[Stoves] Thermal tensions in glass

[Crispin]

Dear Jeff

This is old but I have more info to clarify your question's answer a little.
This is a very important question these days as we get closer to
understanding why clay stoves and components crack.

From: "Jeff Forssell" <jeff.forssell at CFL.SE>
Sent: Thursday, May 27, 2004 3:16 PM
Subject: [STOVES] Thermal tensions

[snip]

>I wondered if the expansion of quartz is radically different from the glass
>that I experienced "blowing".

>An Internet search yielded

>Quartz:
>Coefficient of thermal expansion: 5.5E-7 cm/(cm*K) (average from 20°C to
>320°C)

>Glass:
 >You'll find that the expansion coefficient runs between 4 and 16 x
10^-6/deg. C.
 >Common optical glasses tend to run around 7 and is probably about the
figure that I'd use for plate glass.

>Which means that glass expands about 10 times more.
>I still find it surprising the quartz holds. Are there any
>other tricks in the trade?
[snip]

First, the temperature range over which we measure the expension should be
0-700.  The coefficient is not constant so other ranges are not appropriate/

Next, the quartz problem: it has two phases or crystal structures and it
swaps from one to the other at 573 degrees.  There is a 2% volume change
when this happens which is a Big Deal.  It causes a lot of cracking.

Borosilicate glass can be formulated to have a very low thermal expansion
over the 0-700 degree range.  But it is going to do 'something' at 573 if
there is actual quartz in it (i.e. not melted into other things).  Most
BGS's can take a 300 degree temperature differential (thermal shock) without
cracking.  If you were to heat a piece very rapidly with a 400 degree
difference over a shot distance it would crack.  Making an insulative
component by foaming the product would produce this far more easily.

Soda glass like a coke bottle can only take 100 degrees of thermal shock.
It has a much lower melting temperature.  It is difficult get it to work
across the range of 0-700 because it is full of quartz particles.

I was very impressed to find that astronomical glass for parabolic mirrors
was listed at a thermal expansion of 0.00 +-0.03 which means it contracts a
little and expands a little, and is almost unbelievably stable.  Then I saw
that it was for a temperature range of 0-50 C.  The same material (very
expensive) was 2.0 across the range of 0-700.  Not so impressive after all.

The clay material we are mixing in Maputo is about 1.92 across 0-1200 C and
2.3 from 0-700.  This indicates that it is very stable between 700 and 1200.
Interesting, but not all that useful.

If we make insulative bricks for combustion chambers from the PK11 material,
they are far stronger than anything you have ever seen before.  Apart from
not cracking much even at the microscopic level, it is a really strong
material.  It has the strange combination of being a strong heat-conductive
material that is insulative because of the porus structure.  The connecting
filaments heat each other to prevent localised stresses while on the whole
not being able to conduct many watts because of all the holes.

Garth Whitworth-Williams-Foxcroft managed to make foamed soda glass in his
back yard.  It is very insulative, almost amazingly so, but it can't be used
above 573 C because it will splinter as if cools through that point.

Quartz?  Big future.  As long as it is in the form of really low expansion
components.  Melting at 1600 makes it a trifle hard for the back yard shop
but one day...one day... it will be really worth having.

Regards
Crispin