Gluing and Clamping

mandoboy · Post by **mandoboy** » Sun Oct 28, 2007 7:59 pm

When gluing and clamping wood with a water based glue, like Titebond, is it possible to tighten the clamps too much such that you displace the glue and end up with a weaker joint?

I have always cranked down pretty snug on the joints to get an almost invisible glue line and they always seemed to hold pretty well.

JeffG

charlese · Post by **charlese** » Mon Oct 29, 2007 2:22 am

mandoboy wrote:When gluing and clamping wood with a water based glue, like Titebond, is it possible to tighten the clamps too much such that you displace the glue and end up with a weaker joint?....
JeffG

That's what the experts say! Like you I have "over tightened" my clamps for years. Either I'm not strong enough to squeeze out all glue, or there is not much of a problem with most woods. Maybe there is such a problem with some exotic woods.

When I took a college course in "Wood Gluing and Physical Properties" I cant remember over clamping being mentioned as a problem. That was a long time ago and we were using resorcinol and urea-formaldehide glue mostly. Our main interest was in making laminated beams using softwoods and 12:1 scarf joints at the ends of boards. The professor often pointed out that a good glue joint has no thickness. That was even a question on the final exam. The question was something like this: What is the measured difference in thousands of inches, between an edge joint using resorcinol and a face joint using urea formaldehide glue? This was an essay question. The correct answer was short. - 0.000

I now use mostly red oak in my shop. I use yellow Titebond all the time. I believe the important consideration in a glued joint using oak is the inter-mixing of one narrow layer of glue into the vessels and fibers of both pieces of the oak - thus forming a bond. It just doesn't stand to reason that a thicker layer of glue would have more strength. After all a dried piece of glue is quite brittle.

Ed in Tampa · Post by **Ed in Tampa** » Mon Oct 29, 2007 11:19 am

Years ago I clamped up some cherry for a Baby Cradle and I really clamped them down. Now almost 20 years later the glue joints are failing. I have learned to make good joints by first insuring they were properly jointed. Then applying glue to both sides of the joint and clamping them down until the joint was closed with a good even squeeze out but I hadn't squeezed the glue totally from the joint.

I think most of us tend to glue starve a joint. I try to make sure I have a reasonable amount of squeeze out but not so the joint is dry.

Although your both right the strength is in the glue to wood bond not the glue itself. So if there is too much glue the joint is actually weaker than stronger.

I have a friend that constantly twists the head off screws with a screw driver, I can assure you every one of his glue joints are glue starved. Also you have to be careful with some of these new clamps with big soft rubber handles you can really torque them down too tight.
Ed

charlese · Post by **charlese** » Mon Oct 29, 2007 3:38 pm

Ed in Tampa wrote:Years ago I clamped up some cherry for a Baby Cradle and I really clamped them down. Now almost 20 years later the glue joints are failing. I have learned to make good joints by first insuring they were properly jointed. Then applying glue to both sides of the joint and clamping them down until the joint was closed with a good even squeeze out but I hadn't squeezed the glue totally from the joint. ..."
Ed

Ed- I need a little clarification on your cherry joints failing 20 yrs later. Do you remember what type of glue you used? What kind/type of joints are failing? (mortise & tenon, dowel, edge, haunched, half lap)? Where has the cradle been in the past 20 yrs?

Cherry is supposed to glue very well, but there are quite a few reasons for glue joint failure. Why do you attribute the failures to "joint starvation"?

Thanks!

Ed in Tampa · Post by **Ed in Tampa** » Tue Oct 30, 2007 12:30 am

The wood was edge joined to make it wider. The cradle has endured 6 granchildren and storage in hot humid attics. However the failure to me is due to glue starvation. The cherry was oil finished and basically the wood looks fantastic except for the failures.

If I remember correctly I drilled and dowelled the board together and then cut them to shape. The failure is on one or two glue joints in each piece. Right now they are just cracks but left unattended they will widen. Incidently I have two more pieces built at the same time using the exact same wood, made the exact way except clamp pressure and stored for the same length of time stored my unheated/cooled garage and they are fine. They were drilled wrong but I kept them for patterns.
Ed

charlese · Post by **charlese** » Tue Oct 30, 2007 1:19 am

Thanks Ed! Hmmm, could be starvation. What type of glue did you use? Do you think the cracks occured right at the places where the clamps were located?

Ed in Tampa · Post by **Ed in Tampa** » Tue Oct 30, 2007 1:53 pm

charlese wrote:Thanks Ed! Hmmm, could be starvation. What type of glue did you use? Do you think the cracks occured right at the places where the clamps were located?

Good point! The wood did crack exactly opposite to where the clamps were located. Looking into the crack you can see separation has miminal wood fiber tear out, again this tells me it was glue starvation.
Ed

charlese · Post by **charlese** » Tue Oct 30, 2007 10:02 pm

We still don't know the type of glue used. White glue? Hide glue? Yes the separation at the clamp locations could indicate starvation, however as there is some fiber separation - there could be other causes. It has been known that hide glue gives way under stressful moist conditions. (can fix by adding water and re clamping) My guess is you used a carpenter's glue of the time. Could be Aliphatic resin, yellow or a Polyvinyl, White glue.

Ed in Tampa · Post by **Ed in Tampa** » Tue Oct 30, 2007 11:26 pm

charlese wrote:We still don't know the type of glue used. White glue? Hide glue? Yes the separation at the clamp locations could indicate starvation, however as there is some fiber separation - there could be other causes. It has been known that hide glue gives way under stressful moist conditions. (can fix by adding water and re clamping) My guess is you used a carpenter's glue of the time. Could be Aliphatic resin, yellow or a Polyvinyl, White glue.

It was Elmer's carpenter's glue which is a yellow I think Aliphatic resin.
Again I don't think the separation was caused by the environment. As I said I have two other pieces that were built exactly the same way that show no signs of separation and their storage was in an unheated/un air conditioned garage.
Ed

charlese · Post by **charlese** » Wed Oct 31, 2007 3:22 am

O.K. Ed - We have to agree AND disagree here! Seems we are agreeing that excess pressure caused the problem. However you blame glue starvation and I blame changes in wood structure. (Just went back over old books and notes.)

The ailaphatic glue you used is very good for that purpose. I would like to propose that the theory (we have all read about) concerning joint starvation is a fallacy. The fact is that even under great clamping pressure, the wood fibers and vessels (in hardwood) will absorb enough glue to hold a joint. There is another problem that can occur with too hard of clamping. Without cauls to disperse the pressure, it may crush the wood fibers around the point of greatest pressure, which can result in tearing or breaking of the fibers and vessels (in hardwood). After a while, through wood movement the joint may separate at that point.

I think that is what happened to your cradle. The rest of the joint will probably hold together for scores of decades. I'm not sure your cradle needs to be taken apart. It should stick around for a lifetime in it's present state - just like it is. The crack(s) may change narrower and wider, but it will hold together.