Re: Glue on two sides? -- Woodworkers Club of Houston

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Date Posted: 13:56:46 01/07/09 Wed
Author: Arthur Thompson
Subject: Re: Glue on two sides?
In reply to: Gary, the Web Guy 's message, "Glue on two sides?" on 13:56:55 01/02/09 Fri

I learned woodworking from a man with a PhD in wood technology. This was 50 years ago. He also said apply glue to both faces. You have to be sure both faces are wetted by the glue. You might perform the following experiment. Fully wet one face and press together. Then pull apart. Look at both faces to see if they are equally well covered with glue. Look with a magnifying glass. Do both surfaces look really wet? To make it interesting, glue big faces, like two by two inches on a face. Depending on the preparation of the wood, the time since it was cut, and the type of wood, you may find that wetting is not the same on both faces. Now repeat with glue on both faces. I have done this and sometimes everyting is OK and sometimes not.

He also said you cannot clamp too tighly. I followed his advice and have NEVER had a joint fail. He, back then, said that flimsy bar clamps, like pipe clamps, cannot produce enough pressure. You have to use the steel bar clamps with a big lever arm on the screw. (Because that type of clamp is very expensive, I tend to just use lots of clamps.) In the last year there was a research article published in the scientific literature with the same conclusions about pressure.

There is an additional aspect to the glue joint strength. Compressing with great pressure flattens both sides of the joint so that gaps are filled by wood deformation. The glue extrudes out of each contact point, exposing fresh wood surfaces, as opposed to wood that is oxidized or covered with a coating of dirt or oil.

There is a lot of conventional wisdom about this stuff, but it is not necessary to rely on hearsay. There are folks who do scientifically controled research, including the microscopic controls. However, I have questioned people representing the glue manufacturers and they are often not aware of the research.

Art Thompson

>In the recent issue of Woodworker's Journal a reader
>wrote asking why he has to put yellow glue on both
>boards of a glue up. He want's to put glue on only
>one side. The answer man responded by saying that he
>might be okay using that method but the writer
>recommended taking a little more time and put a thin
>line of glue on each surface.
>
>The answer man proceeds to explain the chemistry of
>glue bonding with wood and glue; the glue/wood bond
>being much stronger than the glue/glue bond. He
>states that when you have very few glue/glue bonds you
>have a strong joint and if your board surfaces are far
>apart you have more glue/glue bonds thus a weaker
>joint.
>
>Perhaps placing a thin line on each surface helps
>assure that you don't place too much glue on one
>surface.
>
>At a club mtg a few years ago I recall a rep from
>Titebond responding that you can either glue both
>surfaces or you can glue one surface provided you
>place enough glue on that one surface.
>
>Soooooooo...I suppose the debate continues.
>
>ANY COMMENTS?

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