Cob done by using potter clay & sand

Hi,
I am planning to build a cob oven and I am realizing (through the jar test) that the earth I have has some fine sand and lots of silt, which initially seemed clay (after 30+ minutes, the dirt had almost all settled, and what was left in suspension in water was not much: ie: the water was almost clear). Actually an interesting thing happened which I had never seen before doing jar tests. The amount of dirt swelled initially, occupying almost all the volume of the water, while submerged in water. I marked the level of dirt in the jar after 30 minutes or so, but after a few minutes I saw that the level of dirt was below the mark and after the night it was even lower.

In case I can't find any suitable clay soil, nearby and want to go for the potters clay, how do I know how much Kg of clay to purchase (and sand)?
I know we want a mix that is around 75-85% sand and 15-25% clay (from Kiko Denzer's book), should I just calculate the volume of the external dome and the volume of the internal chamber and subtract the latter from the former to know the overall volume of dirt? Or is it better to have more of each (clay and sand). I know amounts don't need to be precise, but just in order to know approx how much of each may I need (for a ¡n oven of say 90cm (3 feet?) diameter.
Regards

Hi Antonio;  Welcome to Permies!
I don't have an answer as to how much potters clay you might need, other than to say that having too much is better than not enough.
Have you looked for #50 sacks of dry fire clay? That might be easier to use than potters clay.  I would think that it would be available anyplace there are masons, building fireplaces.  

Have you actually tested some of your soil in bricks? I understand you can have material that reads as silty but performs well enough to use for cob.

If you have clay in your soil, you might want to dig deeper and see if the percentage of clay goes up.  Try going down a meter or more and take a sample from that depth.  You may find that you'll get a much higher clay content if you dig much deeper.  That's generally the rule for most areas.  

Glenn Herbert wrote:Have you actually tested some of your soil in bricks? I understand you can have material that reads as silty but performs well enough to use for cob.

Yes I did, but we were too tight with time so we didn't really get to have a good testing.
Eventually we bought some clay, both potters's clay and another type that is used to do firebricks. But we didn't have a chance to do any testing to check the best mix either.
Eventually we did the first layer, the combustion chamber out of the second clay and sand, and the outer layer out of potter's clay by improvising a little bit on the go.
What I found was that the cob that we did was probably either too wet or too clayee which meant that when building the chamber the cob that was put at the base of the dome started to swell under the weight of what was put on top. We managed to build the oven eventually, but we had to use a "trick", to keep the base form collapsing (especially of the second layer..and it was occupying more and more space).

My grandson and I are in the process of building a 25 inch oven on a 48 inch base...Using some local post hole clay with pottery clay scraps and natural clay kitty litter soaked 3 parts sand to 1 part clay and just enough straw to make a good form...so far so good...if you know of a pottery clay facility near you they may sell scraps which can be easily soaked mixed together...for my dome the inside oven layer am going to use Hawthorne 35M powdered clay can be found on Amazon...not too pricey...for my 1st I went with the 25 inch...starting small and then gonna work our way up to a larger oven....my thermal mass so far has been bottles and perlite, sand...good luck on your building

Isn't that going to be inconvenient with the oven on the ground?  You're going to be kneeling all the time to cook in the oven.

No, Graham not inconvenient...putting two more brick layers on and then a top slab...will be just right so I can sit down and work and so it will be the right height for my grandson to learn to bake in the oven...he's in training...

The one thing I have learned is never to be too tight for time, when dealing with the issues covered in this whole forum.
I actually race motorcycles, but will not allow myself to be hurried at any other time.
Permaculture, homesteading self building needs thought put in to it.
It does not need jobs done twice because they were rushed.
I make the time to think, doodle and plan most things I do.
It means the outcome is usually better, sometimes even cheaper, because I have realised I can use stuff I have laying around or I make the time
to wander around a couple of junk yards I know, or even dumpsters and tips.

Perhaps consider the fine art of doing nothing for a while.
It makes jobs go faster believe it or not.

Ok.  I was under the impression that the glass bottles were just under the oven base to act as an insulation layer as per the image I've linked to.

John C Daley wrote:The one thing I have learned is never to be too tight for time, when dealing with the issues covered in this whole forum.
I actually race motorcycles, but will not allow myself to be hurried at any other time.
Permaculture, homesteading self building needs thought put in to it.
It does not need jobs done twice because they were rushed.
I make the time to think, doodle and plan most things I do.
It means the outcome is usually better, sometimes even cheaper, because I have realised I can use stuff I have laying around or I make the time
to wander around a couple of junk yards I know, or even dumpsters and tips.

Perhaps consider the fine art of doing nothing for a while.
It makes jobs go faster believe it or not.

I am taking my time...have done allot of thermal mass and load math along the way...and $$ outcome, which is pretty inexpensive at this point..adding my base floor pics had found the perfect spot 8 inches of red sand clay and underneath that was a rock table of sandstone :) dug down to the sandstone added water and stomped my way to a level floor added extra water so the floor would level itself and bingo it did...

Just curious how this one came out?