[Cob] How warm must it be to work?

[Shannon C. Dealy]

On Sat, 23 Oct 2004, Amanda Peck wrote:

[snip]
> I don't think that cob, or pure clay/sand or even clay/sand/perlite mixtures
> "set" the way that lime or concrete does.  So the problem is freezing while
> it's still too wet not to get ice crystals extruding an inch up from the
> clay the way I vividly remember from my red clay childhood.

This is essentially correct, when a cob type mix freezes while still
significantly wet, it puffs up like popcorn from the expansion of the
ice crystals and loses integrity.  I don't know the specifics of your
design or mixes, but for many ovens, the simplest thing is to build a
small fire in them to dry them out, this can however have a down side if
your mix shrinks to much the oven will crack (potentially in many places).
Kiko once made an miniature oven in two hours  for me to use in a display
at a local festival (dried it by firing it) it did crack, but was still
good enough for our needs.  Of course doing it this fast is not the best
approach (we were both to busy to spend any real time on the oven), but
for a larger oven and a smaller fire, you may find the results to be a
reasonable trade off.  Another (safer) way to dry it faster is to just put
one or two fans near it, but even this can crack a drying mix if it dries
to quickly.  Cracking is caused in part by uneven drying -- one section
dries and gets hard while an adjoining area is still soft and shrinking,
results in them pulling apart, since the hard area can't move with the
section that is still drying/shrinking.

>
> Or it's too cold to mix barefoot.
[snip]

As one of the few people dumb enough to build with cob in winter in a
moderately cold and very wet climate, it's only to cold to mix if the
mud cracks when you stomp on it :-)


Shannon C. Dealy      |               DeaTech Research Inc.
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