Cob Chimney?

[Don Stephens]

Hi Mike - About that chimney:  Two thoughts - Why not put a hat on it.  A
chimney cap that was made of tern metal, copper or corrosion steel and
extended about out about a foot beyond the chimney all around would
greatly reduce the weathering run-down, could be shapped to reflect the
slope(s) and style of the roof, would keep rain out of the inside of the
chimney, could include a spark screen to keep embers in and
squirrels/bugs/bats/etc out and would be an honest expression of
protecting the cob chimney from a recognized vulnerability.  If you used a
metal that developes surface oxides, these might, over time, deposit some
oxides cown the cob surface, adding to its "sabi".  

Second, let me speak a sacralege by suggesting you might add a little
cement to the chimney mix.  One of my clients has been doing extensive
testing with various clay/sand/cement mixes and finds about 1 part cement
to 2 parts clay and 4 parts sharp sand proves waterproof enough to prevent
freeze/thaw breakdown even in unprotected horizontal paver type
applications in our eastern Washington, USA climate.  These pavers still
breath enough to dampen and dry but never take up enough moisture to allow
internal ice preasure to break them up.  In compression strength tests
they also provide 1500 to 2000 psi readings, depending on how they were
cured.  I try to minimize cement use every way possible but also seek to
avoid failures that require doing things over!  question:  Do you plan to
provide any reinforcement - steel, bamboo, twiggs, whatever - to address
lateral loads?  

P.S. It was quite common in frontier America to build
chimneys of "logs" about 6-8" in diameter lined with cob or just plain mud
- one has been in use again at the reconstructed Fort Clatsap (where the
Lewis and Clark Party overwintered at the mouth of the Columbia River in
NW Oregon) for at least 15 years.  I could fax you pictures. - Don
                          *    *    *    *
(On Sun, 26 Apr 1998, Mike Carter wrote:

Has anyone out there built a Cob chimney (above roofline)? Our flue is
made out of Cob (no straw, high sand mix liner)...we need to extend (it)
above the roof...This means...no "hat")