" Fake fire brick "

O. K. Now that I have your attention, I want to review what is actually what is out there and being contemplated for use as a fire brick in making the core of our
Rocket Mass Heater RMH !

The Best fire brick is a 9'' x 4.5'' x 2.5/3'' and its 'split', 9'' x 4.5'' x 1.25'', the whole brick is light, weighing 28-30ounces, less than a quart (or liter ) of milk !

The second best brick is 100 year old soft, red/red orange, house brick, in good condition,with clean edges and motor removed! Used to make the core of our
RMH and backed with good high temperature insulation or with refractory insulation These 100 yr old bricks can make a surprisingly good RMH.

There is a third kind of brick called fire Brick and sold as fire Brick, it can be found in many sizes and shapes including the nominal sizes above, it is a very hard,
dense brick usually weighing more than 7 POUNDS , This is actually a kiln brick used for its durability, AND its ability to absorb and hold high temperatures
for long periods of time where long heat treating can anneal, strengthen or harden materials "placed in a KILN " !!

Because it is actually a poor insulator as compared to Lightweight Fire Brick, or 100 year old red orange house brick it is definitely a distant 3rd choice for use as
fire Brick in making our RMHs Core !

This is a very important distinction because much of what is presently available for sale in Lowes or Home Depot or other big box Builders Supply houses is
ONLY the hard, Dense 7 POUND BRICKS WE DO NOT WANT ! I Hope this is timely and helps ! For the Good of the Craft ! Big AL

Hi Al; You got my curiosity up, so off to the frozen brick pile I trooped. After thawing ,I have three types of brick here. Two are from a sawmill boiler both I measure at 9 x 4 1/2 x 2 1/2 the (heavy)one weighs in at 7.6 lbs the (lite) one weighs in at 4.4 lbs and last is a newly bought red clay brick from home depot weighing in at 5.7 lbs & measures at 8x4x2.5 . I would imagine that there is a variance in insulative brick as far as the weight goes. Most lighter weight bricks were broken where I got mine (busted pallets ) And not knowing any better I mostly got the heavier (better) ones , I did find & bring home a few of the lighter bricks . My question is should i use both in my next stove,or stick to one or the other ? I'm thinking that the feed tube side might be ok with the heavy bricks and use the insulative bricks for the burn tunnel & start of the heat riser ? I may be able to go back and get more of the lighter brick I'll have to check. Thanks Tom

I think you are right about the feed tube being OK with the heavier brick, as well as the floor. The ones that have more abrasion damage than heat.

I would use them for the entire base and compensate with extra perlite if I had to.

The lighter bricks can be HARD to find, check on knifemaking and hobby blacksmithing forums for local sources. They cost more, but are worth it for the riser at a minimum.

Actually any good brick outfit should keep the lighter true fire bricks on hand. Usually they are light in color as well as light in weight.(not always true) They are also usually the highest temperature rated bricks the brickyard will have on hand.

O.K. What would I do if I had all the lightweight Fire brick I wanted or needed -> Except for being 4 or 5 bricks short ?!

First I would use the 100 year old soft red/red orange house brick picking out my cleanest and best, and always putting the good side in towards the Feed Tube
and finally progressively adding in towards the Burn Tunnel !

If I had to use the 100 year old soft house brick for my burn tunnel floor, I would fall back on the original design and include an ash pit that I would mostly keep
full of consolidated fly ash,* with the floor of the ash pit lined 1 or even two bricks wide towards the opening of the Heat Riser !

If I were still some bricks short, I could easily justify using the heavier bricks at the base or under the base of my Ash Pit, at the bottom of my Feed Tube -
expecting that those heavier bricks would be themselves be insulated from the heat of the hot gases of the combined Combustion chamber -> the rest of the
Burn Tunnel and the Heat Riser! I would also get some satisfaction out of the knowledge that the heavier and harder fire bricks would take the abuse of random
chucks of wood being dropped on them! (especially if further protected by the heat AND impact protecting consolidated Fly Ash!)

* O.K.My own little dirty secret (pun intended) I built my Burn Tunnel for my last 8'' system with a Burn tunnel height of 9'' (9'' by a full 6'') and then back filled
with almost an inch of wood/fly ash, to insulate and increase the longevity of my lighter fire brick !

I hope this is timely and helps - For the Good of the Craft !Think like Fire, Flow like Gas, Don't be a Marshmallow! All comments/questions are solicited and Welcome Big AL

T.R. I expect that the brick that was found to be in the 4 pound region has absorbed more than twice its weight in water and should be carefully dried inside before attempting to use,
other wise I expect it to have a very short life ! A. L.

Hi Guys; Thanks for the responses. Al i'm sure that you are correct about water weight gain , that pile of brick was under ice. I will be going back to the sawmill this spring to see if more of the lighter bricks can be found. The price is right (FREE) but all are in a pile that used to be on pallets & now are in a pile half buried in wood debris currently under 2'of snow. Most of the softer ones were broken and i will have to dig down to see how much buried treasure i can unearth. I am confused about water soaked bricks ? Ianto's book said to be sure and soak your bricks before use so..... should I dry out mine ?

Thomas Rubino : I like free, and the old soft house brick, Ditto Ianto Evans, I am sure he would use free fire brick if it came to him, basically when you lay
your bricks up with mortar if you don't soak them a little they suck all the water out of the mortar before it can make a good bond ! That is still the preferred
way to do the Heat Riser! If you want to lay up your Fire Bricks for your core by dipping them in clay slip and slapping them to place that way (I have not torn
into a Rocket Burner after using this technique) You will still have to dip them into water first 5-10 seconds depending on how thick your clay slip is ! It is a very
good idea to practice build your core a couple of times, just to be sure you have enough bricks and can look at the outside of your stacked bricks and see whats
on the inside ! For the Craft! Big AL

On this point, Al, I beg to differ.

Red brick, old or new, is full of iron oxide and will conduct heat faster than any fire brick. In all of my experimentation with building and burning cores, I found that red brick, no matter what variety, did not preform as well as fire brick. Out was hard to achieve high temps and I had several bricks fracture from the heat stress.

The fact of the matter is that all fire brick is mainly composed of paloma grit which is mostly alumina and silicon dioxide. It's this specific composition that gives fire brick its ability to withstand higher temperatures and conduct heat at a slower rate. The slow transfer rate is essential within an RMH core where higher temperatures are desired. I built two cores using foundry bricks (the "heavy" kind) and not only did they perform beautifully, the bricks showed no sign of fracturing or surface breakdown.

Chris Burge : Goto> 'The Engineers Toolbox' and look up the Thermal Conductivity of Common Materials,

Insulating Brick 0.15

Common brick 0.6--- 1.0

Dense fire(KILN) brick 1.6

Also because the hard dense fire (Kiln) Brick retains heat for so long after your fire, you will continue to thermo-syphon your heat both horizontally and vertically
up the chimney, negating one of the prime benefits of the J-Tube Rocket Mass Heater!

What we want in the walls of our Combustion core are materials that reflect, or refract back the highest volume of the heat back into the Center of the Burning
pyrolyzed gases..

Because the thick Heavy fire(Kiln) Brick is doing what it was created to do absorb the heat and stabilize the heat cycle of a kiln, It actually takes longer to reach
the Freaky high heat promoting a good clean highly efficient burn ! This can be verified by looking at the smoke coming out the vertical chimney ! I hope this helps
and is timely, Think like Fire,Flow like Gas,Don't be a Marshmallow! As always your comments and questions are solicited and are Welcome PYRO Logically BIG AL

Al, it seems to me that you may be misinterpreting the data...

The chart to which you are referring reads as follows:

Brick dense 1.31
Brick, insulating 0.15
Brickwork, common 0.6 -1.0
Brickwork, dense 1.6

The only differentiation between types of brick are "dense" and "insulating". These are obviously two highly generalized categories, but it's safe to assume that all types of fire/kiln/smelter/foundry brick have been lumped into the "insulating" category, regardless of of size, density or application. Therefore, all other types of non-insulating bricks have been categorized as "dense". The values for each are what we would expect-- bricks that are intended to insulate do so, and all the rest do not-- or at least, not as well. These two values also coincide with my qualitative analysis.

The next two lines in the chart are referring to "brickwork", which we can only assume to mean some form of brick and mortar construction, be it a wall, column, etc. The differentiation of these two categories, "common" and "dense", is between the types of work, not the types of brick.

Chris Burge : You are correct in that I did make an error in copying down the true value for dense brick, (Which is ) 1.31(and not the 1.6 I Posted )The chart should read
by weight : This was sloppy investigating And Reporting and you are right to call me on it !

Brick insulating 0.15

Brick Dense 1.31

And Types of brick work

Brickwork Common ) 0.6 - 1.0

Brickwork Dense 1.6

However, though it is your stated opinion that all types of fire and kiln and smelter and foundry Brick have been lumped into the same category- insulating Brick !
I believe that the error lies on your part, by your classification there has to be a class of bricks totally unknown to me that contains materials that make it denser
than 7 pounds if found in a 9'' X 4.5 X 2.5 nominal brick size, And the 100 year old soft Red/Red Orange house brick fails into the same category with this denser
than dense brick !

Your working hypothesis is unwieldily, and postulates a link of Brick type X and that of a much lighter brick than all the rest you name except One!

My stated hypothesis is simple and uses a direct scale that does not threaten our understanding of the rules of mass/density and weight .

There is also the amount of heat energy that each of the bricks can absorb as a function of their density, the denser the material the more energy it can hold,
regardless of its Conductivity and Emissivity It is my position that true light weight (and mass) bricks can hold less heat and their emissivity being higher radiate
more of the heat energy they are exposed to back into the combustion chambers flame path !

It is this combination where related bricks (by mass and weight ) have common functions and attributes, and all other, denser brick fall into their own category
That convince me that its basic simplicity has merit !

I also question the Concept that states that a denser heavier brick by any-name is the best brick to have to line a combustion chamber, and I believe I have stated
my thoughts on this clearly ! Regardless of the way you are interpreting the chart, it clearly says the denser brick conducts more rapidly, You state ''I built two cores-
-using foundry bricks (the heavy kind) and not only did they perform beautifully, the Bricks showed no sign of fracturing or surface break down"

To claim Qualitative evidence you would need to test them against some thing , in this case both for start-up time, and for the presence and for how long of system
thermo-syphoning !

While it has been common practice for 100s of years to call Foundry type brick fire brick, as we use the term in Rocket mass heaters it is a misnomer,

As their are fellow members who are now confused and reluctant to commit to any form of brick work, we must find a way to put this to rest ! Some ones sacred cow
is going to get kicked for good or bad ! How do you think we should put this to bed ! Big AL !

Al,

The evidence is clear, look at the chart, insulating bricks insulate. Their rate of thermal conductivity is determined by their composition, not their mass.

You're creating an issue where there is none and hence, creating confusion.

Fire brick, wood stove splits, smelter brick, kiln liner, foundry brick, all have very similar composition... So similar, in fact, that they have been given one category and have been assigned one value on the chart.

You're splitting hairs on a bald head... A red herring... There is no cheese down this tunnel.

Calm down boys ! I'd like to hear from somebody who manufactures these things. I'm sure that given the temperatures reached and fuel type, they will have a product that fits the bill. It's not rocket science.

Edit --- Oh, that's right, it IS rocket science.

I don't manufacture them, but I do use them.

Two major points of clarification:
1) Building terms differ by region and field. Both "fire brick" and "kiln brick" are ambiguous. Both terms may be, and are, used for either light (insulating) or dense brick.
"Fire" and "kiln" imply tolerance for refractory temperatures (over 1000 F, usually over 2000 F in the case of these brick grades). They imply nothing about the density of the brick.
2) Both types of brick, and varieties in between, are suitable for use in rocket mass heaters. Almost all clay-based materials are capable of handling over 2000 F.

Regarding ambiguous terms:
- Many builders and building-supply retailers don't travel outside their region. It's harder to get a masonry saw on an airplane than, say, a laptop. (This is one reason why we arrive at workshop sites with a tool kit that is inversely proportional to the distance we travel. Believe me, we have extensive experience trying to get the materials and tools we need from locals, who don't know the same regional name that we learned for it back home.)
So, most useful materials have a wide range of common names.
One example is the dense fire brick that has a 1.25" dimension: it is usually called either 'split brick,' or 'half brick,' and I almost always have to try both words at random. The local building supply will know exactly what one term means, but be completely stumped by the other. Kiln suppliers just give dimensions; there are so many types of brick in the catalogs that they don't pretend a shorthand name will be clear to their customers. Pictures are also used extensively.
- Both dense and light bricks are used with fire. Both dense and light bricks are used with kilns. So "dense" and "light or insulating" are probably the best words to continue using, once you are into the general category of heat-compatible bricks. (Concrete pavers are also dense, but not suitable for temps above about 800 F.) Every refractory material's manufacturer provides a rating for working temperature; most provide a range of temperatures for 'normal' working temps, irreversable dimensional changes, and then finally the melting point temperature. Both dense and light fire bricks come in a range of working temperatures. Most are cast from clay, but I have been told that some of the light brick is cut from suitable porous stone.

Dense brick:
What Allen is trying to distinguish as "kiln brick," the dense brick, is what I would call "fire brick" here on the West coast. It is a dense, hard brick cast from burnt clay grog and a small amount of additional clay, to precise dimensions. The Canadians make even more beautiful varieties, in a range of colors including reds, yellows, and black, as well as the more common beige and pink varieties found in the USA.

Its virtues for rocket mass heaters: it is durable, precisely dimensioned, resists heat and abrasion, and serves as thermal mass.
Disadvantages: It is not insulating, so a second layer of insulation must be used. Since a flexible expansion joint around the firebox is needed anyway, the only added expense is the heat riser insulation. It is heavy, so shipping and handling may cost more. When we use this type of brick in a rocket stove, we surround it with a layer of insulation (such as perlite-clay, or refractory wool).

For stoves that are run every day, the residual heat in the thermal mass can keep the barrel warm overnight (in addition to the bench mass), and the residual heat in the heat riser makes it easier to light the following day.
For stoves that are not run every day, especially those that are used only occasionally, the lack of the same residual heat can make for a 'cold start' where the heat riser may need to be primed with a candle or burning newspaper before the main fire is lit.

Insulating brick:
What Allen refers to as "fire brick," and I have seen it sold under that name, is what we find in retailers here listed as "Kiln brick." So indeed, there is a great deal of confusion on this point. Unfortunately, the English language does not reserve for our unambigous use the most convenient, short words; if you want to be specific, you may need several syllables, in this case "Insulating kiln brick" or "Insulating fire brick" would mean the same thing, where "fire brick" would be ambiguous.
Virtues for rocket mass heaters: It is insulating, easy to cut and stack, resists heat, and due to its insulating qualities is quickly brought to very high temperatures by small amounts of fuel. Light weight may also be an advantage for shipping when ordering from a distant supplier.
Disadvantages: It is not abrasion resistant, so wears out quickly in the firebox, creating irregular sides that can prevent wood from self-feeding. It is harder to find in many places, and is easily damaged. When a single layer of insulating brick is used, separate provision must be made for expansion joints, or the brick can wear and crack. Use of unsuitable (too hard) mortars can also cause cracking. The insulating brick absorbs water readily, making it more likely to crack if stored in damp or frozen conditions (including heaters built directly on the ground for intermittent use). Super-insulated fireboxes can exceed the working specs of materials that would perform just fine in fireboxes lined with dense brick; we have seen perlite melt, and concrete spall, when exposed to the intense radiation from a super-insulated cast-refractory burn tunnel.

Inverting the thermal mass properties above:
Light, insulating fire boxes are superior for stoves that are run briefly and occasionally, such as cookstoves and rocket mass heaters for a parlor, guest room, or chapel. They heat up quickly, and may have advantages for reducing the initial smoke from the fire. But they also cool down quickly, meaning that they may not have the 'oomph' to finish burning the last few coals cleanly, compared with a dense firebox that radiates stored heat.

One thing I've noted lately is a tendency to combine dense firebrick feed tubes (because of the abrasion resistance) with light, insulating heat risers (because of the faster heat-up and cool-down, or perhaps just the simplicity of a one-piece material in this tall, narrow structure).
A problem with this approach is that the dense feed tube will get hot and remain warm well into the following day, while the heat riser cools rapidly. This can lead to smoke-back into the room in the cooler parts of the cycle, both lighting the stove and finishing out the burn. It could also conceivably lead to the stove drawing backwards under certain conditions, most likely within the first few hours after the fire has gone out.
While we've worked with a number of stoves where this combination of materials is being used successfully, I just thought I'd warn folks in case you hadn't encountered it yet. A stove built with dense brick in the feed, and insulative materials elsewhere, should probably have back-up draft (a taller heat riser or tall vertical exit chimney) to help overcome this initial start-up disadvantage.

If combining dense and light materials, I'd be inclined to make the entire burn tunnel of similar materials (dense firebrick) to resist wood abrasion and cleaning tools, and then the heat riser must have insulation but may or may not contain dense materials. This should at least put some of the residual heat underneath the heat riser, and help balance out the heat distribution in those cool-start conditions.


Reclaimed older brick:
Ianto loves the hand-formed firebrick and soft-fired building brick or "common brick" that can be recycled from old buildings and chimneys. You will know this brick because it can be used as sidewalk chalk: it will leave a red streak on concrete. Newer building brick is often fired to an almost purple color, and is harder than concrete. This newer brick tends to crack badly in high heats and rocket mass heaters, though it will still serve far better than a concrete paver. Older, hand-formed firebrick has served as well as the newer firebrick in terms of resistance to cracking, especially in the difficult spot that bridges the sides of the firebox and also forms part of the wood feed.
Almost all clay-based building brick can be expected to handle at least 2000 F, if it is dry and in good condition when heated. Most building brick is intermediate in density between the light (insulating) brick, and the dense brick. When using building brick, we use a secondary insualation and expansion-joint layer of perlite-clay or a refractory fiber product such as board or blanket insulation. The more flexible the material, the better protection it offers against cracks caused by unequal expansion between the firebox and the outer masonry casing of the heater.
If using a recycled material, do your best to find out the local history, look it up online or in builders' references, then test it in an outdoor mockup before using it in an indoor installation. Recycled materials may be contaminated by paint, industrial waste / slag, traces of old and unsuitable mortars, creosote, pottery glazes, and other contaminants. Outdoor testing can burn away some but not all of these, especially the heavy metals. We avoid slag-coated bricks as much as possible.

Damage observed with a supposedly suitable brick:
ALL BRICK WILL SPALL IF HEATED WHEN WET. Water boils at 212 F, give or take elevation and salt content.
Steam expands about 1600 times in volume compared to the original water. Dense bricks can't vent steam fast enough to avoid spalling, and light bricks can absorb so much water or ice that they can't vent fast enough either. Both types of brick can be severely damaged by an intense hot/cold shock, especially when wet.
Keep your building materials in a dry storage location, or move them to dry storage a few months before building if you salvage them from an outdoor brick pile.
When building a firebox of dense brick, we often dip the brick very briefly in water and use a wet clay-based mortar. Under most building conditions, we consider the brick dry enough to the stove the following day. Many of our students have seen steam or fog coming from the wet clay and cob on our benches as we heat them up. We use this method both because we have to (we can't light up a student-built stove in a weekend workshop any other way), and because we find it useful to set an expansion joint where the barrel meets the earthen masonry before the masonry is fully hardened. Both dense and insulating brick seems to tolerate this insult. We wait until after the core of the heater is dry to apply finish plasters or tile, and thus any cracks that develop from heating can be stabilized and sealed with the finish layers.
But most builders who work for clients, and need to achieve a higher standard of perfection on the first attempt, advise their clients to wait and let the brick dry out before starting a fire in it. In this case, an expansion joint (such as a braided-fiberglass woodstove gasket) should be provided around the barrel where it is embedded in the masonry; or special attention should be given to a thick, even masonry layer plus instructions for calling the builder or fixing any cracks that may develop in the first few firings. In these cases, the stove will be finished by the builder several weeks before the first fire, and the client will not discover any expansion problems until that time. One builder we know tells clients to call him if a crack develops large enough to slip a credit card into it, or if they ever see smoke or anything at all coming out of any crack.
Be especially careful if wet or frozen brick is used for the floor of a build with a highly-insulated burn tunnel and bridge, as these insulating materials reach intense radiant temperatures much faster than a firebox lined with dense brick. In these cases, wait to fire until the stove is fully dry. If necessary, speed up the drying with a space-heater, or with extremely small fires built on a layer of sacrificial material (like ash or sand).

Ratings vs. Names:
Our goal with clean fire is to stay in the range from 1100 F to 2200 F. Below 1000 F we get carbon monoxide, smoke, and creosote. Above 2400 F we burn (oxidize) some of the nitrogen in the air, creating NOx.
Given the variability of draft and fuel values, it is hard to make a passive-draft stove that consistently runs above 1200 F (even on small/light loads) yet does not occasionally heat up beyond 2400 F (even on large/rich loads).

All materials used in these systems should be rated for at least 2000 F; I am much happier when they are rated for 2400 to 2800 F.
All the fire brick and kiln brick I have seen, whether insulating or dense, is rated for over 2000 F. Most is rated for over 2500 F. So it's all suitable.
But do check when you buy it; if you are building an all-insulation firebox, it will drive performance temps higher, so you want the highest-temp brick you can afford.

The labels 'fire,' "non-combustible," "high heat," are not guarantees of suitability - a 'high-heat' aluminum tape is only rated for 250-300 F. "High temp" paints might be rated up to 500 F, 1200 F, or 2000 F.
"Refractory" and "kiln" are more specific (temps above 1000 F), but still may include materials not suitable for our purposes. Always check the rated temperature on new materials.
NOT SUITABLE:
- Avoid Portland Cement / Concrete: Any product containing Portland cement or lime is not suitable for our purposes, unless rated for 2400 F or higher (a few refractory products do contain trace amounts, but it does a job more like baking soda than like its normal purpose as a binder). Note that 'refractory cement' may have no more in common with Portland cement than it does with rubber cement. 'Cement' is a term for binders / glues, and does not imply anything about the composition of the glue. Likewise, 'concrete' usually means Portland cement plus aggregate (sand/gravel), but can also be used to describe any stuck-together mass of aggregate. Roads can be called asphalt concrete or tarmac, where gravel is glued together with flammable tars and petroleum wastes. Cob could be called clay-bound concrete or 'clay-crete'; (clay content is a critical consideration in earthen dams). And concrete with weird aggregates like paper pulp or Perlite instead of gravel can have very different properties; "pumice" stove liners seem to be a Portland-cement or low-grade refractory cement with perlite aggregate. So you can use rated refractory cements and refractory concrete mixes if you like, just not the ones that have Portland cement in them. Including most concrete pavers and tinted decorative bricks; those are fine around the bench but not in the firebox.

- Avoid Metal: Steel and all metals will warp and degrade in these heats, even if they don't reach their theoretical melting points. Even cast iron gets burnt away by use as a fire grate over time. We avoid metal in the firebox, though we are comfortable with metal used as an outside insulation support around the heat riser where it does not penetrate the first 1" of excellent insulation. We can forge steel and iron in these stoves; therefore, steel and iron will not last long enough to be considered structural within the firebox.

So:
Brick is good - as long as it's clay-based brick, or a refractory brick rated for over 2000 F. 2400 to 2800 F is a better rating if you can get it.
Brick varies in its insulation value.
Insulation is absolutely necessary, but it does not have to come in the shape of a brick. It's particularly critical that the heat riser have reliable insulation and no leaks before the top.

The materials that have been shown by experience to give adequate insulation around the heat riser include:
- Dense firebrick (1.25", 2.5", or 4" thickness) plus 2" of perlite.
- Dense firebrick (""""") plus 1" of refractory-wool blanket (e.g. DuraBlanket) or 2" Roxul rock-wool insulation panel or blanket. (Roxul alone is not able to handle the temps, but it has worked on some stoves when shielded by 2.5" of firebrick)
- Dense firebrick (same thickness) plus 4" of vermiculite, loose-poured into a metal container. Clay-stabilized vermiculite does not retain much insulation value.
- Insulating kiln brick (2" or 3") with a careful, clay-based mortar (and ideally some perlite-clay plaster or chinking to reduce the chance of cracking at the corners). Many types of refractory mortar are hard enough to erode or crack the soft brick during heating and cooling cycles.
- Perlite-clay cast heat risers with sacrificial inner liner (2" to 4" thickness)
- Cast-refractory heat risers using perlite aggregate, or other methods to create insulation value, various thickness (1" to 2" have been successful in short-term prototypes but long-term data not available).

Materials that experience shows to provide enough flexibility to serve as an expansion joint, when properly installed, include:
- Braided or woven fiberglass gasket (as sold for woodstove doors) - useful at corners and around lips and edges, to create a cushion for the highest-stress points.
- Ceramic-fiber blanket insulation - blanket is highly flexible, board can be flexible enough
- Rock wool (spun granite fiber) insulation - 'batts', like a thick stiff felt, have plenty of this kind of flexibility
- Loose perlite or vermiculite: if trapped between two wythes of brick or other masonry, generally the perlite or vermiculite will give before the brick will crack. May cause settling over time.
- Air gaps: Some masonry heaters are built with a 1/8" air gap between firebox and outer casing. This gap can be preserved during building with a cardboard shim, which is expected to burn out during firing.
- Fiber-reinforced casings or fireboxes: Increasing the tensile strength of cast / wet-formed materials with fiber is a great way to reduce surface cracking, but strength is not the solution to heat expansion. If two materials expand differently, one of them will eventually give. The purpose of expansion joints is to ensure that the 'give' comes from a well-placed material that is resilient, even soft, rather than at a random weak point in a too-strong material.
Avoid bridges of rigid mortar that transmit stress across the expansion joint. For example, if the barrel has a gasket around its rim, but is encased in rigid masonry above that rim, there is effectively no expansion joint. Cob and earthen masonry are pretty forgiving for novice builders; you can spot and fill cracks, or carve out relief, as you figure out what the heat stress is doing to your project. If you see cracks that expand when the stove is hot, you can shove a wedge into them; they need to be filled at this expanded size, not when shrunken and cool.

but I digress.

Hope that helps!

Yours,
Erica W

Erica can never be accused of sacrificing clarity for the sake of brevity.

Thank you Erica.

Erica Wisner : Thank you, that is a lot to take in and will take a while, Your efforts, as always is very much appreciated ! Thank you Big AL !

I agree, WOW Erica, what a contribution!

Wow Erica!

That was the best "brick choice pros and cons" or firebricks 101 that i have ever seen. Awesome contribution! You and Ernie rock!

Shit sorry, I may have sold you short there....

You guys ROCK HARD!! Harder than new brick!!

Thank You Erica what an enlightening piece of work...not many people have the experience of you and Ernie! How grateful we are to have you share your knowledge.
-=Greg=-

I dub you - Erica the Cogent. Thank you.

I am tearing down an old school built in 1927 and taking down the chimney it has fired red brick one layer on the outside, two layer soft brick, and one layer hard fire brick. do you think 2 layers of soft brick be insulating fire brick or common. I am guessing insulating but is there any way to tell?

billy larson wrote:I am tearing down an old school built in 1927 and taking down the chimney it has fired red brick one layer on the outside, two layer soft brick, and one layer hard fire brick. do you think 2 layers of soft brick be insulating fire brick or common. I am guessing insulating but is there any way to tell?

Weight. Insulating brick is way lighter than common clay or dense firebrick. If one brick is 4 pounds or more it is not insulating. Or it is very wet And wet is not insulating either... all used brick should be dried before heated use.

Just to add to the mix, if not confusion try making your own fire brick. It is interesting that the author's greatest interest is in making them as dense as possible. He is trying to make them as dense as commercial fire brick.

I'm guessing that many people assume a strong correlation between heat capacity and thermal conductivity, and that weight or density are of utmost importance. Practical experience tells us --- "cold water is heavy and it makes my fingers cold" --- "cold balsa wood is light and after a few seconds, it insulates and keeps them warm". But there is much more to it than this. Many materials of similar density have vastly different rates of heat transfer or conduction and there is wide variation in heat capacity between substances of similar density. Extreme example with red title further down.

Check out these links to the engineering tool box.
Heat capacity --- http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

Thermal conductivity --- http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

I got this from yahoo answers --- The question posed was --- Why aren't thermal conductivity and specific heat capacity related? Thermal conductivity is the rate at which a material conducts heat away. It depends on how easily (some of) the electrons can move inside the material. Specific heat has to do with how much energy is needed to raise an object's temperature, which has to do with the amount of freedom the atoms have in the material. These two properties are therefore quite different.

For example, compare graphite and diamond. Both are pure carbon. The only difference is crystal structure. Graphite has a specific heat that is about 50% higher than diamond, because the atoms are more loosely set in the structure (they are set in planes that can slide).

But diamond has a thermal conductivity that is more than 800
times better than graphite, because the regularity of the diamond crystal favors certain directions enormously.
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Dale again --- With the dense firebrick, the weight is not what matters, so long as it doesn't conduct heat away from the hot surface so quickly that the quality of the burn is negatively affected. And it doesn't matter how hot the whole brick is during a burn. Only the surface temperature of the portion of the brick which contacts the flame could effect the fire. When a fire first gets to roaring, the surface of bricks near the top of the tunnel may reach 1000F, while 4 inches away, the outer edges of those same bricks haven't yet reached 100 degrees. The fire doesn't know about this. It will only react to the surface that it is exposed to. Delayed heat transference is the reason that we need firebricks which are made from a million little broken bits that are mixed with clay slip and re fired. Temperature shock destroys most rocks and bricks. If the whole mass of a solid object heated up nice and evenly, chimneys could be built from glass blocks. They cannot, because glass is very vulnerable to thermal shock when one portion of a block gets much hotter than another.

Dale Hodgins wrote:

With the dense firebrick, the weight is not what matters, so long as it doesn't conduct heat away from the hot surface so quickly that the quality of the burn is negatively affected. And it doesn't matter how hot the whole brick is during a burn. Only the surface temperature of the portion of the brick which contacts the flame could effect the fire. When a fire first gets to roaring, the surface of bricks near the top of the tunnel may reach 1000F, while 4 inches away, the outer edges of those same bricks haven't yet reached 100 degrees. The fire doesn't know about this. It will only react to the surface that it is exposed to. Delayed heat transference is the reason that we need firebricks which are made from a million little broken bits that are mixed with clay slip and re fired. Temperature shock destroys most rocks and bricks. If the whole mass of a solid object heated up nice and evenly, chimneys could be built from glass blocks. They cannot, because glass is very vulnerable to thermal shock when one portion of a block gets much hotter than another.

I am guessing that is where the terminology is getting confused. Traditional masonry heater builders are looking for a very different set of properties than the Rocket Mass Heater builder. This is because the size of the burn area is radically different in size. In the traditional fire box of a masonry heater, there is enough room for all the wood required for a full burn. The best way to burn in such a case is "top down". That is, the fire is started on top of the of the crib of wood and burns towards the bottom. This allows the burn to reach full heat quickly rather than trying to start all of the wood burning at once even though there is not enough flue space to accommodate that much gas or enough intake air to burn clean. Burning from top down allows the fuel to be used at a rate the gas flows can deal with.

In the RMH the burn area is only big enough to fit the amount of fuel that can be burned at any one time. This tends to force the burn rate to be correct for the flue sizes.

So what are the different needs? With the larger fire box, the fuel itself acts as an insulator to allow the flame to get good and hot, but in the RMH, the walls of the burn area must be insulated to keep the heat from escaping. In the case of the RMH, the need for insulation for correct operation is more important than other considerations. The RMH is generally owner built and the owner is therefore responsible for being gentle with the softer insulating fire brick. With the traditional masonry heater, insulation is not needed for proper operation, therefore other considerations become more important. Therefore the masonry heater builder looks for a firebrick that is strong and long lasting as well as dense to store more heat because bricks cost money and masonry heaters are already expensive so being able to use one brick to do two things is important. Some other considerations that favour dense brick are limited space for the heater and the need to build a foundation for the heater for a home that is not "on grade".

The next question becomes "why do they even make insulating brick then?" Most applications use the denser brick, the place I have found the insulating bricks are in iron wood burning stoves/heaters where it is used under the firebox to keep the iron fire box from burning out. There must be other uses for the insulating bricks too... they are certainly not made for RMH

I think there are some places where a RMH could use the denser brick as well. The bottom of the burn tunnel for example. Even if there is insulation under them.

I have done an all metal feed/tunnel/riser RMH (I know, bad) and can confirm that insulation works. I have not done a RMH with dense fire brick, so I can't comment on how well they work that way. Once the fire brick heats up I would think they would work fine, but while heating up the burn may be sub optimal and less complete. The real question then becomes "how long does it take to get to operating temperature?"

This was not meant to be a complete run down on things just some thoughts that may help someone understand some of the whys for these things.

One of these days I'm going to climb that Mountain rewrite this post as Fake fire brick #2 ? and ask Erica Wisner to repost her Thread Extension she
posted last Dec 20th, I would recommend then that everyone read, or re-read it before posting !

Here I just want to make a simple statement about Kiln Brick, covering its use in kilns, and in Rocket Mass Heaters, generalizing about masonry heaters !

First things first, we have inexpensive New and Used fire/Kiln brick not because there are thousands of people building ether RMHs, or Masonry Heaters, but
rather due to the building and routine maintenance of High Temperature kilns. Without commercial kilns most of us would be using 100 year old House Brick !

Dense kiln bricks are used in kilns to absorb and hold large amounts of heat. After an initial firing, this heat retention ability allows for the slow cooling down
of the material being fired in the kiln, both reducing heat stress ( ceramics and glass being to good examples ) and heat treating to provide additional
strengths (mostly Metals).

Many types of materials have the ability to Reflect much of the Heat Energy radiated onto their surface back into the heart of the fire, this is what is meant
by Refractory materials, The hotter the surface of the material is the more heat energy it 'Refracts' back into the fire. Dense Kiln brick will absorb more heat
energy than Lighter Bricks, and therefore will come up to its 'Maximum Refractory Capacity' slower than the quicker lighter Bricks!

This means that that in the Kiln, a Masonry Heater, or in the combustion zone (my word choice ) of a Rocket Mass Heater, Dense Kiln brick will take longer to
reach operating/'Refractory Temperatures, It will also cool down slower, maintaining its 'refractory Temperature longer' !

Erica Wisner points out that with two identical RMHs, the one with the lighter brick may leave a little charcoal behind at the end of the burn as the lighter brick
will cool down below its Refractory -ness faster. The denser Brick may allow a certain additional loss of Heat Energy up the vertical chimney do to the continued
drafting/ Thermo-syphoning of the entire Rocket burner due to its heat retention and continuing heat radiation after a burn cycle.

As far as I know this has never been addressed by careful testing, though the J-Tube construction of the RMH will limit the amount of Thermo-syphoning possible!

It must be said that the dense fire brick seems to be a winner when it comes to the ability to stand up to Knocks, jars, thumps, and thermal shocks, much of this
monolog maybe moot depending totally on the materials you can get in your area ! For the Good of the Crafts !

Pardon me, I got to go look up 'Old Man Rivers ' Big AL !

Erica Wisner wrote:
2) Both types of brick, and varieties in between, are suitable for use in rocket mass heaters. Almost all clay-based materials are capable of handling over 2000 F.

thanks for the thorough, helpful post erica.

i still am unclear whether it's safe to use 44 cent home depot red clay bricks for the entire core (feed tube, burn tunnel, riser).

i understand it's preferable to use insulated firebricks for the core but can i safely use these home depot bricks for the core ?

are any/all bricks sold as "clay bricks" ok (albeit not preferable) for construction of the core?
thanks !

gus miller wrote:
i still am unclear whether it's safe to use 44 cent home depot red clay bricks for the entire core (feed tube, burn tunnel, riser).

i understand it's preferable to use insulated firebricks for the core but can i safely use these home depot bricks for the core ?

are any/all bricks sold as "clay bricks" ok (albeit not preferable) for construction of the core?
thanks !

Generally clay is clay, but a brick is not just clay. There is sand in there as well. Both the clay and sand are able to handle high temperature as was said, but the clay and the sand may expand/shrink at different rates. In fire brick, the sand is chosen for an expansion rate as close to the clay as possible. In common clay brick it is not. So the common clay brick may wear out faster (you may get lucky and have it last for a long time too if everything falls just right). I would try to use fire brick ($1.50 each) for at least the parts where the brick is getting physically hit by wood or cleaning tools. The fire brick is often free from fireplace removal.

Some of the guys at MHA have inspected brick after many years of use and the firebox where the temperature is lower is more worn than the area just above (kind of like the RMH riser) which looks like new even though the temperature is much higher in this area. This would be due both to chemical as well as physical wear. Many of the masonry heaters are now being built with a thin sacrificial layer in the firebox area that can be replaced after 20 years or so.

On the other hand, fire places (the ones that saw daily use for cooking as well as heating in days of old) have been built with common clay brick and still lasted well over 100 years. There are also examples of masonry heaters built of common brick that have done well too. For that matter, the original RMH is built of hand made clay/sand goop for the riser and there have been other burners built of hand made brick as well. Up until maybe 200 years ago (maybe shorter) there was no fire brick, but high heat furnaces were built long before. I would stick to solid brick though and not brick with holes in the middle... though I have seen those used to ... by a certified mason.

Firebrick. I recently purchased a used furnace. It is lined with firebrick. On one side of the wall. Inside where the fire is burning. There are about 4-6 bricks that are broken. Actually broken into about 50 pieces. But still in place. They will fall apart into gravel as soon as I try to remove them. I am planning on replacing them. Does anyone know what could have caused them to break into such small pieces in the firebox? They seem to be the same kind as are available in most stores. Any comments?

I had a box stove that suffered a similar fate to its fire brick lining. The previous owner was known for opening the ash door for extra combustion air to get the stove super hot super fast from a cold start. Worst thing one could do to fire brick. Even the cast iron grates were warped from such abuse.

This is an excellent thread and should be at the top of the *rocket stoves* thread listing.

[bump]

Big Al, anyway to make this thread "sticky" so it stays at the top of the list? This should be required reading for anyone planning an RMH build, IMHO.

Tis the RMH building season. Time to once again bump this thread.

Hello,

I recently contact a refractory material business, and they propose to me 2 kind of brick rated for 2800 F.

- one brick with 70% alumine 9'' x 4.5'' x 2.5'' --> 7 $ each
- Jet DP brick, 9''x4.5''x2.5'' 2850 fahrenheit --> 4$ each data sheet (56.3% Silica, 37.6% alumine)

Both brick have an high density.

Did you think the Jet DP brick should work ?

Thanks

Sylvain; yes, the jet brick will work fine. To bad you can not find these used at a dismantled sawmill boiler, mine originally were located less than 1/4 mile from my house. The sawmill closed , boiler was dismantled and bricks were stacked on pallets ,sold at auction and spent the next 5 years slowly sinking in in debri at the back of a log yard at another sawmill 15 miles from my house ... When I discovered them they were 3/4 buried ,all off the pallets , the softer insulating bricks were almost all broken , as were quite a few of the dense ones. However the price was good .. as in free!!! just go load them up, i got most all of them. This week I got a call from a neighbor, "want some bricks " come and get them.... I got them all sorted and stacked inside the green house today! First pic is todays free find, second is some of the original free firebrick that I " wasted" as a decorative surround of my core. The new bricks will replace the "decorative" firebrick ... freeing it up to be used on another rmh core !

Hello,

Thanks for replying since this post is old but still contain usefull informations. The 4 $ for a JP brick better quality, is still cheaper than buying a lower quality brick in a hardware store where the vendor never really know about the brick they sell... since im not really experimented and can't tell if a custom brick from a random guys are good for what I want to do, that way I'm sure about the quality of the brick

question for the group... i found a product called Old Castle "Concrete fire Brick" sold at lows and home depot. specifications say its good to three thousand degrees. would this be a suitable Jtube material?

I checked the specs and the customer Q&A at Lowe's, and these are not "concrete" as the title says, but standard firebrick. The 3000 degree spec is contradicted by an answer in the Q&A that says they are good for 2000-2300 degrees, which is good enough if not spectacular. 3000 degree firebrick are a specialty high-performance item, not likely to be standard at a consumer big box store. Kiln supply companies offer 2300 degree and 2600 degree firebricks, with the 2600 version distinctly more expensive.

The upshot is that this would be a typical material for a J-tube and heat riser. You might want full thickness firebrick (4 1/2" x 9" x 2 1/2") for the bridge over the burn tunnel, as that shape might be more durable than the equivalent in splits on edge; I haven't seen comparison tests there.

Thanks for the response. Anyone else have any thoughts? We want to get all the info we can before we attempt another build
Bob