[Stoves] Tip from a Potter

Thu Dec 6 17:44:06 EST 2007

Stovers,
There is a lot that can be said about clay. There is the Atterberg 
limits that measure shrinkage, liquid limit and plastic limit. I also 
developed a procedure that does an excellend job on measuring particle 
size. Also a way to measure particle density. If we can find a clay that 
works as we want we can take the readings and use that as our goal. If 
it is of interest i can bring the equipment to stove camp or Ethos.

Frank

Lanny Henson wrote:

>I have been using a simple bar grate folded from a strip of metal and it has 
>been surprisingly durable. I have not burned one out yet and they don't seen 
>to warp like expanded metal. The one I use in my charcoal starter is made 
>from a strip of metal spiral cut from a can. Pallet banding works well too 
>but it is stiff and harder to bend. I believe I posted something to the 
>Bioenergy Website about the "Simple Bar Grate"
>Lanny
>
>----- Original Message ----- 
>From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
>To: "'Discussion of biomass cooking stoves'" <stoves at listserv.repp.org>
>Sent: Thursday, December 06, 2007 3:08 PM
>Subject: Re: [Stoves] Tip from a Potter
>
>
>  
>
>>Dear Charlie
>>
>>You brief message will make my day.
>>
>>I am not overly optimistic about low and medium tech grates made from clay
>>so far.  There are so many fundamental problems to overcome that getting a
>>long life grate (like one comparable to cast iron or glass) if a serious
>>technical challenge.
>>
>>Look for example at the Shuttle tiles.  Thirty years on they still don't
>>have tile that can be heated rapidly on one side, cool on the other and 
>>can
>>also take the impact of even a small piece of ice.
>>
>>Nigel mentioned once a ceramic he saw made by a professor at Queens
>>University in Kingston that was flexible like rubber. That is the sort of
>>thing that can be crucial to overcoming the problem of cracking.
>>
>>    
>>
>>>Does anyone has data on clay grates - rolled clay with holes?
>>>      
>>>
>>Rolling clay cause the introduction of slip zones which increases internal
>>stresses by causing difference in the shrinkage in thickness, diameter and
>>circumference.  It is often assumed that clay shrinks the same amount in 
>>all
>>directions as it dries. No so if it has been rolled, extruded or massaged 
>>a
>>lot by hand. Thus pressing a nearly dry powder in a mould prevents many
>>stresses from being there in the first place.
>>
>>    
>>
>>>How thick?  I can start by drilling holes in clay tiles (baldosa?).
>>>      
>>>
>>Whatever you make will fracture if the stress created by expansion on one
>>side is not matched by expansion on the other, or is countered with 
>>strength
>>that does not mind the strain.  The more insulative it is, the more like 
>>the
>>Delta T will be large, and the stress will be in direct proportion to the
>>difference in temperature and the coefficient of thermal expansion.
>>
>>If I can get a sample grate is would be very valuable in assessing the
>>approach envisioned to remove the main cause of cracking.
>>
>>Best regards
>>Crispin
>>
>>
>>Crispin, do you need a Pyrex grate for testing - I have a very reasonable
>>local borosilicate glass workshop here?  Bring to you at ETHOS?
>>
>>Charlie
>>
>>
>>
>>Craig Gallien <craiggallien at slingshot.co.nz> wrote: If you have trouble 
>>with
>>the durability of the parts of your stove ,
>>particularly the grate, try coating the metal part in liquid clay.
>>
>>I use casting slip ( a mixture of clay, water, and a defloculant) to coat
>>the vertical grate of my sawdust gasifier and it has increased its life 
>>from
>>
>>a week to 2 months and counting. The grate is made from element wire from
>>old electric heaters that is then woven into a mesh with a gap size of 
>>about
>>
>>1cm square.
>>
>>I spray it onto the mesh allowing it to dry between coats to build up a 1 
>>or
>>
>>2mm thickness, but dipping would work to.
>>
>>Thanks for everyone elses tips and ideas
>>Craig
>>
>>
>>
>>
>>
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>>
>>
>>http://improvedstoves.blogspot.com/ - just R&D related to fuel efficient
>>biomass stove issues
>>http://ewbappropriatetechnology4.blogspot.com/ - just posts for the ATDT 
>>of
>>the EWB-SFP; AT for developing countries
>>http://travelswithcharlie.blogspot.com/ - most recent travel posts
>>http://travelswithcharlie2.blogspot.com/ - older travel posts, including
>>Nepal travelogue
>>http://www.flickr.com/photos/12820147@N07/sets/ - best of my travel photos
>>http://huiplesofguatemala.blogspot.com/ - my textile project in 
>>Guatemala -
>>what colors!
>>
>>
>>
>>
>>
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>>
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-- 
Frank Shields
Soil Control Lab
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