various firebrick qualities

[patamos]

Thanks Karl,
By 'higher sensitivity to thermal shock' do you mean greater fragility when subject to thermal cycling?

Thanks for the testing suggestion

[Deleted]

Porosity hinders the propagation of cracks,
therefore underfired bricks with high porosity have a relatively high thermal shock resistance,
regardles of their composition.
High vitrification means low or no remaining porosity,
thus small cracks can relatively easy propagate to large cracks.

To test for thermal cycling just let the spot cool down and heat it again several times.

[pyrophile]

Thanks Karl!
Pat, russian Kuznetsov'bells used to be 13 cm wide, that is one russian brick wide (26 x 12,5 x 6,5cm, if I remember well).
I made a one bell rocket stove built with concrete blocks (protected by clay or brick on edge) which was about 190 cm high, 2 meters long and, inside the bell, 15-20 cm wide.
I built many bells 13 cm wide which worked well but I am not able to say if it would have been better a bit wider...Alex Chernov made many narrow bells about 13cm wide.

Benoit

[patamos]

Oct 30, 2017 5:42:04 GMT -8 @karl said:

Porosity hinders the propagation of cracks,
therefore underfired bricks with high porosity have a relatively high thermal shock resistance,
regardles of their composition.
High vitrification means low or no remaining porosity,
thus small cracks can relatively easy propagate to large cracks.

To test for thermal cycling just let the spot cool down and heat it again several times.

Thanks Karl,

I really appreciate your wealth of knowledge and willingness to share.

So these cheap soft firebricks with high porosity and low (8%) alumina can handle the thermal shocks.  But I worry about their structural integrity - especially spanning the top of a flue run in a bench.

I've placed high quality old red chimney brick over top of a 4" rocket heat riser for hours at a stretch and they holds up fine.  So maybe that is the better option.

Regarding the ringing sound, i've noticed the high heat duty (42% alumina) bricks have a higher pitch than the mediums (30%).  So would this be because they have been fired at a higher temperature?

This issue of vitrification is interesting.  One thing i have been focusing on is how close to the fire chamber i can start using cob as a structural element (ala rocket mass heaters with 6" ducting).
Sounds like the answer is to keep it below vitrification.  I have noticed cob vitrifying and crumbling in cob-and-clay-brick down draft chambers.  So perhaps a little further away.

[patamos]

Nov 1, 2017 15:20:48 GMT -8 pyrophile said:

Thanks Karl!
Pat, russian Kuznetsov'bells used to be 13 cm wide, that is one russian brick wide (26 x 12,5 x 6,5cm, if I remember well).
I made a one bell rocket stove built with concrete blocks (protected by clay or brick on edge) which was about 190 cm high, 2 meters long and, inside the bell, 15-20 cm wide.
I built many bells 13 cm wide which worked well but I am not able to say if it would have been better a bit wider...Alex Chernov made many narrow bells about 13cm wide.

Benoit

Thanks Benoit

10cm.  That narrow huh? 
Okay, good to know.  So I will be fine with 15 to 20cm.
Now to source more decent red clay bricks and get on with this build

[Deleted]

A higher pitch merely indicates higher hardness and higher density.

Kaolinite ~ 39.50% Al2O3, 46.55% SiO2
Kyanite ~ 62.92% Al2O3, 37.08 % SiO2
Mullite ~ 71.79% Al2O3, 28.21% SiO2

Above 1100°C kaolinite is converted into mullite and quartz.
Kyanite requires more than 1400°C to be converted into mullite and quartz
The highest density is reached at about 1500°C.

Mohs hardness:
Kaolinite ~ 2–2.5
Kyanite ~ 4.5-5
Mullite ~ 6-7
Quartz ~ 7

Quartz is slightly harder than mullite and thus makes a higher pitch.

If quartz in a brick causes problems by thermal shock depends on the particle size.
Smaller particles cause usually less problems.

Clays and minerals are never pure in nature.