Interesting Firebrick Discussion

with Jim Buckley

The following is a slightly edited version of a discussion on the Yahoo
MHAmembers list, about firebricks:

To: MHAmembers(at)
From: StevePaisley <poppin8(at)>
Date: Wed, 17 Sep 2008 17:44:05 -0400
Subject: [MHAmembers] High-duty vs. low duty firebrick

      Will high-duty or low-duty brick last longer in a firebox?  It 
would be really nice if there were some more or less definitive 
answer to this question, yet it seems that there is not.  On the one 
hand, we have Albie and Heikki saying (20 years ago, in their book) 
that the commonly available low-alumina, low-duty brick will better 
withstand thermal cycling/thermal shock better than high-duty, and a 
conversation I had with the sales manager for Mt. Savage seemed to 
confirm this idea.  And there are no doubt many 30 year-old heaters 
out there built of low-duty brick that are still doing fine.

     On the other hand, a number of experienced heater masons seem 
to be using high-duty brick for fireboxes lately, and that is what 
Albie uses now too, as a 2" replaceable liner (though that may be 
because you can only get a 2" brick in high-duty).  Have people been 
using high-duty brick long enough to know how long they will last?  
What do the Austrians use?

      Perhaps the answer is "nobody knows which is better", or 
perhaps the answer is "it doesn't matter much, especially if you make 
your firebox and throat replaceable".  In any case, I would welcome 
anyone's thoughts or experiences with this question.

PS  "Low-duty" in my case means Whittaker-Greers.

--Steve Paisley
Archimagical Structures, LLC
Ithaca, NY

At 3:52 PM -0400 9/18/08, Norbert Senf wrote:

Hi Steve:

This topic will undoubtedly have many opinions.

Duty rating is based on temperature limits. These don't have
much applicability in masonry heaters, since temperatures are low by
normal refractory standards.

Of most interest is spalling resistance. One way to determine it is
from manufacturer's data sheets, which gives spalling loss in a standardized
ASTM test. You can do your own version of an
accelerated spalling test by heating
a brick red hot and dunking it in water, and seeing how many cycles it
can withstand.

Here is some quick info from Google:


Best ....... Norbert

From: Jim Buckley <buckley(at)>
Date: Thu, 18 Sep 2008 16:01:13 -0700
Subject: Re: [MHAmembers] High-duty vs. low duty firebrick

Interesting topic.

We sell a lot of Whitacre-Greer firebrick - as well as Alsey and I-XL.  In our Western markets Mutual, Muddox and Pacific Clay firebrick is available so we are familiar with them too.

I use the word "firebrick" loosely.  The old ASTM C27 standard that differentiated between low, medium and high duty firebrick was withdrawn by its sponsoring committee in the early 1990's.  In it's place was established C 1261 Standard Specification for Firebox Brick for Residential Fireplaces which is a less stringent standard than C 27 was.  C 1261 only requires a modulus of rupture of 500 psi and a pyrometric cone equivalent (PCE) of 13.

Alsey makes the best firebrick available.  What they sell for fireplaces meets the old C 27 standard for medium duty firebrick.  They don't even make a low duty brick or a "Firebox Brick for Residential Fireplaces".  Whitacre-Greer and I-XL firebrick meet the C 1261 standard.  Mutual, Muddox and Pacific Clay firebrick don't even meet C 1261 but maybe that isn't even important.  Interestingly I-XL used to sell a red firebrick that we found performed better than Whitacre-Greer firebrick but the I-XL red brick only had a PCE of about 8 and so I-XL stopped selling it as firebrick.

I agree with Norbert that PCE - or the temperature at which the brick melts - is irrelevant.  The practical tests he suggests sound reasonable but I would suggest that how much a brick expands when heated is the only relevant or important characteristic.  Residential firebox brick fails by cracking or spalling due to thermal shock.  It doesn't have to be particularly strong and it doesn't have to withstand temperatures of more than 1800 degrees F.  What's needed is a limit to the linear expansion when heated in order to describe brick that actually performs well as residential firebox brick.  And that is my answer to the question about whether low duty or high duty firebrick performs better in a heater. The difference between low and high duty firebrick is basically the PCE which I think is irrelevant.  The best brick for a firebox will the the brick that expands the least when heated up to 1800 degrees F.

Another interesting tidbit is the difference between dry pressed and extruded brick. Most firebrick is dry pressed but Muddox makes an extruded firebrick that performs differently. Muddox firebrick seems to do okay when laid as a shiner with the grain perpendicular to the firebox wall but it tends to spall when laid as a stretcher with the grain parallel to the firebox wall.

In summation, firebrick is promoted but not actually required by code but we are all interested in which brick or stone performs well in a firebox.  I'd like to see C 1261 revised to include linear expansion and lower or eliminate the PCE requirement but, until then, maybe we can come up with a list of brick that, in our collective experience, does perform well for firebox construction.

Jim Buckley

Jim added the following comments in relation to a discussion about unfired (green) firebricks:

From: Jim Buckley <buckley(at)>
Date: Thu, 25 Sep 2008 20:27:12 -0700

Any brick, including firebrick, is fired carefully, varying the rate of temperature increase through several quartz inversion levels, to a specific temperature, usually over 2,000 degrees F, determined to be just right for the particular mix of clay and/or shale the brick is made with.  Brick manufacturers spend a lot of time and money buying computer controlled kilns and testing their clays to get the brick to have the optimum characteristics while using the least fuel.

Using green, unfired brick, to build a heater and count on firing it in situ is not a good idea.  Very likely you'll never get it hot enough - firebrick is fired to about 2,300 degrees F - or you will get it too hot too fast through one of the quartz inversion levels and it will explode.

Modern refractories are a good thing.  Build your heater fireboxes with firebrick or castible refractories, perhaps with an insulated backing, and then build the enclosing heater body walls with any stone, brick, rammed earthen block or concrete you like.

Jim Buckley

Jim Buckley
Buckley Rumford Co.
1035 Monroe Street
Port Townsend, WA 98368
360 385 9974 fax 360 385 1624