Humidity and Raised Glue Lines in Furniture

      A closer look at detectable raised glue lines that appear in solid wood joints, related to humidity changes. December 16, 2011

Question
I have used Titebond, Titebond II and Titebond III for solid wood gluing. After I complete a piece, everything is nice and smooth. Then the glue expands with changing humidity levels, and you can feel the glue lines. Is there a glue that is easy to use, strong and doesn't expand with humidity?

Forum Responses
(Adhesive Forum)
From contributor J:
I'd say it's likely you're misdiagnosing the problem. The glue isn't expanding with the humidity. Instead, your lumber is too wet before or during assembly. The glue line is rigid and dimensionally stable, but the wood around it shrinks as it dries out, which makes the glue line stand proud of the surface. Make sure the wood you're building with is appropriately dry to start with, and check the humidity in your shop and lumber storage areas.



From the original questioner:
I would typically agree if I didn't just finish this piece. My wood supplier's kiln dries everything and I test the moisture level. The only problem is the humidity in my shop. The day in question the humidity was terrible. However, I have the same problem with pieces that I built 7-8 years ago. If the air was dry and the wood shrinks, then I could see feeling the glue line. It still doesn't make sense to me. One person sent me a message about maybe too much glue in the joint. I always load it with glue because everyone says not to starve the joint.


From contributor O:
I have seen this many times. The regular wood glues do not lock the edge wood joint because they are always a little flexible. Plastic resin (Weldwood) cures rock hard and locks the adjoining parts tight.


From contributor E:
You're adding moisture to the joint with the glue. If you let the squeeze out dry completely before scraping off excess, it adds more moisture to glue joint line. Then when the humidity dries out of the wood panel, the joint area dries less, making it raised from the surrounding area.

Glue should be scraped off before complete drying, when it has set some and will not be spread around by scraping.



From contributor P:
The glues you mentioned using are plastic and will creep up when they can. Such glues will always show up in otherwise well made and properly dried and joined flat work. Use a rigid glue line glue - PPR or similar, and you will not have the problem. Try a few tests, include the conditions listed by others above, and you will see what the cause is.


From Professor Gene Wengert, forum technical advisor:
Although the PVA family adhesives are indeed somewhat plastic, they are also quite rigid and their plasticity is not an issue except when they are under a large load. As stated in the first response, the glue joint area itself is fairly rigid, even with PVA and more so with some of the other adhesives mentioned, which are also plastic, incidentally.

However, the wood is dynamic and as the RH changes, the wood's MC will change and that means its size will change. So you have the wood changing size in response to the RH changes in the air around the wood. However, the glue line is fairly rigid and so it does not change size as much as the rest of the wood.

Note that there is a special case when a glue line is sanded smooth in less than 3 days after the glue joint was made. The water in the adhesive which caused the wood right at the joint to swell is still there. After 3 days or so, this water is gone and the wood around the joint shrinks a bit. This localized shrinkage causes a small depression right at the joint (called a sunken glue joint). The archives have a few articles about this including the one linked below.

Woodworking 101: The Basics



From the original questioner:
The glue joints don't stand out on the furniture at the store. Does anyone know what glue the big manufacturers use?


From Professor Gene Wengert, forum technical advisor:
They use an adhesive in the same family as TB.


From contributor W:
What is the bonding strength (lbs/sq. inch) achievable for PVA wood glue and UF glue at normal room condition (without through water soaking or heat stress)?

Can the wood joint expansion or sunken phenomenon be alleviated if employing higher bonding strength and moisture resistant wood glue?



From Professor Gene Wengert, forum technical advisor:
The bond strength is larger than the wood itself when properly made. In response to the second question, "No."

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Have you reviewed the related Knowledge Base areas below?
  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Adhesives, Gluing and Laminating

  • KnowledgeBase: Adhesives, Gluing and Laminating: Glues and Bonding Agents

  • KnowledgeBase: Furniture

  • KnowledgeBase: Furniture: General


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