[Stoves] RE: Glass stove parts

[Crispin Pemberton-Pigott]

Dear Frank

>Could you please explain to me in a way I can understand the units of
>expansion? Is it for every deg. C increase the glass expands +/- 3  um ?.

The number is a rate of expansion expressed as a proportion of its length. 
I am avoiding the word 'percentage' because it is not divided by 100.

Fired clay has a typical rate of thermal expansion of about 7.5 x 10^-6 
which is to say 0.0000075 per degree C.  Multiply that number times the 
length of the piece and you will get the increase in length per degree.  For 
example 0.0000075 x 100mm x 50 degrees = 0.0375mm expansion for that 100mm 
long sample.

There are (of course) a couple of issues to keep in mind.

First, the rate of expansion is not linear.  Certain structures expand 
rapidly from 0 to 200 C but slower afterwards.  Quartz changes its crystal 
structure suddenly at 575-585 or so, getting bigger when it does.  Thus it 
is important to look at the expansion curve of a material to know what it is 
going to do when heated.  It might not expand at all for some considerable 
range of temperature.  Water shrinks when heated from 0 up to 4 degrees and 
expands thereafter.

Second, the rating of a material is not 'for the material', it is for that 
material 'over a particular temperature range'.  For example the expansion 
rate for a certain clay may be 2.0 when calculated for 0 to 1200 degrees, 
but 3.15 when calculated for 0 to 700 degrees, if all or most of the 
expansion takes place between 200 and 900.

You get my drift?  So it is important to get the rating for the material in 
the range you plan to use it.  For a stove it is about 0 to 700 C.  There is 
no point having a low rate of expansion, overall, between 0 and 1600 because 
you are never going to see that temperature.  Further, really high 
temperature materials with low rates of expansion tend to be very constant 
in size at the high end.  That is of no help to us.

The glass someone mentioned which is used in optical things like telescope 
mirrors is fantastically stable at 0 to 50 and has a rate of 0.00 +- 0.30 
indicating it is negative sometimes and positive at others in the narrow 
range of 0 to 50 C.  Attractive number, right?  But if you check that same 
glass in the 0 to 700 range, it is 2.0 not 0.00 because it starts to expand 
at higher temperatures.

By working with the chemistry and lithium feldspar, we have managed to get a 
fired clay for the Maputo stove of 1.92 which is even better than the 
super-duper glass _across that higher temperature range_.

I hope this is clear enough so you know what to look for in the techie data.

Best regards
Crispin