REPAIRING FURNITURE JOINTS
By Jeff Jewitt
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 severalgenerations, but even the best joint may eventually need to be repaired, particularly onhard wearing items like chairs. Re-cutting the original joinery or replacing an entirepart may not always be the right course to follow, especially if the furniture has anintrinsic historical value. Discovering how the piece was originally made is sometimeshalf the battle. This article will take you through the basic steps to repairing the mostcommon furniture joints - mortise and tenon, dovetail and dowel.
DIAGNOSE AND DISMANTLE
Besides accidental breakage, a joined piece of furniture may fail for a number ofreasons. 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 glueline. A joint may also have been improperly cut when originally constructed with one ofthe components either too large or small.
To properly repair a furniture joint you should completely dismantle it and replaceworn or damaged wood with wood from the same species. This advice is perhaps the mostdisregarded by well-meaning novices and even poorly-trained professionals. Nails, screwsand metal brackets are often installed on loose joints in an effort to repair them. Gluedribbled into a partially opened joint and hot melt glue are also encountered. Besidesthese added fasteners, many production furniture pieces are pinned with small finishingnails which held the glued joint together until the glue dried, eliminating the need forclamps. Glue blocks are often employed to counteract racking on chairs and to re-reinforcejoinery. Whatever fasteners you encounter, they need to be removed so that the joint comesapart easily. To pry out small nails you can regrind the outer jaws of end nippers so thatthey can pry out nails set flush with surface. On nails set below the surface you can tryto push them through but I find it best to leave them. This will split the wood on themating joint, but this is easier to repair than show wood gouged to access a small nailhead. On old flat head screws, make sure the tip of the driver fits snugly in the slot toavoid stripping the slot. I keep an old driver that I re-grind to customize the fit forold screws. For frozen screws, hold a screwdriver in the slot and heat the shank of thescrewdriver with a propane torch to transfer the heat to the screw. After the screw coolsit 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 shouldcome apart easily by hand. If not, there's probably some hidden fastener. Look the jointover carefully and look for any tell-tale holes. If you can, slip a metal feeler gaugeinto the joint. In some instances, screws are counter-bored into a show side and the holeplugged with wood from the same species. These can be hard to spot under a finish. If youencounter one, drill it out and re-plug it after repairing the joint.
Glues
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 applicationcharacteristics 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 meansthat joints originally glued with hide glue do not have to scraped to bare wood to get thenew glue to stick. Just re-apply some new glue after moistening the old glue with hotwater. You can use either hot hide glue made from dry granules or pre-mixed hide glue likeFranklin'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 withdoing this on antiques. PVA glue is not considered reversible and will make any futurerepairs difficult.
Hide glue can be "de-activated" on joints that are still stuck by saturatingthe joint with alcohol. Squirt some alcohol (I use denatured alcohol) along the edges ofthe joint and it will wick in by capillary action. After several minutes the joint will beloose enough to pull apart.
PVA glues like Titebond and Elmer's are very difficult to remove. If you suspect thatone of these glues was used, wetting the joint in hot vinegar loosens the joint enough towiggle it apart. Unlike hide glue, PVA glue does not re-bond to itself so you must scrapeoff the old glue to bare wood. If you are gluing a broken piece of wood with irregularedges, 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 hotwater on the glue and wait several minutes. Hide glue will become sticky and PVA glueswill turn white.
Other glues you may encounter are epoxy, urea-resin and super glue. All of these gluesshould be treated the same as PVA in that they are non-reversible. However, none of themcan be softened to aid in disassembly. Since most of these glues are brittle, a sharp blowwith a hammer usually breaks the glue line.
Stubborn Joints
In some situations a joint that is still properly or partially glued may need to bedismantled.
If the joint can be wiggled, lightly tapping it with a hammer and a piece of soft woodis usually enough to persuade it apart. If this doesn't work, placing it between benchdogs 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 (notthe 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 repairpurposes you should acquaint yourself with the different types of joints. Although thereare exceptions, the most commonly used joints in furniture construction are the mortiseand tenon, dovetail and the dowel.
Mortise and tenon
This is the most commonly used joint in furniture construction and the one most oftenin need of repair. It is used to joint wood with grain at right angles to each other, andbecause of this, the contrary expansion/shrinkage of the different members causes the glueto fail - loosening the joint. Cabinetmakers have been aware of this for centuries, sovariations of this standard joint have been devised to keep the joint together when theglue fails. These include the pegged, offset pegged, through wedged, and fox-wedgedmortise and tenon.
When a standard mortise and tenon joint fails it is easy to disassemble byde-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 youneed to remove the pins or wedges to get the joint apart.
Through Pegs
Blind Pegs
Offset pegs
Wedged Through Tenons
Blind (Fox) Wedged Tenons
Windsor chairs
Rebuilding a Mortise and Tenon
If the mortise does not make good wood-to-wood contact when it's re-glued or you had toscrape away a lot of wood to remove glue, you need to build up the cheeks of the tenon toget a good fit. Simply glue two pieces of veneer cut slightly oversize to the tenoncheeks, 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 isbroken off, you must rebuild the end of the tenon. Cut away the broken parts flush to theshoulder and drill a series of holes 1"-1-1/2" deep using a drill bit the samediameter as the width of the original tenon. (Hold the piece in a padded vise to avoidsplitting the wood when drilling and chopping out the waste.) Then cut a piece of wood tosplice 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 goodfit with the insert piece. Make sure the grain is the same orientation, then glue theinsert in.
Round tenons broken at the shoulder present a problem. Rarely does the design presentenough "meat" below the shoulder to accept a dowel of the same diameter as thetenon hole. The best way to repair these are to cut off the tenon end below the shoulderat an angle of 30 degrees or less. A new piece of oversized wood is glued on (this iscalled a scarf joint) and then planed and spoke-shaved to the original profile. Roundtenons can be enlarged to fit into oversized mortise holes by either wrapping the tenon ina 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 sothat the glue will stick. If not, a new piece of wood should be spliced in and the mortisere-sized to fit the tenon.
Dovetails
Dovetails are another classic joint that form a mechanical lock in addition to the gluebond from the mating wood surfaces. Like the mortise and tenon there are many variationsof this joint. The most common versions found on furniture are through, half blind andsliding. Through dovetails are found on many case pieces and drawers. Half-blind dovetailsare the traditional favorite for drawer fronts and sliding dovetails are used for legs andcrests of chairs.
Through and half blind dovetails
Sliding dovetails
Dowels
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 dowelingcreates a very strong and durable joint. Like any other joint, stresses and contrary woodmovement will invariably loosen the dowel in at least one of the components and it shouldbe re-glued or replaced.
Many times a dowel will simply loosen when the grain of the dowel is at a right angleto the grain of the component. The joint can be tapped apart with a soft faced mallet andthen re-glued. Other times the dowel will break and the old dowel must be drilled out andreplaced. If the new dowel does not seat exactly like the old one, misalignment of thejoint 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, drillout the center of the dowel. Hold the part in a padded vise. When the bit reaches thebottom 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, parethe excess dowel away from the sides of the hole. To clean the hole run a drill bit thecorrect 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 becomedifficult to remove. Use dowels that have been pared or sanded undersized. These areeasier 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 mostrestorers and conservators agree that hide glue is the best choice for antiques because ofit's reversibility. Hot hide glue allows a quick initial tack and the pre-mixed cold gluewill allow for a much longer open time for complex re-assemblies like chairs. There arearguments that PVA glue is stronger, but both hide glue (hot and cold) and PVA's form aglue 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 canbe toned to match the surrounding finish by mixing some dry pigments with shellac orlacquer and toning the replacement part to blend in. Working the color in thin layers tobuild 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.
Jeff Jewitt is a finisher, writer and teacher from North Royalton Ohio. In addition torunning a full-time finishing shop, he has written numerous articles on finishing for FineWoodworking, American Woodworker, Popular Woodworking, Woodshop News and ProfessionalRefinishing Magazine. He is currently a technical editorial advisor forProfessional Refinishing magazine, acted as a consultant for large finishing companiesand has developed finishing products which are sold all over the world under the Homesteadname. He is the author of Hand Applied Finishes and two videos, ColoringWood and Applying Topcoats (Taunton Press) and is currently working on a new book dueout in late 1999.
Jeff Jewitt owns and operates Homestead Finishing Products – featuringhard-to-find traditional finishing products. 5 Grades of dry shellac, oils, varnishes,brushes and their own exclusive line of dyes make Homestead Finishing Products a must forevery restorers supply needs. Excellent technical advice. Please contact us by:
Homestead Finishing Products
PO Box 360275
Cleveland Oh 44136-0005
Phone: 216-631-5309
E-mail jbjewitt@gwis.com
Also available are Jeff’s Book Hand Applied Finishes, and two videos, Coloring Wood and Applying Topcoats – winners or the 1997 Stanley Award for best How-To book and video.
For stuff to dye for............try
http://www.homesteadfinishing.com
- The biggest problem with these are when they are used on legsjoined 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 notdue to the amount of long grain on the pedestal. Drilling small holes down the outermostpoints of the male portion of the joint and injecting alcohol or hot water will usuallycoax the joint apart. - These two joints are found most often on drawerconstruction and the biggest problem is a broken pin or tail. After disassembling thejoint, a new piece is spiced in, then pared down until it fits with the mating joint. - The undercarriage and seat of Windsor chairs are traditionallyassembled 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 bedismantled by drilling a series of holes with a small drill bit to waste away wood at thecenter of the tenon to collapse it. The joint is re-assembled using a fox-wedge technique. - These are very difficult joints to spot. If you canpull some of the joint out then it abruptly stops, it probably is fox wedged. If you canspot the bottom of the wedge, you can usually get a drill up into the wedge to drill itout to collapse the tenon. Make a new wedge from a very hard wood like maple andre-assemble. Do not use a thick wedge since it may split the grain of the tenon beyond theshoulder. - If a through tenon does not pull apart easily when the glueis de-activated the tenon may be wedged. In most cases the wedges will be of a contrastingor slightly dissimilar wood and be easy to see. You can pull them out after drilling smallholes into the wedges In other cases, particularly glue-less Oriental joinery, the wedgesare made from the same wood and are difficult to spot. You'll need to drill two sets ofholes with a 3/32" drill bit from each end of the tenon which should be enough tocollapse the tenon as you pull it out of the mortise. - Pegs that are driven in offset holes in the tenon are impossible todistinguish from blind or through pegs unless the joint is taken apart. This joint willrarely loosen enough to be a structural problem unless the surrounding wood becomesweakened through rot or woodworm. If you run into holes that don't line up when the jointis re-assembled, they're offset pegs so do not re-drill the holes to line them up. - Pegs that do not go through to the other side must be drilled out ifthey cannot be pulled out with pliers. On valuable pieces, this should only be done ifrestoration of structural integrity is the primary consideration. Use pegs of the samespecies and hand whittle them to duplicate original construction. - Pegs that go completely through the joint and come out the otherside can be tapped out from the other end. On old pieces these pegs are usually taperedand are usually driven from the show side so tap from the opposite side. If the pegs can'tbe tapped out easily, drill them out