Questions on gluing and wood density
22WPELan gene-wengert DB

Gluing panels, joining glass and metal to wood, all about wood density.

Q: We are making edged-glued panels. Our question is: “Can we glue using a sawn edge?” We have heard several opinions. Can you steer us straight and give the reasons? Thanks.

A: Great question and the answer is, “It depends.”  o, let’s go back to the requirements of an adhesive. Most woodworking adhesives need the two pieces of wood to be very close. For maximum strength for PVA adhesive, 0.002” to 0.006” gap is ideal. For PUR, perhaps 0.009” is acceptable for many types of PUR. For, epoxy, we can easily go over 0.020”. In common language, PVA is not a good gap-filler; PUR is a better gap-filler; and epoxy requires a larger gap so that enough internal heat is generated to cure the joint.
But there is another factor. 

The glue joint we make, if done correctly including a freshly prepared, flat surface, is 1.5 times or more stronger than the wood. For an edge-glued panel, if the joint is equal to wood strength, that is plenty good. In other words, the extra strength in a glue joint is not useful. So, we essentially need only 2/3 of a surface of a joint to be between 0.002” to 0.006”. The rest of the joint can be too close or too far apart. In fact, in most edged glued panels, the required strength in the final product is perhaps only 50% of the maximum wood strength, so, from a practical point of view, only 1/3 of an edge joint has to be within 0.002” to 0.006”.

The concern about a sawn surface is primarily the roughness created by the saw blade teeth variations. So, I ask you: “What part of the tooth actually touches and cuts the edge?” The answer is the side of the tooth and not the top. So to get a better surface on the edges, use a saw that has been sharpened on the sides of the teeth and also has the teeth sides aligned. With each other. The key word for this process is “side-dressing.” Some saw shops do this well and other do not, so look for someone that does it right. With the right saw blade, you can easily get a joint, if all else is correct, that is at least 1.25 times stronger than the wood.

We have talked about proper gluing techniques before, so let me mention only two common concerns.

First, the strongest joint happens when surface is sawn no more than about 30 minutes before gluing. Any longer and we risk getting moisture change that will make flat surfaces distort before gluing and gaps will develop. If we do not adhere to this time guideline, then it becomes critical to have wood at the correct MC. Another issue with a time delay is that surfaces age and become less reactive, meaning the adhesive cannot hold on as well.

Second, use the correct pressure. Low pressure means that joints will likely have bigger gaps; high pressure means that oftentimes too much glue is squeezed out and there is no longer enough adhesive to do the job. The role of pressure is not to straighten warped strips.

Q: We are gluing some glass and wood together, but our joints do not seem to have much strength. Any guidance?

A: Gluing glass and metal to wood can be a bit tricky and different compared to gluing wood to wood.

First, the adhesive is different. Use urethane, not butyl or epoxy. Also, there are many urethane adhesives, so use one specifically intended for glass. Although I do not suggest a brand name, I suggest an adhesive made in USA and that is a bit expensive. Your adhesive salesperson is usually a good advisor.

Urethane (sometimes called polyurethane or PUR) cures quickly and is very strong, but it requires moisture to catalyze the cure. Atmospheric moisture (or moisture at 7% MC in the wood) is okay, so application in early mornings is better as the humidity is higher. Avoid using the adhesive in the sun or heated area as that will drop the relative humidity. Full curing time for urethane is 24 hours or maybe more. Do not stress the joint until fully cured. 

The first step after purchasing the adhesive is to clean the two surfaces. Do this within 15 minutes of the application of adhesive — not longer. Remove oil, paint, etc.

Next step, after applying the adhesive, following directions, is putting the two surfaces together in as close to a perfect position as possible. Opening or moving the pieces after joining might weaken the joint.

A urethane joint will lose strength if the amount of adhesive in an uncurled joint is reduced by using more than modest pressure. With too much pressure, the adhesive needed for a strong joint is squeezed out of the joint. Excessive pressure means a weak joint. (Implicit in this approach is that you have enough adhesive in the joint to begin with.) This pressure is lower than when working with Elmer’s glue or Titebond and gluing wood. (If you have an adequate amount of adhesive, and maybe a few spots that have a bit more, modest pressure will squeeze out the excess but not less than the required amount. So, a little squeeze out is likely a good sign.)

Now, urethane cures slowly over the next 24 hours. It might take longer if the relative humidity is low, so avoid working on this in the afternoon when outside humidity is low…as I write this in Georgia in the afternoon, the outside humidity is dry, 39% RH, but it was more than 90% in the early morning. Keep the new joint out of heat (above room temperature) for the first day.

Incidentally, epoxy also needs a thick glue line, but this is because there must be enough adhesive to generate the heat needed to cure the joint. Epoxy needs 24 hours to cure, even if it says 5 minutes on the label.

Q: Can you please explain to me the connection between the density of wood and the values I see sometimes referring to the specific gravity of red oak as 0.64?

A: To answer your question, I will have to be somewhat technical. If you become bored, jump to the last sentence to see why all this is important. For some reason many decades ago, when referring to wood density, researchers began to use specific gravity instead of density.

WOOD DENSITY. The density of wood or any object is an expression of the current or present weight (in pounds or grams) divided by the volume (cubic inches, cubic feet, cubic centimeters, etc.). For reference, the density of water is usually quoted as 62.4 pounds per cubic foot.

When an object is heated or cooled it expands or contracts, so it’s density changes. For wood, this heat movement is minuscule. However, when moisture is added or removed to or from wood, the wood expands or contracts, so its density changes by a significant amount. This density change with moisture content (MC) is why density for wood should include the MC as well.

As an analogy, wood cells are like miniature, long soda straws, with most cells running up and down in the tree. The wall of the cell is 1.5 times heavier than water, so if the hollow space in each cell (technical term: lumen) did not exist or was quite small, then wood would not float in water. In fact, some tropical species, as well as bacterially infected wood, do not float as the overall wood density is higher than the density of water. Ever hear of someone logging a lake or river bottom for logs?

SPECIFIC GRAVITY. As mentioned, many decades ago, researchers began to describe the density of wood by comparing it to the density of water. This comparison number is called specific gravity (SG) and is always given as a decimal rather than percent. So, if the SG is 0.50, the wood is half as heavy as water; if 0.33, one-third the weight of water. We could also say that 0.50 SG wood is one-half the density of water; and 0.33, one-third.

Sounds straightforward until we factor in the MC. The density or SG of wood changes when the MC changes, increasing as the MC drops. So, decades ago, the SG values of wood at 12% MC were determined to be an engineering standard, with the weight of wood at 0% MC. That’s not too handy for furniture and cabinets that average around 7% MC in use. The volume was determined at 12% MC. It is these SG values that we will find in the US Forest Service’s Wood Handbook. As a result, each wood species has one SG value in the tabular listings: oven-dry weight and volume at 12% MC, compared to water at 62.4 pounds per cubic foot. You can make an estimated correction to the tabular values to achieve the SG at 7% MC by multiplying by 1.08.

USEFULNESS. The actual SG of wood can be used, especially in design, to calculate strength and stiffness, nail, staple and screw holding power, lumber weight (pounds per BF), and many other properties and characteristics. It is very helpful when dealing with the differences in processing between the dozen or so species of oak lumped into “red oak” and the same for white oak, plus the eight hickory-pecan species, red and silver maple differences, and pine differences. SG can be as important as other factors such as wood color and grain. 

Q: In response to your recent answer to using southern pine for interior millwork, my company has been providing SYP (mostly loblolly) millwork for several decades.  We learned very early that the reason many potential millwork users avoided SYP was the lumber was at the incorrect moisture content when the lumber left the kiln.  We dry our SYP as carefully as other operations dry oak.  We dry to about 6-8% MC and use conditioning at the end.  We agree with your statement that wood changes size and shape ONLY WHEN THE MC CHANGES.  So, by careful drying, our SYP does perform well as millwork.

A: Thanks for your response. I agree with your approach.  The moisture content concern you show speaks well for your company.


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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.