[Cob] a cob bathtub

[Shannon C. Dealy]

On Mon, 28 Mar 2005, Amanda Peck wrote:

[snip]
> Somebody knows this!  Tell us!  How do we add oil to a floor, plaster--or
> bathtub--mix?  Add it to dry sand, mix well, and go from there?  How much do
> we add? something on the order of a cup--around .25 liter--to a 5-gallon
> (around a hundred liters) pail of sand? I've never tried it, and don't have
> a book handy--it might be quite a bit less than that, more like a cup to
> three buckets of sand, and one clay.

I've never found anything written up on this, normally people apply the
oil after the floor or plaster has been applied and dried.  Because of
the lack of information, I did alot of experimentation myself as I was
trying to create a one step floor that would dry as quickly as possible.
I wasn't expecting it to be sealed when I was done, simply much more
durable than a straight earthen floor that had not been oiled (I was and
still am living on the floor surface at the time I was applying it, so I
needed something that was usable in days rather than weeks).  The problem
is that clay will not dissolve in the oil, and you really need
clay in the mix both for it's fine particulates to fill in between the
sand particles, as well as it's binding strength, technically, the oil
when dry would probably provide all the binding strength you need,
however, without the clay it would require massively more oil which is not
only more expensive, but would take MUCH longer to dry.

Unless you have a special reason for doing this, I would highly recommend
going with the more conventional approach of applying layers of oil to the
dried, finished cob surface.

Anyway, the best approach I was able to come up with to date was to do the
usual base/rough surface with a strong good quality cob mix, then figure
out your best ratios for a very high sand cob mix (just enough clay to make
a good binder, but nothing more), you will also want plenty of straw in the
mix, I've been using short straw, though some people I know have started
using longer straw in their recent floor and plaster mixes.  Once you have
your ratios, measure an appropriate amount of your clay (or clay soil)
into a bucket and add enough water to just wet it completely through.  You
do not want a slip here and there shouldn't be any standing liquid or
slip on the surface once it has time to absorb the water, you just want
enough to completely wet the clay while keeping the mix as stiff as
possible, you will probably want to add water a little at a time, and let
it sit at least overnight to give it time to fully absorb the water for
the first batch or two until you know how much water it will take so that
there are no dry lumps of clay left.  In a separate bucket you will want
to make an emulsion by mixing one part bentonite clay (I bought a box of
of generic clumping cat litter which listed only bentonite clay in it's
list of ingredients), six parts water, and eight parts oil (this was off
the shelf boiled linseed oil from the local hardware store).  Mix this
thoroughly, it should end up with a consistency similar to mayonaise,
probably will do better if after you mix it, let is sit overnight, then
mix it again.  Finally, mix your sand and clay together, and in place of
additional water, add the emulsion to get your usual pre-straw cob working
consistency, and finally add the straw.  I mixed this in a five gallon
bucket, making batches of about three gallons at a time (using a heavy
stick to do the mixing).  Keep in mind this is intended to be a finish
coat, so it is typically going to be less than an inch thick and you
won't need that much, three gallons would cover six to twelve square feet
depending on the thickness.  If you dry this quickly (as I did, using a
fan), expect cracking, though of course you can fill in the small cracks
with a fine mix.  Once this is dry it is usable and much more durable than
straight cob, but still somewhat fragile, ultimately it will still require
multiple coats of linseed oil to finish it.

> Using it on top is easier, and you probably want to do it this way even if
> you did use oil in the mix.  Just wipe it on.  Mixed with mineral spirits if
> you have it.  Let dry, repeat--probably three coats in all.  Then leave out
> in the rain for a couple of months!
[snip]

The way I was originally taught this was to use uncut linseed oil for the
first coat, then add thinner to linseed oil in progressively greater
quantities to the succeeding coats (seven was the recommended number as I
recall).  A number of people I know have just been using the linseed oil
without thinners in multiple coats.  I think one person used thinner in
all their coats in order to get deeper penetration into the floor.  The
basic rule in applying linseed oil to anything is to apply it, give it
some time to absorb into the surface (ten minutes comes to mind), then
wipe off ALL the excess (a single drop of linseed oil left standing on
the surface will not only take forever to dry, but will create a gummy
mess if you step on it).  With a floor, you can just work slowly,
and migrate the excess from the treated areas to the untreated areas,
supplementing with more linseed oil as you go.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 929-4089 |                  www.deatech.com