different types of fire brick

[jeb21962]

I have some really heavy brick which came out of a wood burning stove. I also have some very light brick which I do not know the source of, it was given to me. I was told it was fire brick. These two types of brick are night and day different in weight and texture. What do you suppose the difference in material composition is for and which would you say would be better for a rmh?

[Deleted]

Without knowing more about specific properties:
The very light ones have low thermal conduction ( Insulation ).
The heavy ones are good for harvesting and storing heat and in areas with high abrasion.

[satamax]

Logicaly, the light weight ones are insulating fire bricks. Good for the end of the burn tunel and the heat riser. No good for the feed tube and begining of the burn tunel, where they might be poked by wood or else.

[jeb21962]

I made a j-tube without mortar and did a burn with the light ones. It took a little experimenting with the length of the burn tunnel and height of the feed tube but even without mortar I had a real nice sideways fire going inside there. At first I got some soot on the inside of the heat riser but it burned off quickly and was clean by the end of the small bit of wood I burned. I also had some fire creep up the wood inside the feed tube when I had a fair amount of wood piled inside it. This was my first experiment and was just a test, hence the no mortar and no outside barrel. I read the fire creeping up the wood was probably because the j-tube was just assembled with out mortar and was sucking air through all the cracks. With a small bit of wood inside the feed tube there was almost no heat coming up from the tube but the heat riser was hotter than blazes. I could only hold my hand above it for a second or two but I could hold it indefinitely above the feed tube. I think it was a pretty good first experience. I think I'll use the light stuff for my heat riser. The light stuff had real good insulative properties as the outside of the burn tunnel hardly got warm. Thanks for the replies.

[matthewwalker]

I'm going to respectfully disagree with Max and say that having built both heavy brick combustion cores and highly insulated ones, the heavy brick feed tube and burn tunnel don't make for a good stove in my opinion. I'd much rather trade a little durability for the better burn obtained from a highly insulated feed tube/burn tunnel. My most recent project was made from heavy brick entirely for durability reasons. Once again I was reminded how much I prefer insulative materials for the feed tube and burn tunnel. I think performance suffers greatly from heavy brick in those areas, more than anywhere else in the system, in my opinion.

[satamax]

Matthew, i didn't say feed tube and burn tunel entirely. But i think heavy bricks at the bottom of the feed tube and begining of the burn tunel wouldn't be a bad idea, where they can be poked. and i have noticed, i like the feed tube towards the "user" side to be non insulated, it avoids creep. JMHO. But i agree with Ernie, who hangs at permies, "planelle" bricks are better. You know the half ones, which are about 1 1/4 in thickness. Put theses sideways = less mass to heat. And you have more room for insulation.

[matthewwalker]

I understand Max. It's been my experience that the heavy bricks lead to more ash/coal build up while they heat up and that can lead to poor performance. Additionally I've noticed a distinct disadvantage in having the heavy bricks for the top of the feed tube. They can get hot and create a chimney effect which works against the draft of the stove, as well they heat the fuel up high once they are hot and encourage creep in my experience. I know that there are a lot of folks out there who use them to good effect, but I wanted to put it out there that better performance is possible with a combustion core that is highly insulated everywhere. That's been my experience having built both insulated and heavy brick designs. Just thought I would share since it does go against some reading I've done on the subject as well.

[satamax]

So, user side of the feed tube in metal, and may be the whole higher part, so it cools down and there's no creep. Bottom of feed tube with thin heavy bricks, heavily insulated. Burn tunel, where it can't be poked anymore, light insulative bricks insulated. Beginiong of heat riser too, then switching to stainless steel, well insulated too

Ideal core?

[matthewwalker]

Well Max, I believe there are many ideal cores depending on the user and application. For me, no metal at all. The radiative losses there lead to a poor burn, and no heavy brick as it sucks the heat from the wood and again leads to a poor burn. My best running stove by a long ways is the one I built for my home. Fireclay/perlite super insulated combustion core everywhere. Just a thin tile lining for the inlet port to keep it from eroding with feeding abuse. Otherwise, all super insulated clay everywhere, the equvalent of light firebrick. It's surprising how much abuse the mix can take inside the firebox now that it's been basically fired to bisque hardness. You really must see it in comparison to a heavy brick or metal system. The difference in performance is no small thing. It goes incandescent almost instantly, and produces no significant coal bed. Just ash. The downside is there is no flywheel, if you let it go out, it's out.

[randymax]

I am curious about your fireclay perlite, also what coarseness of perlite do you like?
recipe and molding techniques...
I am looking forward to my first build and want to go the super insulated route.
Thanks.

[matthewwalker]

You definitely want to go with super insulated over heavy brick if at all possible. I used masonry grade perlite in my first core but have since switched to Horticultural grade since it's cheap and available locally for me. The media is around the size of a BB on average, where the masonry grade was finer. I prefer the larger grit. I'm working on a video of my process now, but basically I used fireclay and perlite in a pretty coarse, dry packable mix. It makes a soft core, although after a year of firing it's now pretty hard. Initially I just patched any wear spots with more of the same for the first month or so, and didn't sweat it too much. It is important to encase it in a good cob shell to keep it all together, think of it like a liner.

You might also try working in some wheat flour paste to add some stickiness, and a bit of sand might help with cracking. All in all though, it's a pretty forgiving process. Just treat it like a liner, not as a stand alone stove, and you'll be fine. Don't sweat the cracks and stuff, just pack more in where needed and seal from the outside with cob.

[Rick Thornton]

Matthew, I anxiously await your video and to learn more about your mix and forming/molding advice. Best--Rick

[kmccune]

So since I do not own any fire brick I'll have to go get some. And if I,m looking at some fire brick on a shelf how do I know if they are an insulative type or not?

[morticcio]

Weight...
Insulative fire bricks are lightweight (approx 4lbs or 1.8kgs for a standard brick).
Standard firebricks are heavier and very dense (approx 10lbs or 4.6kgs for a standard brick).

You may want to check the temerature rating for the bricks. They should be rated for a minimum of 2200 F (or 1200 C).

In addition, insulative bricks can be cut with a hand saw.

[matthewwalker]

I did finish the casting video a few days ago, I'll post a new thread for those that are interested.