[Stoves] Clay Stoves - Columbia University

Tue Apr 8 12:52:11 CDT 2008

Thank you very much Crispin for such a detailed information on this  
subject. It is valuable information that will be very helpful to my  
project.

If I may, I would like to ask a few more questions.

First is, does the heat capacity of the material make any difference?  
I looked through the "Design Principles for Wood Burning Cook Stoves"  
paper on the Aprovecho website and read that retaining energy in the  
stove body to cook food is a common misconception. With that said,  
does the heat capacity not make any difference in the efficiency of  
the stove? But how about keeping food warm or simmering by placing the  
food inside the stove rather than in a pot on top of the stove. Would  
this provide enough incentive to create a stove with high heat capacity?

Second, it seems that insulating the stove from the fire is quite  
important. I am looking into adding burnout fillers to create air gaps  
within the clay to make it more insulative. In comparing the increase  
in insulation to the decrease in strength of the stove, is this an  
appropriate method to improve the stove? Or are there better ways to  
improve the insulative properties and strength of clay?

Lastly, is there any way of avoiding failure due to the free silica  
when working with clay stoves?

This topic is becoming more and more interesting to me and I  
appreciate any and all advice/suggestion/information. I am  
particularly interested in an all clay stove desgin because clay can  
be easily found in any community and will also be relatively low in  
cost. By the way, does anyone know a common soil content found in  
Africa? Or if anyone knows any papers listing soil content from  
different areas, please let me know.

Thank you. i truly appreciate everyone's input.

Sincerely,

Tae Young Lee

Quoting Crispin Pemberton-Pigott <crispinpigott at gmail.com>:

> 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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