Anatomy of a glue joint

Q. We attended your seminar on gluing with Jeff Pitcher. We think that our problem is with the wood surfaces. Can you quickly review those comments?

A: To help us understand a glue joint, we use an analogy comparing the joint to the links of a chain, as illustrated here. The strength of the weakest link is the strength of the joint.  In this illustration, links #4 and #5 are the strength of the wood’s surface which is where you think that you are having a problem.

Links #2 and #3 are the chemical bond between the wood and adhesive (see water drop test below for this analysis); and link #1 is the strength of the adhesive itself, which, if the glue has not been frozen or mishandled, is never an issue with adhesives made in the United States.
So, specifically in response to your question, sometimes the thin surface of the wood that the glue is in contact with is not well attached to the solid wood underneath. (Analogy: Peach fuzz on the skin of a peach is not well attached to the skin.)

This surface layer or surface wood fibers can be damaged due to excessive sanding pressure, due to excessive heating, due to excessive roller pressure when machining, due to inherent weakness in the wood, due to chemical treatments of the wood prior to gluing, and similar.

One quick test for a good surface strength is to put a piece of tape on the surface and then lift the tape off. The tape should not have very many wood fibers on it, check with magnification, if the surface is well attached to the substrate.

Sawn wood surfaces

Have you ever asked yourself, “What part of a saw tooth prepares the wood surfaces that you will glue?” The answer is not the top of the teeth (which is often the part that we sharpen), but the instead it is the sides of the teeth. For this reason, the rip saws used, when gluing off of a ripped edge, should be properly “side-dressed.” Side dressing is not always well done by every saw sharpening operation.

Surface activity

The wood surfaces to be glued must be chemically available, called “active,” for attaching to the glue. A quick test for the activity of a surface is to put a drop of water on the surface. On a good, active surface, the drop should disperse or soak into the wood within a minute or two. An inactive surface would let the droplet remain on the surface like a droplet on a freshly waxed hood of a car.

For example, a surface may be inactive due to aging (oxidation), MC changes, dusty environments, oil contamination (from tools perhaps) or chemical treatments of the surface. Some wood composites use a bit of wax in manufacturing; this makes gluing very difficult. Hitting the surface with one pass of sandpaper often restores activity. (Note that damaged wood surfaces, mentioned above as weak links #4 and #5, will show up good in this water drop test.)

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.