Cob: lime/waterproofing

[Dena Marchant]

I've heard that an application of lime is enough
to protect the walls of a cistern?  Does anyone
have any thoughts on this?  I'm building a cistern
next summer ( in heavy clay soil ) and would appreciate
the input.

Dena

> > >-----Original Message-----
> > >From: owner-coblist at deatech.com [mailto:owner-coblist at deatech.com]On
> > >Behalf Of Patricia Kerns
> > >Sent: Friday, January 19, 2001 1:55 PM
> > >To: Scott Gregorie Howard; coblist at deatech.com
> > >Subject: Cob: Re: outdoor plaster
> > >
> > >
> > >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
> > >>
> > >>
> > >>
> > >>
> > >
> > >
> > >