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Question: I recently glued up some boards for a project and some of the joints failed. I used yellow carpenter's glue and left them clamped overnight like I always do. I've never had this happen before. Can you give me any ideas??
Answer: I can think of a couple of reasons why PVA (polyvinyl acetate) glue would fail. The most obvious reason is if the glue is older than one year. I like to write the date on the glue bottle when I buy it. When I was in business, I used so much glue that dating wasn't necessary, but now that I'm a happy hobbiest I've had several batches of glue go bad. If the glue freezes, becomes extremely thick or stringy, or starts to smell sour, it's past its prime and shouldn't be used.
Dull jointer knives can also cause glue to fail. If the knives on your jointer aren't keen, they can crush the wood fibers closed, which keeps the glue from getting into the pores. I like to run a jointer plane over machined joined boards to get rid of the knife marks and to get perfectly mated boards. I'm also assuming you're edge gluing. PVA glue only works long grain to long grain. If you're trying to glue end grain to long grain without joinery the joint will fail. The same goes for end grain to end grain.
In addition, all glues have a recommended "open time", the amount of time you can safely leave the glue exposed to the air before assembling the joint. However, open time varies with environmental temperature and humidity. Hot, dry conditions will cause the glue to set quicker, while PVA's will not perform well at all in cold conditions. Most manufacturers recommend that the temperature, glue and materials be above 45-55°F before using glue.
Another reason why glue fails is called glue starving. Glue starving happens when you don't use enough glue in the first place, or when you clamp so tightly that the clamps squeeze all the glue out of the joint. Glue should be applied over the entire surface to be joined - spread a thin layer on both mating surfaces. Don't rely on a squeeze and think it'll get distributed during assembly. When clamping put just enough pressure to bring the joint up tight with a little glue squeeze out and then stop. Keep in mind that TOO MUCH glue can also cause problems. A thin, even layer of glue will form a strong bond between two pieces of wood, but a thick cushion of glue will do just the opposite and weaken the joint.
Finally, allow glue to set for at least an hour before removing clamps, and don't stress joints until the glue has fully cured. Any one of these reasons or combination of reasons can cause PVA glue to fail.