers I use sairset which was recommended by another heater builder. It is rather hard on skin so wear rubber gloves. I have also used heat stop 2 but it is rather pricey and comes in a bag so not so good for storage between jobs.

In the past I experimented with fireclay sand mortar with sodium silicate as a setting agent with mixed results.

Yes, good call on the Sairset over sodium silicate. Sairset contains colloidal silica, whereas sodium silicate introduces a lot of sodium, (which is a ceramic flux best to avoid or at least reduce as much as possible.) Having sodium already in the mortar will make it degrade faster when more sodium (and potassium) is introduced to its surface by wood ash.

You can get away without using an air-setting, bonding rigidizer/hardner (which colloidal silica and sodium silicate both are) in the firebox and secondary burn chambers if your design allows for the bricks/slabs and mortar joints to be held in place by gravity alone. With plain clay mortar, it's easier to design a rebuildable firebox liner that can be replaced when/if the bricks wear out. ...but with cantilevered overhangs, corbelling, etc. it's best to use a bonding mortar to keep them set in place and sealed, before firing cycles can harden clay mortar. 

It's also a trade-off between keeping joints sealed by setting them with a rigid bond —vs.— allowing for thermal expansion stresses to flex the mortar joints to avoid destructive cracking somewhere unexpected.