[Cob] cobbing over a woodstove

[Barbara Roemer and Glenn Miller]

I'd be a little leary of cobbing over willow on both sides unless you've
done some testing.  Ditto, though for different reasons cobbing over
woodstove.  We have a bread oven, and just as Kiko Denzer says in his book,
it gets mighty hot - as high as 700 degrees - and holds the heat overnight
so that in the morning, even though you've swept out all the coals the night
before and the oven is empty with just a fire screen over the opening, the
screen's metal handle is still to hot to touch the morning following a pizza
baking session.  At Black Range several years ago, there was a fire adjacent
to a cob stove.  It's not exactly analogous, but the heat carbonized some
nearby straw so that it eventually caught fire.  There was an air path,
which you wouldn't have in cob, but the problem of cellulose materials
hidden in a wall, carbonizing over long periods, and then suddenly igniting
is well known.  That's the reason you have to have sheetrock over the studs,
an air space, and then something like cement board behind a woodstove when
you frame conventionally.

When we built our bread oven, the first 2-3 inches of the cob mix had no
straw but did include insulative material (vermiculite), with more
insulation up around the chimney at the hottest points.  Successive layers
on the oven included lots of straw, but we kept the chimney area free of
straw in the hot zone.  I'd worry about the willow eventually combusting,
though it is unlikely with no air path.

We are cobbing around our triple wall stove pipe for further heat retention,
and considered cobbing around the woodstove, though ours has openings on
top, side, front and back, so it proved not to be feasible. In putting cob
around the woodstove, keep in mind that metal expands with heat and shrinks
as it cools, most likely leading to cracks in the cob.  Those old log houses
with catties often saw chimney fires....There's probably more than one
reason they are no longer built and masonry has been substituted.  If you've
ever seen the difference between a fireplace with a firebox lined with fire
brick and mortared with fire clay mortar, and a firebox lined with brick and
regular masonry mortar, you'd be convinced that the high fire material
stands up much better to intense heat.  You can expect brick deterioration
and mortar failure in less than ten years if you don't use material to
withstand the heat.

Barbara