[Stoves] Tip from a Potter

[Lanny Henson]

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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