[Clayart] Need an adhesive that will withstand a hot, dry climate

Snail Scott claywork at flying-snail.com
Mon Jul 24 11:25:34 EDT 2017


Actually, ‘hot' is not usually an important consideration unless the work is placed 
in areas where temps can top 250F (such as your summer dashboard, or behind 
a heat-trapping south-facing wall, or near your kiln), and ‘dry’ is seldom relevant.

More often, the critical consideration is the surface area of the joint. 

Strength of adhesives is rated in psi (pounds per square inch), or in metric 
units of mass per area.  For any given adhesive, you can hold a huge weight 
easily if the contact area of the joint is sufficiently large, while the same 
adhesive may not even hold a single pound if the contact surface is too small. 

Let’s say a hypothetical adhesive is rated for 100 pounds per square inch. 
A three-inch by three-inch joint surface is nine square inches, so it’ll hold 900 
pounds of direct pulling force! (Of course, not every application job is perfect, 
but not all forces are in the most vulnerable orientation.) Now, let’s take a 
smaller example, like an earring back. It might be equivalent to 1/4” x 1/4" of 
total surface area, so now that same glue is only capable of about 6 pounds 
of total strength, and that’s assuming that adhesion conditions were perfect.  
You can accidentally exert that just by pushing with your thumb against the 
earring post. Most adhesives are rated higher than that, but most aren’t 
perfectly applied, either, or not to appropriate surfaces, and thus never attain 
their rated capacity. 

Also, consider leverage; ‘moment arm’ is the term. Gluing two flat things together 
is easy, because there’s no real leverage pulling them apart. Tile murals weigh 
(sometimes literally) tons, but they are flat to their wall, and tile mastic or grout 
or construction adhesive are sufficient to hold them for decades or more, because 
the only force is gravity, acting parallel to the bond surface. Neither of these will 
hold a measly little earring to a post, because the post is probably longer than 
the  backer is wide, and gives (even if the post is chintzy) a whole lot of leverage 
relative to the strength of the bond.

Surface area and surface properties are possibly the two most important and 
most under-considered factors in adhesive selection.

Not really talking about Deb’s earring project here - it just seemed like an 
opportune moment to bring up a generally ignored topic. I sometimes hear 
someone going on at length about their ceramic processes, but then they go to 
the store and buy whatever glue 'looks strong', or comes in a nice friendly package. 
Lots of people make wall-hung work, or work in parts for post-firing attachment, 
or do repairs, and the forces acting on the adhesive ought to be understood at 
least as well as the other aspects of the process. 

               -Snail


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