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