Building furniture with traditional joinery is fairly straightforward: cut, glue, clamp, and let it dry. A well made mortise and tenon or dovetail joint will last several generations, but even the best joint may eventually need to be repaired, particularly on hard wearing items like chairs. Re-cutting the original joinery or replacing an entire part may not always be the right course to follow, especially if the furniture has an intrinsic historical value. Discovering how the piece was originally made is sometimes half the battle. This article will take you through the basic steps to repairing the most common furniture joints - mortise and tenon, dovetail and dowel.
Diagnose and Dismantle
Besides accidental breakage, a joined piece of furniture may fail for a number of reasons. The most common are wear and tear which produce racking stresses on the joint (like the back legs of a chair) and normal expansion/shrinkage due to seasonal changes. These two forces may operate independently or together to produce failure at the glue line. A joint may also have been improperly cut when originally constructed with one of the components either too large or small.
To properly repair a furniture joint you should completely dismantle it and replace worn or damaged wood with wood from the same species. This advice is perhaps the most disregarded by well-meaning novices and even poorly-trained professionals. Nails, screws and metal brackets are often installed on loose joints in an effort to repair them. Glue dribbled into a partially opened joint and hot melt glue are also encountered. Besides these added fasteners, many production furniture pieces are pinned with small finishing nails which held the glued joint together until the glue dried, eliminating the need for clamps. Glue blocks are often employed to counteract racking on chairs and to re-reinforce joinery. Whatever fasteners you encounter, they need to be removed so that the joint comes apart easily. To pry out small nails you can regrind the outer jaws of end nippers so that they can pry out nails set flush with surface. On nails set below the surface you can try to push them through but I find it best to leave them. This will split the wood on the mating joint, but this is easier to repair than show wood gouged to access a small nail head. On old flat head screws, make sure the tip of the driver fits snugly in the slot to avoid stripping the slot. I keep an old driver that I re-grind to customize the fit for old screws. For frozen screws, hold a screwdriver in the slot and heat the shank of the screwdriver with a propane torch to transfer the heat to the screw. After the screw cools it should come out easily. If the slot is stripped, a screw extractor is a last resort.
If the joint was loose to begin with and you've removed all fasteners, the joint should come apart easily by hand. If not, there's probably some hidden fastener. Look the joint over carefully and look for any tell-tale holes. If you can, slip a metal feeler gauge into the joint. In some instances, screws are counter-bored into a show side and the hole plugged with wood from the same species. These can be hard to spot under a finish. If you encounter one, drill it out and re-plug it after repairing the joint.
The type of glue used on the original joinery is important. Prior to the mid forties, hot animal hide glue was the traditional glue used in furniture assembly. After that time, PVA glues eventually replaced hide glue. Hide glue has some annoying application characteristics but it's redemption is in the fact that it is reversible. It can be "re-activated" with water and heat and it will re-bond to itself. This means that joints originally glued with hide glue do not have to scraped to bare wood to get the new glue to stick. Just re-apply some new glue after moistening the old glue with hot water. You can use either hot hide glue made from dry granules or pre-mixed hide glue like Franklin's. The pre-mixed variety will give you more open time to work than the hot type. You can also use a PVA glue to re-glue an old hide glued joint, but be very cautious with doing this on antiques. PVA glue is not considered reversible and will make any future repairs difficult.
Hide glue can be "de-activated" on joints that are still stuck by saturating the joint with alcohol. Squirt some alcohol (I use denatured alcohol) along the edges of the joint and it will wick in by capillary action. After several minutes the joint will be loose enough to pull apart.
PVA glues like Titebond and Elmer's are very difficult to remove. If you suspect that one of these glues was used, wetting the joint in hot vinegar loosens the joint enough to wiggle it apart. Unlike hide glue, PVA glue does not re-bond to itself so you must scrape off the old glue to bare wood. If you are gluing a broken piece of wood with irregular edges, soak the glue with hot vinegar and remove it with a brass bristle brush.
If you're not sure which glue was used you can do a simple test. Place a drop of hot water on the glue and wait several minutes. Hide glue will become sticky and PVA glues will turn white.
Other glues you may encounter are epoxy, urea-resin and super glue. All of these glues should be treated the same as PVA in that they are non-reversible. However, none of them can be softened to aid in disassembly. Since most of these glues are brittle, a sharp blow with a hammer usually breaks the glue line.
In some situations a joint that is still properly or partially glued may need to be dismantled.
If the joint can be wiggled, lightly tapping it with a hammer and a piece of soft wood is usually enough to persuade it apart. If this doesn't work, placing it between bench dogs and running the tail vise in reverse will pull apart even the most stubborn joints. If the joint doesn't respond, snug the vise as far as you can and then whack the dog (not the joint) with a hammer. This will dissipate the blow of the hammer.
Techniques for Individual Joints
The design of different joints necessitates different techniques in repair. For repair purposes you should acquaint yourself with the different types of joints. Although there are exceptions, the most commonly used joints in furniture construction are the mortise and tenon, dovetail and the dowel.
Mortise and Tenon
This is the most commonly used joint in furniture construction and the one most often in need of repair. It is used to joint wood with grain at right angles to each other, and because of this, the contrary expansion/shrinkage of the different members causes the glue to fail - loosening the joint. Cabinetmakers have been aware of this for centuries, so variations of this standard joint have been devised to keep the joint together when the glue fails. These include the pegged, offset pegged, through wedged, and fox-wedged mortise and tenon.
When a standard mortise and tenon joint fails it is easy to disassemble by de-activating the glue and pulling the joint apart. When the joint is pegged or wedged, the joint will be loose, but will still hold together. To disassemble these joints you need to remove the pins or wedges to get the joint apart.
Through Pegs - Pegs that go completely through the joint and come out the other side can be tapped out from the other end. On old pieces these pegs are usually tapered and are usually driven from the show side so tap from the opposite side. If the pegs can't be tapped out easily, drill them out
Blind Pegs - Pegs that do not go through to the other side must be drilled out if they cannot be pulled out with pliers. On valuable pieces, this should only be done if restoration of structural integrity is the primary consideration. Use pegs of the same species and hand whittle them to duplicate original construction.
Offset pegs - Pegs that are driven in offset holes in the tenon are impossible to distinguish from blind or through pegs unless the joint is taken apart. This joint will rarely loosen enough to be a structural problem unless the surrounding wood becomes weakened through rot or woodworm. If you run into holes that don't line up when the joint is re-assembled, they're offset pegs so do not re-drill the holes to line them up.
Wedged Through Tenons - If a through tenon does not pull apart easily when the glue is de-activated the tenon may be wedged. In most cases the wedges will be of a contrasting or slightly dissimilar wood and be easy to see. You can pull them out after drilling small holes into the wedges In other cases, particularly glue-less Oriental joinery, the wedges are made from the same wood and are difficult to spot. You'll need to drill two sets of holes with a 3/32" drill bit from each end of the tenon which should be enough to collapse the tenon as you pull it out of the mortise.
Blind (Fox) Wedged Tenons - These are very difficult joints to spot. If you can pull some of the joint out then it abruptly stops, it probably is fox wedged. If you can spot the bottom of the wedge, you can usually get a drill up into the wedge to drill it out to collapse the tenon. Make a new wedge from a very hard wood like maple and re-assemble. Do not use a thick wedge since it may split the grain of the tenon beyond the shoulder.
Windsor chairs - The undercarriage and seat of Windsor chairs are traditionally assembled using green wood. This design produces a locking tenon that resembles a ball. Though loose, this joint can be swiveled around like a ball and socket. It can only be dismantled by drilling a series of holes with a small drill bit to waste away wood at the center of the tenon to collapse it. The joint is re-assembled using a fox-wedge technique.
Rebuilding a Mortise and Tenon
If the mortise does not make good wood-to-wood contact when it's re-glued or you had to scrape away a lot of wood to remove glue, you need to build up the cheeks of the tenon to get a good fit. Simply glue two pieces of veneer cut slightly oversize to the tenon cheeks, taking care to orient the grain the same way and using wood of a similar species. Don't glue on one side only, this will change the offset the tenon When the tenon is broken off, you must rebuild the end of the tenon. Cut away the broken parts flush to the shoulder and drill a series of holes 1"-1-1/2" deep using a drill bit the same diameter as the width of the original tenon. (Hold the piece in a padded vise to avoid splitting the wood when drilling and chopping out the waste.) Then cut a piece of wood to splice into the old wood, using the original mortise to size the width.
Clean up the drill holes by paring the holes with a sharp chisel until you have a good fit with the insert piece. Make sure the grain is the same orientation, then glue the insert in.
Round tenons broken at the shoulder present a problem. Rarely does the design present enough "meat" below the shoulder to accept a dowel of the same diameter as the tenon hole. The best way to repair these are to cut off the tenon end below the shoulder at an angle of 30 degrees or less. A new piece of oversized wood is glued on (this is called a scarf joint) and then planed and spoke-shaved to the original profile. Round tenons can be enlarged to fit into oversized mortise holes by either wrapping the tenon in a glue soaked plane shaving or by expanding the tenon diameter with a wedge.
Mortises that are cracked or split can be re-glued as long as the wood closes snugly so that the glue will stick. If not, a new piece of wood should be spliced in and the mortise re-sized to fit the tenon.
Dovetails are another classic joint that form a mechanical lock in addition to the glue bond from the mating wood surfaces. Like the mortise and tenon there are many variations of this joint. The most common versions found on furniture are through, half blind and sliding. Through dovetails are found on many case pieces and drawers. Half-blind dovetails are the traditional favorite for drawer fronts and sliding dovetails are used for legs and crests of chairs.
Through and half blind dovetails - These two joints are found most often on drawer construction and the biggest problem is a broken pin or tail. After disassembling the joint, a new piece is spiced in, then pared down until it fits with the mating joint.
Sliding dovetails - The biggest problem with these are when they are used on legs joined to turned pedestals. When the leg is racked or some other type of stress applied, the grain of the pedestal cracks. Repairing the joint is easy, but getting it apart is not due to the amount of long grain on the pedestal. Drilling small holes down the outermost points of the male portion of the joint and injecting alcohol or hot water will usually coax the joint apart.
Since the mid 1850's dowels have been used as replacements for the mortise and tenon, dovetail, and other traditional joints. Though despised by purists, proper doweling creates a very strong and durable joint. Like any other joint, stresses and contrary wood movement will invariably loosen the dowel in at least one of the components and it should be re-glued or replaced.
Many times a dowel will simply loosen when the grain of the dowel is at a right angle to the grain of the component. The joint can be tapped apart with a soft faced mallet and then re-glued. Other times the dowel will break and the old dowel must be drilled out and replaced. If the new dowel does not seat exactly like the old one, misalignment of the joint will result. The technique below solves this problem
Replacing a Dowel
Begin by cutting the dowel flush to the surface of the component with a sharp saw. Using a sharp brad point bit 1/32nd-1/16th smaller than the diameter of the dowel, drill out the center of the dowel. Hold the part in a padded vise. When the bit reaches the bottom of the dowel hole, you will feel the bit "slip" a bit and you can stop. Using a sharp gouge with a sweep that matches the curve of the dowel circumference, pare the excess dowel away from the sides of the hole. To clean the hole run a drill bit the correct diameter backwards. (A new bit can catch and rip the hole apart if run forward.)
Don't use new dowels to check the fit. These can seize in the joint and become difficult to remove. Use dowels that have been pared or sanded undersized. These are easier to remove after a trial fit.
Reassemble and Touch-up
The choice of glue that you use to re-assemble the pieces is up to you, but most restorers and conservators agree that hide glue is the best choice for antiques because of it's reversibility. Hot hide glue allows a quick initial tack and the pre-mixed cold glue will allow for a much longer open time for complex re-assemblies like chairs. There are arguments that PVA glue is stronger, but both hide glue (hot and cold) and PVA's form a glue line which is stronger than the structure of the wood, so either type can be used.
On some exposed replacement parts like pins, tails, or round tenons, the new wood can be toned to match the surrounding finish by mixing some dry pigments with shellac or lacquer and toning the replacement part to blend in. Working the color in thin layers to build up to the original color works better than trying to hit the color all in one shot.. When the color is right, apply a clear topcoat to protect the touched-up area.