Cob: Re: outdoor plaster

[Patricia Kerns]

I would not plaster a bench with lime.  To be accurate, I should say, I've
done it, and I wouldn't do it again.  Lime is not waterproof.  While it is
highly water resistant on vertical surfaces, water soaks through it quite
readily on a horizontal surface.  I tried this first over a straw bale
encased in cob, then over just cob with sand/mud filler.  In both cases, the
wetting/drying of the inside was at a different rate than that of the lime,
and I got continual cracks all around the corners and edges.

I recently attended a workshop at the Steen's, where we did some experiments
with clay/lime plaster mixes.  Everyone there (including me) who had ever
tried lime/clay mixes had bad experiences - mostly ending up with very
crumbly plaster.  We did a few tests, using different proportions of
lime/clay, and it was amazing how much a tiny difference in the mix could
make - the difference between a disaster and a good result.  I wouldn't just
go out and try making your own lime/clay plaster without doing some
experiments such as this with small amounts first.  One thing that did
improve the plasticity of the lime/clay plasters we tried was to add in a
small amount of prickly pear juice (which you can make by simply chopping up
the pads and letting them soak awhile).

I have experimented with coating adobes with linseed oil, with good success.
I later soaked the blocks in water for two days, and the portions
impregnated with linseed oil were impervious (the remainder of the blocks
melted completely).  I wouldn't do this for a building where you want it to
breathe, but it should be OK for an outdoor bench.  Use the linseed full
strength for the first coat, then add a few more coats, cutting the linseed
oil with progressively more thinner (mineral spirits or turpentine), until
you end with a coat of 25% linseed and 75% mineral spirits.  By the way, if
you are going to coat a mud structure with this type of sealing finish, you
need to be absolutely sure the structure is completely dry throughout first.
I heard one story (I believe from Athena Steen, but not sure) of a cob or
adobe bench that was sealed up before dry, and it literally exploded, as the
water vapors inside tried unsuccessfully to escape.

I heard of one other finish for outside cob, also a sealant (i.e., not good
for building walls), but have never tried it.  I believe it was melted
paraffin wax, mixed with linseed and turpentine, painted onto the cob.  The
wax apparently gets carried into the structure a little ways, giving more
protection.  I saw an example of this on an outdoor cob structure in Germany
during a slide show by Frank Andressen, the earth plasterer.

Patricia
>
> I have a question about the best way to completely waterproof outdoor
> cob. I have built an outdoor bench that will not have a covering.
> Also, it is near to a garden beds and so cannot drain and leech any sort
> of toxic chemicals.
>
> What is the best way to waterproof my bench? Lime plaster?
>
> I own the cobber's companion, and know of several techniques, but I have
> no experience in plastering and no experience in finding the proper
> materials to make a lime-sand plaster for example.
>
>
> Thank you,
>
> Scott Howard
>
> Whitman College Organic Gardening club
>
>
>
>