3 Cracked Rungs Bushed on a Ladderback Chair with Tulipwood Dowel and Cascamite

Three rungs, all on the same side, all cracked where the tenon meets the leg mortise. That pattern tells you something before you touch it. Someone leaned the chair back on two legs for years, and the racking load went straight into those front-to-side rungs. The wood did not fail in the middle. It failed at the shoulder, where a tenon that has gone slightly oval keeps working against the mortise wall until the grain splits.

So the job is not really gluing three sticks back together. The tenons are undersized now, the mortise holes are bell-mouthed, and any glue you push in has nothing to bite. That is where bushing comes in.

Why the old holes are the problem, not the rungs

Pull the chair apart before you decide anything. On a ladderback the back legs are usually the ones that hold up, and the side stretchers between front and back leg take the abuse. Wrap a strap clamp round the seat frame, tap the joints with a soft mallet, and watch which ones move first. If a tenon slides out with a dry rattle, that hole is done.

Measure the mortise with a set of drill shanks or a cheap digital caliper. A 12mm mortise that now reads 13.5mm at the mouth and 12mm at the bottom is bell-mouthed, and that is the classic sign of years of loading. You cannot glue a 12mm tenon into a hole that only touches it at the deepest 4mm. The joint will feel tight for a week and then click again.

Bushing means you drill the tired hole out oversize, glue in a plug of new timber, then re-drill a fresh clean mortise back through the plug at the original size. New wood, new walls, full contact along the whole tenon. On a beech ladderback leg you would typically open a 12mm mortise out to 16mm, plug it, and re-bore 12mm. The 2mm wall of new timber all round is plenty.

Tulipwood as the bushing timber

American tulipwood, liriodendron, is the stuff most people reach for here and there is a reason. It is stable, it is close-grained, it glues beautifully, and it takes a drill without tearing out. It is soft enough to shape but hard enough to hold a tenon. On a dark-stained beech or elm chair the pale green-brown of tulip disappears entirely once the finish goes back, because you are staining over it anyway.

Buy dowel oversize if you can get it. A 16mm tulipwood dowel to plug a 16mm bushing hole wants to be a hair proud, so you often size 16mm stock down through a dowel plate to get a snug drive fit. If you only have square tulip offcuts, a dowel plate and a mallet turns them into round pins in a couple of minutes. The plate shears the fibres cleaner than a router does on short lengths.

Grain direction matters. Run the tulip dowel long-grain parallel to the leg so the new mortise is boring into end-grain-ish walls the same way the original was. If you drop a plug in cross-grain, the re-bored hole has weak short grain on two sides and it splits out the first time the chair racks.

Cascamite, and why not PVA here

Cascamite is a urea-formaldehyde powder you mix with water to a double-cream consistency. It sets by chemical cure, not by losing water into the wood, and that is exactly what a gap-prone chair joint needs. It has real gap-filling body, it dries rock hard with no creep, and once cured it does not soften if the chair sits in a conservatory in July.

PVA, even the aliphatic yellow ones, stays slightly plastic. Under the constant small racking of a used chair it creeps, and a tenon glued with PVA into a marginal hole will loosen again inside a couple of years. That is why the joint you are fixing probably failed in the first place. Somebody ran wood glue into it.

Mix Cascamite fresh. It has a pot life of maybe 15 to 20 minutes at 20C and much less if the workshop is warm. Mix only what you need for the plugs, then a second batch for the final tenon assembly. Ratio is roughly 2 parts powder to 1 part water by volume, stirred until there are no dry lumps and it ribbons off the stick. Butter the plug and the oversize hole, drive the tulip dowel home, leave it overnight.

Next day, re-drill the mortise. Mark the original centre before you plug, or you will lose the angle. Ladderback side rungs almost never sit at 90 degrees. They splay, so the mortise is angled, and if you bore the fresh hole straight the rung will not reach the far leg. A cheap angle jig or a sliding bevel set against the leg before disassembly saves the whole job.

With the new mortise cut, dry-fit everything. All four legs, both side rungs, back slats, the lot. If the frame stands square on a flat bench dry, then mix your second Cascamite batch, glue the tenon walls and the mortises, and clamp with the strap clamp plus a couple of sash cramps pulling the splayed legs together. Wipe the squeeze-out with a damp rag before it skins, because cured Cascamite is a swine to scrape off a moulding.

The finish once the joints hold

A chair that has been apart usually shows its history round the repair. Bright new tulip, sanded-back beech, and forty years of grime everywhere else. Colour the raw wood back before you seal anything. Van Dyke crystals dissolved in warm water and a splash of ammonia give a walnut-brown wash that you can build in coats, and it sits well over both the new dowel and the old timber because it is a dye, not a pigment. Two thin coats beat one heavy one.

For the topcoat on a chair that gets handled, Osmo Polyx Oil earns its place. It is a hardwax oil, it soaks in and cures to a wipeable surface, and it does not build a film that chips at the repair line the way a lacquer would. Two thin coats with a lint-free cloth, buffed off after ten minutes each, cured overnight between. On an already-finished chair you are just feathering the new area in.

If the rest of the chair still has its original French-polished sheen and you are only touching the repaired zone, skip the oil and use Liberon Burnishing Cream to blend the new work into the surrounding shellac. It is a mild abrasive paste that knocks back and evens sheen without stripping. On teak or oiled hardwoods that live indoors, Renaissance Wax over the lot gives a low protective sheen and does not go milky.

While the chair is apart, check for worm

Look inside the mortises and the underside of the seat rails for the fine round exit holes and gritty frass of common furniture beetle. If you find live flight holes, that is the moment to inject a woodworm treatment fluid straight into the timber through the holes, because you will never have better access than with the frame in pieces.

Getting the angle right the second time

The part that catches people out is not the glue or the dowel. It is the re-boring. You plugged the old hole, so the original centre and angle are gone the moment the tulip goes in.

Before you drill the bushing oversize, drop a panel pin or a scriber point into the exact mortise centre and prick the leg surface, or better, take a photo with a rule laid across showing the splay angle against the leg face. Set a sliding bevel to that angle and lock it. When the plug is cured and you come to re-bore, clamp the bevel to the leg, line the drill against the blade, and go slow with a brad-point bit so it does not wander into the softer tulip. A lip-and-spur bit in tulipwood cuts cleaner than an auger at these small diameters.

Bore to the original depth, no deeper, or a long tenon bottoms out before the shoulder seats and you get a gap you will spend an hour chasing. A depth stop, even a wrap of masking tape on the bit, sorts it.

One question worth sitting with before you reach for the dowel plate: if all three rungs failed from the same racking habit, what stops the chair racking the same way once it goes back to the same room and the same person?