[Cob] Lava Rock

[Anna Young]

We are one of the ones building with pumice cob in Victoria, BC. It is quite 
doable, though a little different from plain cob. We are using a digger to 
mix as well, as it is just me cobbing most of the time. I think foot mixing 
would be quite uncomfortable with pumice in the mix. We are using pumice 
that is sized 2" and down so we can compact it for underfloor insulation as 
well, but I don't think pumice size would matter much if you are just using 
it for added insulation in the cob. The other folk that built with pumice 
cob in Victoria have a website www.eco-sense.ca and I think there is 
something on there about the R-value of their cob, as they are all set up 
with sensors in the walls.

I don't think having pumice in the mix will make it much easier to 
machine-mix or handle; ours is still heavy, wet ornery stuff. Once it is on 
the wall and I am working it with my thumbs, I can't whip them in and out 
real fast to stitch it in, as I end up whanging my fingers on pumice lumps. 
I just go a little slower and use lots of wooden thumbs to spare mine. Also, 
I find it is worth while taking the time to shape the wall and seal the 
outside closed, rather than piling it on and cutting off the extra later, as 
all the pumice lumps make it hard to cut bits off and seal up the wall. 
Makes it a longer and more frustrating job.

For a mix, we are using the backhoe scoop as a measure, and doing one of 
clay, two of pumice and one of sand, with a bit of our fine earth mixed in 
somewhere. We get much better results if we put the scoop of clay to soak in 
the bucket for a week or so first. Then we dump out the clay, spread the 
sand and pumice out in the mix area, and scoop the clay on top before mixing 
hugely with the scoop. My husband is doing a video of the process for 
Youtube, and promises to get on with it now someone out there might be 
interested in it. You are welcome to come by for a visit if you are across 
in Victoria; we will be cobbing till at least September inside.

Anna
Cob/strawbale timberframe in Victoria BC


----- Original Message ----- 
From: "uli hamann" <uli.hamann at gmail.com>
To: <Coblist at deatech.com>
Sent: Sunday, July 17, 2011 10:20 PM
Subject: [Cob] Lava Rock


> Hi  i'm getting ready to finally build the walls on my house. Did the roof
> first   35ft inside diameter  round.   Roof is supported by center log and
> 12 posts around the circumference sitting on concrete foundation and
> stemwall.Since i am doeing this alone i am considering adding some 1.5 to
> 2inch lava rocks that are locally available (bellingham wa) Some folks in
> victoria bc have buildt one using 3/4 inch lava rock which is not locally
> available.So my thought is to try the larger rock here to maybe increase 
> the
> R value, speed up the mixing and make the cob lighter and easier to 
> handle.
> I have a kubota backhoe (running on biodiesel)to do the mixing. Anyone 
> have
> any pros or cons to add to my already overloaded brain as to how viable 
> this
> plan is???      thanks       uli
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