[Cob] Earthen floor cracks--Help!

[littlehouseantiques at att.net]

------------------------------------------------------------------------
Dorethy wrote:  
 
"I feel sure the mix did not have too much clay. My recipe was 4 parts 
finely screened sand to 1 part clay, plus a couple good handsful finely 
screened chopped straw and a couple handsful strained horse & cow manure, 
with just enough water to be trowelable. It's a dark, reddish chocolate 
brown, and I love it, but am so disappointed it cracked at every section 
line, almost; and I was so careful to roughen the edges and force enough 
mix into those joined edges. Any ideas? Thanks!"
-------------------------------------------------------------------------

Dorethy:

I'm  a newbie to cob but do know a little more about clay as I've been playing with it for awhile now doing experiments w/ regard to adding ingredients to the natural red clay from my yard to keep it from cracking.    

The trick to getting clay to dry with no cracks is mainly a function of how slowly you dry the clay.    

Adding sand to clay helps it crack less.   (Sand helps to bind the clay particles together AND it also it retards the drying process).  But you already know that, right?  Did you also know that thicker clay is less likely to crack (because it dries out more slowly) and conversely the thinner the clay the more likely it is to crack and curl (because it dries out way way too fast)?    Guess ya discovered that too, since you describe the thicker layers being less cracked than the final thinner layer.    

The results of my experiments to date are this:   

-Mixtures of clay and sand crack less than any other mixture.   

-Whenever I added OTHER ingredients (straw, wood chips, etc.) my clay cracked, no matter how small the proprotions of the other ingredient OR the length of drying time.  

-The only other mixture that worked (and produced no cracks) was when I mixed clay, and paper mache pulp without using ANY sand.  This didn't produce any cracks BUT the texture is rather on the "lumpy" side, probably not one would want for a floor surface.  

So what to do in your case?  Well I'm not sure because you've already mentioned that you "roughened" the edges and forced mix into the joined edges (and this is how one repairs cracks and/or how one joins extra parts when working with clay) but when repairing cracks in clay in addition to scoring the edges, one usually paints a layer of slip* over the roughened/scored edges before applying more clay.  

Did you paint your roughed/scored edges with slip prior to applying more mix?   

Also, this question is a little out of order, but did you test your mix for the final layer before applying it to the floor?  Before you plop another layer on there, TEST that mixture my making a two small tiles (say 5/8"x4"x4") and dry one in a closed plastic bag and let the other dry uncovered.  If drying it slowly in a plastic bag doesn't keep it from cracking, it sure isn't gonna work for the top surface of your floor no matter how carefully you join it to the previous layer.  

Kathy