All too often, wood furniture that has aged and been exposed to the rigors of family life will begin to show some signs of wear and tear. The glue dries out, the wood shrinks, the various pieces move against each other, and a loose joint is often the result.
Left unattended, the joint will worsen until it fails completely, with unpleasant results for both the piece of furniture and for anyone using it at the time.
The repair begins with an examination of the original joint used. Chairs and light tables often use mortise and tenon joints (one piece has a square end--the tenon--which fits into a square hole--the mortise--on the other piece). Heavier tables may use this or other types of joints, often with additional reinforcement underneath where it can't be seen.
To repair a loose mortise and tenon joint, it is best if you can completely separate the two pieces. Try and pull them apart with your hands, or use a small rubber mallet to gently knock them apart. Use a scraper or light sandpaper to remove all of the old glue from both the mortise and the tenon.
With a fine-tooth saw (a hacksaw will also work in a pinch) cut a horizontal slot into the end of the tenon. The depth of the slot should be equal to about three-quarters of the length of the tenon. Next, cut a small wedge out of scrap wood (most hardware and lumber yards also carry precut wooden wedges of the types used to repair hammer and ax handles).
The wedge should be slightly smaller in width than the width of the tenon, and should be just slightly longer than the length of the slot. Insert the wedge into the slot until it just begins to spread the tenon apart.
Coat the tenon with a good grade of yellow woodworker's glue, and insert it into the mortise. Use the rubber mallet or a bar clamp to draw the joint completely together. As the tenon enters the mortise, the wedge will bottom out and spread the tenon, ensuring a tight joint. Leave the joint clamped for at least 24 hours.
If you are unable to completely separate the joint, here are a couple of other tricks you might want to try:
First, use a bar clamp or a strap to pull the loose joint together, then drill a one-eighth-inch hole through the side of the mortise and into the tenon. In order to keep the repair as unobtrusive as possible, drill from the back or the inside surface of the joint, and do not drill all the way through.
Use a glue injector--an inexpensive plastic applicator available at most hardware stores--to force woodworker's glue into the joint, then drive a short length of one-eighth-inch hardwood dowel into the hole to lock the mortise and tenon together. Keep the joint securely clamped for at least 24 hours, using a bar clamp, a web clamp, or simply a length of cord that has been wrapped around the chair several times and securely twisted tight.
When the joint is dry, sand the end of the dowel flush with the surface of the wood. To conceal the dowel even further, you can drill it down slightly below the surface, then fill the hole with putty to match color of the surrounding wood.