temp. rating of used fire brick?

We can get 250 used fire brick tomorrow from a chimney that was made in the mid 50s. They are in very good condition. How can we tell if they are high temp. bricks good for burn tunnels and risers? We need to know right away. thanks alan

Look for markings on the brick. They will be large enough to see if you knock off the mortar. For example, 'EMPIRE' could be in 2" letters in bas relief in the brick. If you can identify the brick manufacturer, you may be able to identify the product.

The good news is that firebrick technology was well developed long before the 50s. Coming out of a chimney supports the notion that the bricks will be suitable for direct exposure to flames in a burn chamber. You can test them out: Dry stack the brick to form a burn chamber, set up a roaring fire. Add a couple of brick directly to the fire. If they come out fine, it should boost your confidence in their strength.

Today we bought 275 used fire bricks in near perfect condition for $250. They are 4.5 x 2.5 x 9 and weigh 7 pounds. They say Builders on them and were made in Canada in the 50s.

The size is right for standard firebrick.
The size is wrong for any standard size residential construction brick.
I can't find anything on 'builders firebrick canada' but it's been 60+ years since these were made.

What color are the brick?
Red steers toward residential construction brick.
Tan, yellow, white, light steers strongly toward refractory.

How thick is the mortar joint?
1/4" or more suggests residential
1/8" or less suggests firebrick

Describe the edges
rounded suggests residential
sharp and square suggests firebrick

Grain
uniform can be either
dark flecks suggest a high performance firebrick

Got a picture?

If I had to guess, the fact these were in a chimney for decades and are still holding up supports the idea they are a fireclay firebrick.

Light tan, sharp corners, uniform grain with black flecks, less than 1/8" of motar, 7 pounds each.

You got firebrick, Baby!
Even a low rating of 1800-2000 degrees will do whatever you need.

The light tan color suggests an elevated silica content. This is good
The dark flecks result from the alumina.
Can't tell much from the weight. Depends on what the proportions of material is used.

For less than a buck each, that's a touchdown!
You probably have enough there to make a brick pizza oven in addition to an RMH.

Even a low rating of 1800-2000 degrees will do whatever you need.

This issue brings me some questions: if RMH are supposed to reach temperatures high as 2800 F on flame's path (as 1200­ to 2800 F on Ernie & Erika's home 8" system) why old red brick (about 2100 rating as some ceramist told me about old bricks here in my nearby) or lesser-grade rating bricks are supposed to work??
Is this because these temperatures are reached only in the middle of the flow in the chamber, and bricks donesn't get so much heat??

Please, if someone can help me with this doubt I'd appreciate very much.

Yesterday I drove 200km to go to a factory of refractary bricks and bougth some 2534ºF (1390C) ones to begin building the "J" (today, I hope, on the backyard, of course).

Anyway, should my RMH work properly and built it in the hosue, I'll make another one J chamber, with isolation and barrel but without mass, on the backyard, using the 1652F (900C) grade bricks I bought before, just to know what happens to them...

Thank you all for sharing:

Manuel

Firebrick is made with high levels of alumina and silica whereas red shale clay bricks have elevated levels of iron. The key difference is the rate of thermal transfer through the brick. Firebrick moves the heat through rapidly so the temperature on one side of the brick is close to the other side. The red shale brick does not transfer the heat well. One side will be significantly hotter than the other. Bricks expand when heated. IF the temperature is uneven, the brick will crack as one side expands more than the other. This is called thermal fracturing. It can occur with each heating or cooling cycle. You can use red clay brick but it will disintegrate. You'll be replacing it after a couple of hundred firings. The firebrick could last for decades.

Thank you for your explanation, Ken.

So firebricks won't be so susceptible to thermal fracture but, what happens when fire gets temperatures higher than they're supposed to stand well?

The brick have already been baked nearly to the point of vitrification during production. This is a heating process which takes them almost to the point of melting into glass in which the material fuses together, giving the brick strength.
Exceeding the temperature limit will melt the brick. It's not going to turn into a liquid lava, but the bonds between the component molecules will be rearranged. Some parts will turn into glass. Some parts will fuse in a different manner. The effect changes the properties of the brick. It will lose structural strength, lacks abrasion resistance, and often crack and crumble, which is the dead giveaway telling you the brick is pooched.
All you can do is tear it out and replace it.

edit>>>see the 3rd link below

This is the sort of work I do for a living. I work for a refractory contractor. I've seen all kinds of firebrick in all sorts of situations. Sometimes it's so hot when we enter a vessel the brick is still glowing red underneath the scale. It'll melt your boots. There are all kinds of different bricks out there for different applications. Carbon brick is black like a hole in the world. It is acid resistant. It is used in conjunction with furfuryl alcohol resin mortar which is also acid resistant. Mostly I deal with acid resistance. There are shale brick for low acid uses. Chrome bricks are green and heavy as hell. They are used in coal gasification power plants to take the force of the constant blast from ignition. We use a hexavalent chromium based mortar, get to wear full body suits on that job. There are insulating bricks which are as light as a feather. Bricks are used primarily for heat, acid, and abrasion resistance, although there is some call for non-reactive materials in some industries. Along with materials, shape is a key feature. Kilns, for example, are long tubes through which material flows. They constantly turn while heat is applied at the low end. The material enters on the high end and works its way downhill. The entire tube is lined with brick. These brick are arched. Here's an idea of tearing it out and installing kiln arches.

Compare the kiln brick in that 2nd video to this video clip: Why It Is Important To Check The Fire Bricks In Your Stove.

I don't actually lay the brick. I say to lay brick and other guys do the work.

Thank you very much, Ken, you have cleared my mind on that subjetc. I'll try to download the vídeos you linked so I could watch them at home later.

Hey what about using kiln bricks for the J tube and burn chamber? Thoughts

You can. They'll hold up for sure. But the pricetag of $25/brick is prohibitive.