[Clayart] Need an adhesive that will withstand a hot, dry climate
robertgharris at gmail.com
Mon Jul 24 17:47:59 EDT 2017
Great to hear some applied and practical engineering, Snail! Thanks for
encouraging people to think as hard about adhesion as any other part of
On Mon, Jul 24, 2017 at 9:25 AM, Snail Scott <claywork at flying-snail.com>
> 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
> 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
> 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
> adhesive may not even hold a single pound if the contact surface is too
> 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
> 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
> 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
> their rated capacity.
> Also, consider leverage; ‘moment arm’ is the term. Gluing two flat things
> is easy, because there’s no real leverage pulling them apart. Tile murals
> (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,
> 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
> 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
> 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
> 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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