Cob: Couple Of Questions

[Ocean]

The issue of bonding layers of cob and "sewing" straw with "thumbs" is a 
bit overblown as far as I can tell.  I know Ianto vehemently insists it 
is necessary, but he in his wonderful global slide show demonstrates how 
almost no other earthen building tradition worries about it!  And he 
readily admits that they "invented" the use of the thumb to sew straw, 
and well as the making of small individual "cobs" which by their nature 
need to be "thumbed" and kneaded.

Now, when we built our Kiva and the restaurant wall, we just piled the 
cob on the wall by the forkfull or bucketful, then shaped it by hand 
into the shape we wanted the wall, then moved on!  No thumbing, 
kneading, etc.  If you work with sufficiently wet cob and at an even 
rate, the shear compressive load of the material compresses the bonds, 
and the water tends to do as water does, flow downward with gravity, 
through the "dry" layer immediately below.

A problem I see with our listserv here:  we have lots of theory, not 
much experience, and obviously for those of us on the internet, lots of 
time to postulate.  But when building a structure, time isn't always so 
must time available, and even less energy.  So any technique which isn't 
essential for a sound building (e.g. thumbing and making cobs) is 
actually quite costly in terms of human labor, which is the most costly 
item on the cob building budget spreadship.

Ocean


On Monday, July 22, 2002, at 07:31 AM, Sarna Salzman wrote:

> When bonding clay for ceramics and pottery, I use a vinegar-slip to help
> connect two pieces of drying clay together. I'm not really clear on the
> chemistry of it, but it is supposed to be more effective than a 
> water-based
> slip. Wouldn't the same apply to cob? (Though, I guess that might 
> involve a
> LOT of vinegar!)
> Sarna
>
>
>>
>> If the wall dries before you get back to it for the next layer, the 
>> bond
>> between the old and new layers will be significantly weaker because you
>> will not be able to get as good a mechanical bond since the layers will
>> not be intermixed.  To improve the quality of the bonding, whenever you
>> are done for the day, it is best to put a lot of holes in the top using
>> your thumbs or a stick of similar diameter.  By doing this, you 
>> provide a
>> means for interlocking new cob with the old cob even if it dries.  It
>> won't be as good as if the wall were built continuously, but for most
>> cases should be more than adequate.  If the wall has dried before you 
>> go
>> to put on the next layer, it is important to thoroughly wet the surface
>> where you will be adding new cob (preferably repeated soakings over at
>> least an hour or two), this won't soften the old cob to any significant
>> extent (unless it is a really poor quality high clay mix), but it will
>> significantly improve the bond and reduce problems with cracking along 
>> the
>> joint between the old and new layers.
>>
>
>
>