[Cob] Lava Rock

[Anna Young]

This particular pumice mix only has about 1-2% of the big ones; the rest 
grade right down to coarse sand size. Our mix has about 1/4 to 1/5 pumice 
in, and if I feel there is too much pumice in a bit when I am working it 
onto the wall, I just put it in the reject pile. I don't have concerns about 
structural integrity, we are careful not to crank the proportion of pumice 
up too high though. Our cob is not load-bearing and is still about 12-14" 
thick at the wall tops, so I am being less conservative than I might be if 
it was holding the roof up. I did have to remove some last year to 
accommodate some resized windows, and it was reassuringly hard work to do 
it. Our house site is quite shady and marginal for cob, so we were keen to 
incorporate added insulation where possible, and pumice seemed like a good 
way to go. Pumice is also quite fragile so it's easy to break up too-big 
bits by hitting them with a hammer, my favourite problem-solving technique.

As always, the mix proportion is something to keep an eye on. It would 
easily be possible to add in so much pumice that structural integrity and 
thermal mass were trashed and it all fell down when the big one hit. The 
completed pumice cob house already in Victoria is apparently performing 
well, so I think adding a certain proportion of it doesn't affect thermal 
mass too much.

Anna
PS You will need to get extra pumice if you have kids, as they will make off 
with loads to check it all floats...


> Hi Anna, Glad to hear you are on your way with your place. The size  of 
> your pumice aggregate raises a question about the 2" part. Around  here 
> that means there could be a large quantity of bigger than golf  ball 
> chunks. Other than the inconvenience, my concern would be the  integrity 
> of the wall. It  entirely possible it's nothing to worry  about but have 
> you asked anyone knowledgeable about that?
>
> It would be less of a concern if there aren't very many pieces that 
> large, OR you could get a piece of hardware cloth, I think that's  what 
> it's called, it's like a screen but made of something like  quarter inch 
> wire. It's an extra step but it seems getting out those  large pieces [ 
> which could be used elsewhere ] would benefit the  integrity of your wall.
>
> Ron