[Stoves] Clay Stoves - Columbia University

Fri Apr 4 22:48:18 CDT 2008

Dear Crispin,

Your reply to Tae describes perfectly the variations and properties of 
natural clays.
My only question is: "What do you mean by 'free silicon'" ?
In nature there is no such thing as free silicon. What there is is 
silicon dioxyde, SiO2, usually called silica.
Silica has a high melting point. As far as I know when things are made 
of fused silica it is called Quarz. A property of quarzware is its low 
change in volume with temperature eg its high heat shock resistance.

The short question is:"Did you mean silica?"

Kind regards

Peter Verhaart

Crispin Pemberton-Pigott wrote:
> Dear Tae
>
> You ask an interesting question to which there are numerous answers, none of
> which seem to be perfect.
>
> The highest quality material is boron silicate glass which can makes stoves
> with zero thermal expansion and nearly unlimited thermal shock in the
> temperatures which stoves run at.  I have not yet see a small stove made
> from this material however it is only a matter time before someone makes
> one.
>
> There is so little known about the vast, almost a universe of diversity in
> the formulation of natural clays, that one wonders why more names nave not
> been given to them. They can be characterized as kaolinitic and high-mica
> and so on.
>
> Something recently discovered by Bruce Berger in Johannesburg is that the
> availability of free silicon in the clay is a very important indicator of a
> propensity to crack.
>
> Silicon changes from a Phase A to phase B with a 2% larger volume as it
> passes through 573 degrees.  That is why glass has to be cooled very slowly
> through that temperature so that the entire object changes size at the same
> time, or it will fracture.
>
> Free silicon in clay does the same thing. If it is taken through 573
> degrees, up or down, there is a size change in the silicon, which can induce
> fractures which can joint together after a few more thermal cycles and the
> component fails.
>
> JIKO Stoves, which have a ceramic liner with a surrounding metal shell are
> made so that when (not if) the ceramic fails, the metal will hole it
> together. The metal also prevents a certain amount of mechanical shock. The
> reason they are not made entirely of clay is because it simply would not
> last.  On the global scene, 1 year is considered very good lifetime for a
> clay stove.
>
> That said, it depends on the amount of thermal shock the clay receives.  It
> is it a charcoal stove lit with kerosene it is likely to be faced with
> sudden changes in physical size on the inside of the stove. The inside
> expands, cracking the outside surface, if the compressive strength is much
> higher than the tensile strength (which is usually is). Making the clay
> porous can weaken the structure, making it 'spongier' and this can reduce
> the fracturing for two reasons: it has a lower compressive strength and it
> is slightly stretchy in tension.  The problem is that getting it into that
> conditions usually means not having something strong enough to make a stove
> from.
>
> The Baldosa tile made traditionally in Central America is an interesting
> exception to this general approach. It has nearly no free silicon in the mix
> - quite how is not clear, but I had it tested by Bruce recently.  The
> expansion coefficient is quite high, more than 5 x 10^-6, but it exhibits
> very little evidence, if any, of free silicon changing size at 573 degrees.
> It is important for researchers to find out why.
>
> The Baldosa tile is known for being able to take quite high temperatures as
> a flat tile, I think provided that it is given room to expand on one side
> and not on the other, which would make it curve a bit.  It is a little big
> spongy, quite strong, but quite expansive.  Normally this would be death to
> a stove part, but the lacy of a sudden change in size on one side as the
> surface goes through 573 degrees points to a possible way to avoid the
> development of low thermal expansion materials.
>
> The clay stoves fail mostly because of thermal stress from differential
> expansion.  The is expectable because you have a material that is quite
> rigid, brittle and changing dimension on the hot side faster than the cold
> side.
>
> Making a ceramic 'insulative' (reduce the heat conduction coefficient)
> invites additional stress because there is a high temperature difference
> between the two sides, however the pores introduced to reduce heat
> conduction often make it spongy enough to partially of fully counteract the
> increased stress.
>
> Some natural clays have a thermal expansion coefficient of 3.0 x 10^-6 or
> less however even these, unless fired to a low temperature so they do not
> form a lot of ceramic bonds, will fail quite quickly when made into a
> bucket-shaped vessel.  Baldosa tiles, formed into a ring, are likely to have
> a short life, unless they are heated very slowly.
>
> Another possibility is to increase the heat transfer coefficient so the
> entire stove heats evenly or nearly so.  This loses heat so the stove should
> have a heat-recycling design where the heat is picked up and taken back to
> the fire.  However it reduces the physical stress by making the two sides
> more or less the  same temperature.
>
> The features of a clay that will make a stove are:
>
> Low thermal expansion coefficient
> Low free silicon content
> Low porosity including quasi-porosity
> High tensile strength relative to the compressive strength
> High heat conduction coefficient
>
> There are many 'refractory' ceramics that have the characteristic of having
> a low heat conduction coefficient (for building kilns etc) but these are
> usually very soft and can even be cut. Others have high 'temperature
> resisting' properties but conduct heat very well.  Others can take massive
> thermal shock, but tend to be very low thermal expansion mixes.
>
> The problem is that most of these are fired at very high temperatures,
> usually far above their 'working temperature'.  That proves to be impossible
> to produce in a low tech environment.
>
> I am very interested to see what you discover in our work.
>
> There is a very interesting paper showing that the clay:non-clay ratio is
> very important to finding a mix of clays that will survive well. One the
> members of this list sent it to me so it might pop up again.
>
> Best regards
> Crispin
>
>
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