Clay Cracking.

[raj]

Ok, really getting fed up and could do with some help!!!

Made my base, then my first test clay floor, on top of it, it cracked horrendously, re-tried with 1 clay 2 sand mix, seemed ok for a few days and then….. crack, crack, crack. Not small cracks, large craters.
So back to the drawing board, read the book, read more forums…. Re-built the base in case it was causing issues. Now the base is absolutely solid, no chance of movement.
Re-started the clay base, mixed the quarry clay I obtained, really wet stuff, with dry powered clay from the previous build and mixed with straw and sawdust.
Left to dry, 24hours later…. Crack, crack, crack…. (see pictures)
I’m really fed up of this, what am I doing wrong?? We mixed and mixed the clay, sand, sawdust and straw for hours, got the right consistency, not too dry, not too wet, doesn’t splat when dropped from a height etc…..
What next?? (sorry for double posting, also in Cambridge thread).

[cannyfradock]

Hello Raj

The pictures are too blown up to actually see anything. Try posting some pics from further back. Maybe that would help in seeing exactly where the clay is cracking. You mention the floor. Normally the floor or hearth would always be of clay brick or fire-brick...or even terracotta clay pavers.

It sounds like you have mixed the perfect cob from your clay so perhaps it's the method of applying it is the problem?...or maybe the "firing" or..."non firing" of your oven is the problem. It's a shame to see someone making so much effort....having so many problems.

I'll look out for your further posts.....if you can add any further info, I will try my utmost to help you.

Terry

[downunderdave]

Unfired clay is not durable enough for a floor, you need firebricks.
All clay shrinks, typically around 10% on drying.The addition of sand will reduce shrinkage.
Clay typically takes around two weeks per inch of thickness to dry completely, longer if the underside is hidden from the air.
When build the dome in cob the former needs to be removed as soon as the dome is completed, otherwise the dome will crack on contraction over the mould.
The easiest mould is a sand castle you can easily dig the sand out, but use wet newspaper as a slip layer.

[raj]

Hi all,

Sorry went quite, i've been busy with other work.

Back on this now. I've looked at the first layer of Cob I made last year and whilst still damp, it was OK. I let it dry and a few cracks started to appear!!!!!! Haven't opened it up again since, but plan to start working on it again this weekend coming subject to weather.

The plan was to build the first layer - Clay, Sand, Straw and saw dust as the insulating layer to about 10cm thick, then make a lip around the edge, approx 10cm high to host bottles, fill with sand and flatten, then place firebricks on top ready for dome.

Any help / advise appreciated, especially on trying to prevent the cracking.

Cheers

[raj]

Excuse the crude diagram, but that is what I am trying to get to. The orange layer above the bottles is Sand (ideally Klim dried). Then once I have this done I will build the dome. 

According to my reading this is the advised method of build to retain as much heat as possible. 

Any advise / help appreciated.

Thanks in advance.