[Cob] RE: Adhesion/bonding of PVC and Cob

[James Thomson]

Ok, I'm going to weigh in on the cob/PVC question...

I don't buy the vacuum theory, for a couple reasons:  there's no change 
in airspace when you pull an open piece of pipe through cob, and also 
cob is not air-tight.  Any vacuum that did occur would be quickly filled 
with air.

For me, the described example is somewhat analogous to pulling a nail 
out of a piece of wood.  I have two theories on what's going on:

1st and most likely, is that the shrinking of the cob that occurs when 
it dries clamps down on the pipe and simple friction makes it difficult 
to pull out.  The same thing happens with glass, metal, and other kinds 
of plastic.

2nd, and maybe little far-fetched, is that there is actually a molecular 
bond between the clay and the PVC.  Clay molecules carry a slight 
negative charge, which is why they bond so nicely with water.  I don't 
know anything about the chemistry of PVC, but if it has any potential 
for a positive charge, there could be a slight molecular bond.

I'm always amazed by how well cob sticks to just about anything: glass, 
metal, rubber, shoes, skin... clay is amazing stuff!

just my $0.02,
James

BTW- See the movie "Blue Vinyl" before you use any more PVC... Yikes!


littlehouseantiques at att.net wrote:

>Not a scientist by any stretch of the imagination but seriously doubt that COB and PVC form a bond 
>of any kind (molecular or otherwise!)  
>
>I, like David, believe that the reason that PVC wouldn't come out is because a vacuum was created
>between it and the cob.  
>
>Reason I think this is that I have many times used 4 inch PVC pipe as a "form" when making pots and
>planters with hypertufa and believe me it is the VERY dickens to "break" the vacuum that is created
>between the surface of tufa and PVC...just when you think nothing short of dynamite is gonna get that PVC
>to move, you will hear a little "wooshing" sound of the air rushing in there and then and ONLY then can
>you pull the PVC out of the hypertufa (and even after the vacuum seal has been breached it is still very
>tough to remove!)    The smoothness of the PVC should make it an excellent material to use as a form because other materials will not "stick" to it (tufa, clay, cob, etc.) however, I think it is the rigidity of the PVC that causes the vacuum and the removal difficulties.  
>
>Kathy in PA
>
>  
>
>-------------- Original message from "David Knowlton" : -------------- 
>
>  
>
>>sounds like a garden variety scientist is needed to study this. 
>>
>>conventional wisdom says clay will bond better with rough objects. 
>>is this something like water tension, or suction? you know, the steamy 
>>hot saucepan lid that gets 'stuck' to the smooth counter? 
>>
>>any materials scientists out there? 
>>
>>dave in tampa 
>>
>>    
>>
>>>From: "Marlin" 
>>>To: ap615 at hotmail.com,coblist at deatech.com 
>>>Subject: RE: RE: RE: [Cob] sinking logs in cob? 
>>>Date: Fri, 01 Oct 2004 14:24:50 -0400 
>>>
>>>...snip..... 
>>>
>>>An anecdotal story...... I put a cutoff piece of PVC water pipe in a Cob 
>>>model as a form that was to be pulled out later (I've used PVC sleeves in 
>>>cob before for water/elec. thru's the wall), it was supposed to be just a 
>>>form for the model's arch. I couldn't get the PVC (how slick is that?) out 
>>>of the wall after it dried and couldn't tap it out with a chisel and 
>>>hammer. I didn't want to start making big blows on the little 2 inch thick 
>>>cob wall but it impressed me that the PVC was probably gripped in by the 
>>>cob at a very microscopic level. 
>>>
>>>Marlin  
>>>      
>>>
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