[Cob] Good clay?

[Dave Brown]

This is my first venture into the world of cob.  I intend to build a 
cob oven in the back yard.  This area of WI has plenty of good 
clay.  A fraction of a foot of topsoil, then clay, and clay, and more 
clay ... right in my backyard.  Or so I think.  This area used to be 
big brick making, predominately red brick from the local red clay.

I tried the shake and let it settle test.  After 24hrs I find that 
there is almost no sand or grit in the bottom of the jar.  I don't 
recognize any silt at all, and the slurry is still thick as a melted 
chocolate/strawberry McShake.  It appears to me as if I've got instant slip.

I know that there are some doubts cast about the shake test, but I 
think it's at least given me an idea of what I have to work 
with.  What's the next best step to take to determine if what I have 
is good clay or is it silt masquerading as clay?  Or would it just be 
best to simply use it for what I think it is and make a series of 
trial cobs (varying sand amounts and with/without straw?

Suggestions?

Dave Brown
Green Bay, WI