FW: [Cob] a cob bathtub

[Shannon C. Dealy]

On Tue, 29 Mar 2005, Maria Morehead wrote:

> From: Bex <cobalot at cpros.com>
[snip]
> We have done plaster experiments adding the oil to the mix and I like the
> results. It didn't seem to matter how the oil was added to the cob.....just
> throw some in at any point in the mixing. Amounts? Hmmm My unscientific
> records do not say but I would just follow my instincts. I agree that more
> oil added when the cob is dry is a good idea.
[snip]
> Becky

Thought I should follow up on my posting to clarify what/why I was doing
things the way I did, not disagreeing with Becky, but there were reasons
for my particular (and significantly more complicated approach), as my
requirements were a little different than most.

The goal I had was to reduce the time from application to usability to the
absolute minimum as I was actually living on this floor (the unfinished
parts) at the time I was applying the mix.  I knew that a straight cob
floor wouldn't hold up, so I wanted to get enough linseed oil into the
floor to provide some additional strength and be usable (under light
use) even without the finish/sealing coats of linseed oil.  Here is
some of what I found as the result of various experiments (I could have
built a sidewalk with all the experimental mixes I had drying on rocks,
chunks of old cob and scrap wood, next time I probably will :-)

   - Faster overall drying required that the amount of water be minimized,
     but I still wanted enough to get a good clay bond, so I was trying to
     have just barely enough water for the clay to begin to disolve, and
     make all the additional liquid in the mix be linseed oil which would
     make the mix workable.

   - Increased linseed oil also slows drying

   - Basic chemistry - Oil and water don't mix - so any time you put oil
     and water into a mixture, you need an emulsifying agent to help them
     mix together (otherwise you get a much less uniform mixture and it
     will immediately start separating the moment you stop mixing), the
     agent essentially binds to one and goes into suspension in the other.
     Clays can in some cases be used to do this since it will bind to the
     water, so your cob mix is already part way there, but I found that
     the clay soil I was using did a poor job of this, so I made the
     emulsion separately from the cob mix and used bentonite clay which
     does a very good job of this (it is also the emulsifying agent in
     asphault emulsion which is where I got the idea).  It  sounds like
     from Becky's experience this may not generally be necessary, though I
     expect that the more uniform mixture you get with the emulsion will
     probably dry a little faster and more evenly.

   - To much linseed oil in the mix traps the water and significantly
     slows overall drying time (and you don't want water trapped in the
     mix, you'll get mold).  I suspect Becky probably has never pushed the
     linseed oil content as high as I did in my experiments.

   - Pure linseed oil does not disolve clay to any significant degree so
     your clay becomes in essence just smaller sand particles (still
     useful, but a clay bond in addition to linseed oil would be better).

   - A pure linseed oil mix takes FOREVER to dry, my small sample out in
     the sun during the middle of summer took many weeks to harden.  A
     floor inside a house would have taken far longer.  On the other hand,
     it is very durable.

Ultimately, I was using this floor only a few days after I applied it.

FWIW.


Shannon C. Dealy      |               DeaTech Research Inc.
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