Glue Joint Failure in Heat-Treated Wood

      Wood that has been "roasted" at 300-plus degrees Fahrenheit loses some of its adhesive bonding qualities. January 19, 2012

We have been using roasted wood (heat treated) for some time usually gluing it to another type of wood. Lately we have gluing it to itself making panels. We have experienced some joint failure. We are using regular Lee Valley GF glue and clamping time is sometimes overnight. I am not new to woodworking, I have 30 years in making and gluing up panels as a professional but this has me stumped. It is not every joint and not every time.

Forum Responses
(Adhesive Forum)
From contributor D:
The best suggestion I have is to use a finger joint or a glue joint with several zigzags if you aren't already. A lot of folks swear by a butt joint. Tooling manufacturers make these finger joints for a good reason, and manufacturers of engineered wood products use them for these same reasons.

From Gene Wengert, forum technical advisor:
Although such wood can be mechanically fastened well, the chemical bonding is poor as the heat has destroyed the bonding sites for adhesives.

From contributor S:
Gene: Roasted wood - what exactly is that? How hot does it get? I've heard of people cooking spruce guitar tops to "set the pitch" and stabilize it. They take it up to around 210 degrees F for around an hour. Is this it? Would epoxy work? Or joining a new gluing surface?

From Gene Wengert, forum technical advisor:
In today's discussions, "roasted wood" is usually referring to wood that has been heated to well over 300 F. I cannot imagine that the heat used to set the pitch is close enough and long enough to cause any changes within the wood (other than evaporating some of the pitch).

From contributor S:
I appreciate the reassuring words; I never take wood up to that temperature.
Luthiers have been baking guitar tops for years now, explaining variously as "setting the pitch", or "pre-stressing" (Collins guitars) and most report benefits. I'm not so sure, especially if the woods aged.

From Gene Wengert, forum technical advisor:
There is no question that going over 160 degrees begins to slowly destroy hemicelluloses. Short treatments will not have much effect. What is more likely the effect of heating is the rapid loss of moisture. When wood is dried, things change. We see the wood getting more brittle, for example. Once moisture is lost, it is difficult to add moisture back and even when added, the wood does not return to its original condition.

This is important for all of us today, as we need to be so aware that over drying of lumber needs to be avoided at all costs. For softwoods, generally, do not go under 9, maybe 10% MC. For hardwoods, 5.5% MC is the bottom and maybe even 6.0% MC. Wood that is not over dried is better wood for the ultimate user in most cases.

From contributor S:
I am very curious about this: Could you recommend a source where I could look over some of the studies on this? I'm sure the pieces were pretty dry when they came out of the oven, but most tonewood dealers claim their boards are 7-8% rmc, so the boards were fairly dry going in. This and the fact that the boards are milled fairly thin seemed to my mind to minimize occurrence of honeycombing or similar stresses. Maybe I'm assuming too much?

From Gene Wengert, forum technical advisor:
Please define "This" as the work on all the topics discussed is widespread.
As one example of MC, I sampled over 150 pieces of lumber from over 35 mills and the wood was all at 7% MC. What I found was 3.8 to 11.3% MC. It is rare that mills can get all their wood between 6.5 to8.5% MC Which is 7 to 8% MC. This would be a standard deviation of 0.3, but most mills are twice that at best. Of course, thin pieces will change MC quickly in an oven, etc - compared to lumber.

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