Richard Wood : If you look at the
Thread I posted in the Rocket Mass Heaters Forum :::-->
''
Fake Fire brick ''
I fell into a similar trap, part semantics,and part a cary-over of terms from The days when Potters/Ceramists were naming
and deciding the definition of the terms used.
Because I freely admit the fact that my Thread contains errors,
let me direct you to the Thread Extension posted by Erica
Wisner, whose
answer is the clearest one posted yet . Her Thread Extension was posted Dec 20 2013, near the bottom
Actually there are Two types of fire brick,* with similar but different jobs and abilities they can bring to the same task !
We have to talk about all Fire bricks ability to be Refractory ! That is- to reflect a high percentage of Radiant Heat Energy back
into the Combustion Zone of the RMHs Burn Tunnel and Heat Riser ! This ability, shared by both types of Fire brick that creates
the High Combustion Temperatures (time, temperature and turbulence) that give us our great efficiencies.
Of the two types of F.B. The lower density 'insulating' F.B. will Reach its Maximum Refractory or reflective state a little sooner
and thus sooner create the High temps and High Efficiency state!
While the denser firebrick does Have a higher thermal capacity/will hold more heat, and Conducts far more heat through itself,
it's ability to provide much more uniform constant combustion Temperatures for the entire burn cycle, and even for hours after
helps all of us maintain the clean burn state ( but especially New
RMH users ).
Here to clarify my point (I Hope) I want to talk about a potters kiln
after firing By carefully controlling the temperatures
of the Kiln -long after the fire is out, the Potter hopes to have very little or no 'Thermal Shock' breakage!
(S)he lines the inside of the kiln with dense firebrick to allow the whole thermal mass , f.b. and pottery to slowly cool down. The
dense f.b. are only doing 1/2 of the Kilns job and there is an outer shell of some type of lighter but also refractory insulation
to slow the cooling of the kiln !
So- it is the Refractory-ness of the two Fire brick types that we prize, Dense fire brick still hot and refractory from an earlier
firing will be the easiest to relight ! The low density Fire brick will light nearly as easily and reach a refractory state sooner than
cold fire brick.
In a well made RMH system with plenty of insulation in its outer shell- the difference in time to reach refractiveness will be One
minute or less, this seems to tip the scales towards dense fire brick !
It is also very important to remember that part of the job of the insulation is to insulate and protect the
Cob, and the surrounding
exposures ! For the good of the Crafts !
Think like fire, flow like a gas, don't be the Marshmallow ! As always, your comments and Questions are solicited and Welcome!
Pyromatically Big AL
* Available as a reasonable Substitute for 1st time RMH builders/experimenters, and those on a budget who want good, acceptable
results, are old dead soft Building bricks, the kind that can be used like 'sidewalk chalk' and leave a clear red make behind ! These
take the longest to perform at maximum efficiencies, seeming to need to get hot before becoming Refractory !