Fire brick or cast with fire clay?

Still researching and reading on building the core. So far there are two methods that stick out. The typical build with fire brick which is well documented. There are also others who have cast the core and the riser using fire clay. A quick search brought up a fire clay that you can purchase called Hawthorn Fire Clay. Looks like 50 lb. bags shipped come out to about $40 a bag. Would casting the core and riser be a cheaper option than building with brick? If the riser is cast into a thick tube shape with say a 6" or 8" ID and an outer OD of something like 12" to 14" negate the need for insulating around it since it would have such a thick wall?

Casting the riser with a perlite-clay mixture is well documented. If you make the outer form 4" larger than the inner, you will have two inches of good insulation. Mass alone (solid fireclay, for instance) will not work well for the riser; it needs insulation and a low-mass inner surface for best performance. The perlite-clay method will stretch the expensive shipped component with the cheap locally purchased perlite. There is a very nice video of a high-quality riser build in this thread:
http://donkey32.proboards.com/thread/1298/fireclay-perlite-barrier-furnace-cement

As far as the original cast vs. brick cost question, my research has found them to be relatively similar in overall cost for my location. If you have a local source of cheap firebrick, or a local building supply center that will order the refractory and have it delivered with their regular shipments at minimal extra cost, that would tip the scales.

There are factors that may be relevant - bricks can easily be disassembled and reconfigured while you decide on the best layout, while castable can be formed to any precise dimensions you want. Also, castable can be formed thinner than firebricks so as to have less mass in the combustion zone, improving efficiency (at the possible cost of durability).

Josh ; I have built both. The cast core will burn hotter and faster than the brick core. But... the cast core is fragile. If your rmh is in an insulated home(mine is in the greenhouse) with careful adults adding wood then a cast core is a good choice. However you can totally destroy the feed tube side as well as the roof of the burn tunnel if you treat it like a regular wood stove. I've included a couple of pictures that show how my feed tube went from 7.5x7.5 to the size of a 5 gal pail, over the course of a winter, the feed side of the burn tunnel roof also caved in on me. I rebuilt using firebrick. Now it takes longer to come up to temps and until it gets there I can only get apx. 800 degrees on the barrel top ,after a while i can push it over 1000. My cast core regularly ran at 1100 degrees, FAST. Keep calling around on looking for fireclay, I called many/many building supplies and most did not know what fireclay was much less where to get it ! Finally called the right place and wala a whole pallet full at 20.00 a bag , later I found it even closer at 15.00 a bag I also was told in spokane it can be got at 10.00 a bag. The cast riser however is AWESOME !!! Use a 16 gal barrel as the outer form,use sonotube or just thin wall stove pipe for the inner form . Mine is on its second season and its good as new ! I was able to just lift it off when I did my rebuild and just set it back in place when I had the new brick core ready! Good Luck keep us posted and send pictures !

Heres a few more pictures, from brand new with nice square hole to what I had at the end of winter! By the way ... if you don't have a copy of ianto evans book rocket mass heaters ,you should invest in one. It and reading and posting here at permies will help you to have a working rocket in your life and not a youtube flaming unit of death !

Josh whatever you do DON'T order that fireclay. That's a huge ripoff. Call around all of your local mason supply stores. Trust me you'll find it WAY cheaper. I live in Oregon, and I can get 50lb bags of fireclay for roughly $8 a bag. $40 is just insane....

Also, you can cast your core and heat riser with a homemade recipe for "refractory cement"
3 parts perlite
3 parts portland cement
4 parts silica sand
4 parts fireclay
Handful or two of loose fiberglass for added strength

I cast both my burn chamber and heat riser from this mixture. Dirt, dirt, dirt cheap and works extremely well. It is brittle, very easy to chip or break, but once it's in place if you have a steel feed tube they work like a dream

thomas rubino wrote:Josh ; I have built both. The cast core will burn hotter and faster than the brick core. But... the cast core is fragile. If your rmh is in an insulated home(mine is in the greenhouse) with careful adults adding wood then a cast core is a good choice. However you can totally destroy the feed tube side as well as the roof of the burn tunnel if you treat it like a regular wood stove. I've included a couple of pictures that show how my feed tube went from 7.5x7.5 to the size of a 5 gal pail, over the course of a winter, the feed side of the burn tunnel roof also caved in on me. I rebuilt using firebrick. Now it takes longer to come up to temps and until it gets there I can only get apx. 800 degrees on the barrel top ,after a while i can push it over 1000. My cast core regularly ran at 1100 degrees, FAST. Keep calling around on looking for fireclay, I called many/many building supplies and most did not know what fireclay was much less where to get it ! Finally called the right place and wala a whole pallet full at 20.00 a bag , later I found it even closer at 15.00 a bag I also was told in spokane it can be got at 10.00 a bag. The cast riser however is AWESOME !!! Use a 16 gal barrel as the outer form,use sonotube or just thin wall stove pipe for the inner form . Mine is on its second season and its good as new ! I was able to just lift it off when I did my rebuild and just set it back in place when I had the new brick core ready! Good Luck keep us posted and send pictures !

Thomas--if you ever need more fire clay, it might be worth a trip down here to Hamilton...Donaldson Brothers has Lincoln 60 for $7 a bag. I bought the last of what they had, but they were looking to get another pallet in, so they've probably got more by now.
Josh--I'll echo everything else that Thomas said... I went with a brick core, and being able to play with them like legos was a big help in finding a configuration that would work well with my setup. I also cast my heat riser...12" sonotube for the outer form, 6" steel pipe for the inner. Wish I could have found one of the small grease barrels, but apparently they're just about impossible to find (around here anyway). For the heat riser mix, I followed the recipe I found on another forum...fireclay dusted perlite, mixed with furnace cement thinned with (home made) sodium silicate. I filled about half the form on my garage floor, and the other half with it stuck in the barrel. I had to put it in there by myself, so I wanted to get it in before it got too heavy/unwieldy. It took about 4 hours of burn time to really cook the sonotube, and even then it's only about the top 2/3 of it that cooked off. I also made a "hot face" of fireclay/sand at the top as a sort of "cap". When I started out I was temporarily using a piece of insulated Class A chimney (which worked fine), but would really start to lose it's draft after about 3 hours...I think the whole thing was heating up to the point the temperature differential between the inside and outside wasn't enough, and it would start to stall... Even after 4 hours, the draft on the new insulated cast riser is just as strong as when it's getting going, which I think shows just how well insulated it really is.
Good luck with the build!

Hi scott; Thanks for the tip on donaldson bro. I haven't been down to hamilton in years, might be time. Those grease barrels commonly get taken home as trash barrels at the mechanics home shop. Ask at quick lube shops or a school bus shop / county shop / farmers, anyplace that has equipment needing regular greasing and you will find barrels.

Wow, thanks guys. I am not sure when the build will start. I was hoping to get it done this winter, but it may just have to wait. It seems the brick core and cast riser makes a good combo. I haven't looked around really locally for fireclay. There is a refractory supplier about 40 minutes away.

How many bags of fireclay would one need? Is it better to cast in place or off site and move to it's permanent resting place?

Josh; One 50# sack of fireclay and apx 2'of perlite should be plenty for a riser. however I would buy at least 2 bags(or more) of fireclay and a 4 cubic foot bag of perlite just to make sure. Also that way you will have a start on materials for your second rmh,lol. Build your core first ,in fact, fab it up(with no mortar) outdoors and try it first. Then build it indoors. After your core is ready then pour your riser next to it on the ground and lift it into place when ready. I've included some pics of mine. The first is brand new, next is at the end of the first season covered in ash and the last is after its mounted on the rebuild. Go to youtube look up (broaudio) this is matt walkers page (a respected rocket builder) watch his videos of cast core and cast core part two.

Hi everyone, I am new to the site... I am working on a solution to heat my uninsulated studio.. We are renters so insulating is not really an option. I just wanted to lend some of my professional experience as a potter.. Also, to you folks living in MT you should call the Archie Bray Foundation its an old brick factory turned ceramic mecca. They do sell fireclay by the ton and should have a never ending supply at a good price. On average, nationwide, a bag of fireclay should cost $12-$15 per 50lb bags. Less if you buy more..

I wanted to give you some info on castable mixtures, I have been building kilns with this stuff since undergraduate school. In areas that are not going to reach (Quartz Inversion approximately 1000 degrees F in ceramic jargon) the fireclay will never make the magical switch to fired clay. So anywhere you put this stuff that will not reach Quartz it will always be susceptible to being crumbly and it will not be waterproof/freeze thaw proof. So if you expect your heat riser to actually reach these temps then you can reduce the amount of cement needed to form it. So when you are building your suitable mixtures you can keep that in mind. This bond is called a thermal bond. In areas where you will not be reaching Quartz you will need to have a hydraulic bond (like the bond you get from plaster and cement) in order to maintain its rigidity.

In the ceramic world we differentiate the bricks mainly by density and thermal properties. Firebrick is a specific term for Hard Brick. Insulated Fire Brick is a specific term for Soft Brick. Soft brick is relatively new... However, it is made with the same materials as Hard Brick. Its duty rating (heat resistance) is based on its purity and frankly I've never understood why they are more expensive. If you want to create Insulated Fire Brick, or Soft Brick, you need to create air voids in the mix. You can achieve this by adding carbon based material, sawdust, spent grains from a brewery, etc. The particulate size will give you variable size holes in your refractory mix. A word of caution here I have made castable with bigger sawdust and the size of the void does make the mix even more fragile. Now this will work if your mix, fireclay (or any clay actually--I can answer any questions on this topic) sand, cement reaches at least 451 F (this is when wood burns, all chemical carbon with burn out at Quartz Inversion). I see that a lot of you are using perlite which is a igneous rock--good choice as a fill material but it does carry with it issues too. One, it has a very low melting point. Two, chemically it releases its H2O with heat and expands which may be another reason I see that you all have a lot of steam/water collection. It does lend itself to being refractory so it is a good choice as a fill. Again, we potters totally geek out on this stuff so I could go on..

If you really want cheap insulation for heat risers and insulation is really what you want/need for a riser personally Id cast a core of a fireclay, sand, sawdust, cement (just enough to make it hard enough to move around before it is "fired" then wrap the whole unit in Kaowool. That stuff is amazing. I can line a galvanized garbage can with 1" and fire it to 2000 F in less than 20 mins with a propane burner and the can doesn't lose it zinc... The fireclay skin will prolong the life of the Kaowool... Just a side story, this is the stuff the "tiles" of the space shuttle were made of... Very fragile.

Thanks for all your info.. I'd love to hear more of some of the room temps you all achieve with your heaters. I hope this info helps you guys!

thomas rubino wrote:Josh; One 50# sack of fireclay and apx 2'of perlite should be plenty for a riser. however I would buy at least 2 bags(or more) of fireclay and a 4 cubic foot bag of perlite just to make sure. Also that way you will have a start on materials for your second rmh,lol. Build your core first ,in fact, fab it up(with no mortar) outdoors and try it first. Then build it indoors. After your core is ready then pour your riser next to it on the ground and lift it into place when ready. I've included some pics of mine. The first is brand new, next is at the end of the first season covered in ash and the last is after its mounted on the rebuild. Go to youtube look up (broaudio) this is matt walkers page (a respected rocket builder) watch his videos of cast core and cast core part two.

I notice your rebuild picture the riser has a much thicker lip on the top. Is this on purpose? Why the rebuild?

Sean Fitzgerald wrote:Hi everyone, I am new to the site... I am working on a solution to heat my uninsulated studio.. We are renters so insulating is not really an option. I just wanted to lend some of my professional experience as a potter.. Also, to you folks living in MT you should call the Archie Bray Foundation its an old brick factory turned ceramic mecca. They do sell fireclay by the ton and should have a never ending supply at a good price. On average, nationwide, a bag of fireclay should cost $12-$15 per 50lb bags. Less if you buy more..

I wanted to give you some info on castable mixtures, I have been building kilns with this stuff since undergraduate school. In areas that are not going to reach (Quartz Inversion approximately 1000 degrees F in ceramic jargon) the fireclay will never make the magical switch to fired clay. So anywhere you put this stuff that will not reach Quartz it will always be susceptible to being crumbly and it will not be waterproof/freeze thaw proof. So if you expect your heat riser to actually reach these temps then you can reduce the amount of cement needed to form it. So when you are building your suitable mixtures you can keep that in mind. This bond is called a thermal bond. In areas where you will not be reaching Quartz you will need to have a hydraulic bond (like the bond you get from plaster and cement) in order to maintain its rigidity.

In the ceramic world we differentiate the bricks mainly by density and thermal properties. Firebrick is a specific term for Hard Brick. Insulated Fire Brick is a specific term for Soft Brick. Soft brick is relatively new... However, it is made with the same materials as Hard Brick. Its duty rating (heat resistance) is based on its purity and frankly I've never understood why they are more expensive. If you want to create Insulated Fire Brick, or Soft Brick, you need to create air voids in the mix. You can achieve this by adding carbon based material, sawdust, spent grains from a brewery, etc. The particulate size will give you variable size holes in your refractory mix. A word of caution here I have made castable with bigger sawdust and the size of the void does make the mix even more fragile. Now this will work if your mix, fireclay (or any clay actually--I can answer any questions on this topic) sand, cement reaches at least 451 F (this is when wood burns, all chemical carbon with burn out at Quartz Inversion). I see that a lot of you are using perlite which is a igneous rock--good choice as a fill material but it does carry with it issues too. One, it has a very low melting point. Two, chemically it releases its H2O with heat and expands which may be another reason I see that you all have a lot of steam/water collection. It does lend itself to being refractory so it is a good choice as a fill. Again, we potters totally geek out on this stuff so I could go on..

If you really want cheap insulation for heat risers and insulation is really what you want/need for a riser personally Id cast a core of a fireclay, sand, sawdust, cement (just enough to make it hard enough to move around before it is "fired" then wrap the whole unit in Kaowool. That stuff is amazing. I can line a galvanized garbage can with 1" and fire it to 2000 F in less than 20 mins with a propane burner and the can doesn't lose it zinc... The fireclay skin will prolong the life of the Kaowool... Just a side story, this is the stuff the "tiles" of the space shuttle were made of... Very fragile.

Thanks for all your info.. I'd love to hear more of some of the room temps you all achieve with your heaters. I hope this info helps you guys!

While a lot of this is mumbo jumbo to me, it has a lot of keys for Google and that means learning! I need to reach out to the local university as it is has a pretty good fine arts department and I'm sure they have a ceramic guru on staff that would get pretty hot and heavy for this stuff. Networking is what I need to do.

Hi Josh; Rebuild was caused by the cast core not holding up to heavy winter greenhouse use. I went with the brick core instead. The riser difference was the 55 gal barrel being a few inches lower after the rebuild and i had to (shave down) the top of my riser to get a 2-2.5" clearance. I've included a few rebuild pics and the last pic that shows the (peter chanel) I added , a BIG rocket booster and I highly recommend adding one to all rocket stoves!

"WARNING !" Please be advised "KAOWOOL" is as dangerous as asbestos. The tiny ceramic strands that make up the wool have little barbs on them that make them impossible for your body to expel. This was told to me by an insulator supervisor that worked in the power plant where I worked. He died a few years later from his exposure to asbestos during his early years in the trade.