All about gluing, part 2
Wood gluing diagram

Besides the adhesive, there are four other links to consider in a successful gluing chain.

Second of two parts

Last month we talked about different kinds of glue. Now, let’s explore the factors that affect woodworking glue joints. First revisit the diagram we showed last month. We previously discussed the first link in the chain, the adhesive. Next, we must consider the other four links in the chain.

Viscosity and pressure
The role of pressure when making a glue joint is often misunderstood. Pressure squeezes glue into the nooks and crannies of the wood surface but does not squeeze the glue deep into the wood itself. The pressure also assures that the glue is spread to both surfaces and evenly between the surfaces. The pressure also squeezes out any excess glue.

Here is an often-overlooked point: The ability for the glue to spread and for the excess to squeeze out depends on the viscosity of the adhesive. Colder means thicker and less fluid, while hotter means easy flowing and potentially too much squeeze out. So, it is important that BOTH the wood and the adhesive be at the same temperature, at the time of adhesive application. As little as 20 degrees F makes a big difference. This also means that the adhesive should be stored at a controlled temperature day after day. When cold wood from a cold environment is brought into the plant without a chance to warm up, poor gluing can be expected.

Squeeze out is a good thing. There should be squeeze out the full length of the joint; if not, then the joint may be starved for glue.

For pressure to work properly, the pressure along the joint must be uniform. Uniform pressure from a clamp extends for some distance beyond the clamp. Clamps should be close enough to apply uniform pressure along the joint fully. Also, because the ends of pieces often dry and shrink just a bit in the wintertime, it is advisable to move the clamps out close to the ends. This assures adequate pressure where the pieces might have a small gap otherwise. Moving the clamps away from the ends in the summertime would be reasonable. It is poor practice to join two pieces of wood that do not fit together perfectly; pressure used to straighten them will create a joint that is too thin in some spots and too thick in other spots; overall, a weak joint.

Some gluing operations, after the edge pressure is applied and the excess glue is squeezed out, then apply top pressure mechanically. If the edges are not perfectly at 90 degrees (which can happen with cupped lumber), then when the pressure is applied, the joints will open. Obviously, open joints have no glue in them, as the excess glue has already been squeezed out, so they have no strength.

A good practice is, once pressure is applied, to never allow the pressure applied to drop later. With a drop in pressure, the joint will open, but, as the excess glue has been squeezed out, there will be a lack of glue and a weak joint. Similarly, moving the pieces of wood after the pressure has been applied is usually not a good idea as the glue spread will be affected. Further, some adhesives will lose much of their strength if the joint is moved.

A dowel or biscuit that needs to be pounded in to install it will likely have very poor strength as the joint has too little adhesive. One trick is to overdry the dowel or biscuit so it shrinks in size and fits easily. Subsequent moisture gain in gluing can assure a tight fit.

Activation test for gluing
To test wood for gluing, put a few drops of water on the surface. If the drops are absorbed within a minute or two, the surface is ready to glue.

Interphase (Links #2 and #3)
Two things happen in the interphase zones: mechanical bonding and chemical bonding between the adhesive and the wood.

Mechanical bonding. The adhesive, with pressure before curing, goes into the nooks and crannies that exist in wood. Then when the adhesive hardens, this provides mechanical strength, much like the pieces of a jigsaw puzzle interlock with each other mechanically. To do this, the wood surface must be open. Stated another way, the surface cannot be burnished or squished. There must be accessible nooks and crannies.

Unfortunately, mechanical bonding, especially with dense woods that have fewer nooks and crannies, does not develop tremendous strength. However, in many cases, high level of strength is not required. But it is important to understand that the stress trying to break a glue joint can be because of external forces or it can be due to stress that develops when wood shrinks or swells. In other words, incorrect moisture content creates stress on the joint which thereby decreases the stress left for external forces before the joint fails. Adhesives that primarily bond mechanically have been used successfully, but only when the moisture content was carefully controlled before gluing.

Chemical bonding. Chemical bonding can be amazingly strong. As an example, the conversion of liquid water to ice is a chemical bonding phenomenon. As a simple analogy, consider that the adhesive has chemical magnets that are attracted to the chemical magnets in the wood. Unless the adhesive has been mishandled, its chemical magnets are plentiful. On the other hand, the wood’s magnets are searching for anything to attract, so if the surface is not freshly prepared, the wood’s magnets can be occupied by dust, moisture, the wood itself, or other similar debris.

From a practical standpoint, wood to be glued must be fairly dry, and freshly (less than an hour old) prepared in order to achieve the maximum strength. Further, oils from the wood can interfere with the bonding ability. Heat from cutting the surface (such as from a dull sawblade) can destroy the availability wood’s magnets. A surface that is fresh and ready to glue is sometimes called an activated surface.

A simple activation test for a surface is to put a few drops of water on the surface. If the drops are absorbed within a minute or two, the surface is activated — ready to glue.

An older surface can be reactivated by one swipe of sandpaper, followed by cleaning of the dust. With an oily wood like teak, a swipe across the wood with a solvent, will remove the oil for a few minutes.

To achieve maximum strength, the wood surface also needs to be accessible to the adhesive, as with mechanical bonding.

Magnified glue joint with squished cells
This magnified end grain picture of a wood joint shows the dark glue line between two pieces of wood with squished wood cells, making for a poor connection between the surface cells and the wood below, thus a weak joint.
Magnified good glue joint with no squished cells
This is a close-up of end grain in a good joint with no squished wood cells, making for a solid connection between surface cells and the wood below.


Wood anatomy (Link #4 and #5)
Adhesive attaches to the cells or fibers that make up the surface of the wood. To get a strong joint, surface fibers must be well attached to the wood beyond the surface. Dull tools or dull sandpaper, especially when a lot of pressure is used during machining, can damage the connection between the surface and the solid wood underneath. Compare the pictures of end grain with two pieces edge glued above and below with dark colored adhesive between them. Compare the squished cells (bad joint) with the good joint showing now squished cells.

What this all means is that the surfaces need to be prepared with sharp tools, with minimal pressure. Note that with a circlular saw, it is the sides of the teeth that contact and prepare the surface; sharpening the sides of the teeth is called side dressing, but some saw shops do not do this well. Likewise, we do not want a lot of sanding dust, so usually a surface sanded with 80 grit is better than a 220 grit. But, ordinarily, sanding before gluing should not be needed on fresh surfaces if tools are sharp.


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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.