[Stoves] Tip from a Potter

[Crispin Pemberton-Pigott]

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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