Most structural furniture failures are glue failures. And most glue failures are predictable.
The joint didn't fail because wood is weak. Wood is strong. A well-cut mortise and tenon in dry hardwood, with clean surfaces and appropriate glue, will outlast whatever you ask it to do. It failed because something in that sequence was missing — cold shop, wrong glue, surfaces that weren't clean, fit that was never right in the first place.
Your job is to figure out which one before you reach for the clamps.
Read the old glue first.
Before you reglue anything, identify what the original builder used. It tells you the age of the piece, your options for getting the joint apart, and what you're cleaning off the surfaces afterward.
Hide glue — amber to brown, slightly translucent when thin, chalky white when thick. Brittle. Smells faintly like a wet dog when you get it hot. The standard furniture adhesive through most of the 19th century and the first half of the 20th. If it's an antique, it's almost certainly hide glue.
PVA — white or off-white, plastic feel when cured, flexible rather than brittle. Dominates furniture built after about 1965. If the piece came from a furniture store and the dovetails look routed, it's PVA.
Epoxy or urea-formaldehyde — harder, more glass-like. Usually shows up in production furniture or anything that spent time near water.
The distinction that matters most: hide glue releases with heat and moisture. PVA does not. Apply a damp cloth and a hot iron to hide glue, work the joint open slowly, and you get bare wood. Try that on PVA and you get a damp joint that still won't move.
Getting the joint apart.
For hide glue: moist heat directly to the joint. A damp cloth and iron works. A dedicated steaming setup works better. Go slow. The goal is to soften the glue, not steam the wood. Let the joint open under hand pressure before you force it.
For PVA: mechanical separation only. Careful prying with thin tools, accepting some risk of damage. Or you work around it — cut the tenon back, clean off what you can, regrab with fresh surfaces. There's no elegant option on a PVA failure.
Either way, once the joint is apart, you have to remove all the old glue. Not most of it. All of it.
Surface prep is the repair.
This is the step most often skipped. It's the only step that matters.
PVA will not bond reliably to cured PVA. If you reglue without stripping the old adhesive, you're gluing plastic to plastic. It will feel solid for six months and fail the same way the original did.
Hide glue is brittle — a sharp chisel scrapes it clean. PVA takes a card scraper or sandpaper and more patience. Work carefully. You're cleaning to bare wood, not refitting. The joint was cut to dimension. You're just removing the film.
When you think you're done, fit-test dry. Snug by hand pressure. If it's loose after cleaning, the joint needs a patch or a careful refit before glue touches it. A loose joint with glue in it is still a loose joint.
The reglue.
For museum work, anything with value beyond its utility: hot hide glue at 55–60°C, both surfaces, clamp within 90 seconds. It sets rigid, doesn't creep under load, and leaves the door open to future repair without damage.
For most shop repairs: PVA on clean, dry, close-fitting surfaces. Apply to both sides. Thin and even. Not thick. Glue does not fill gaps. A thick coat in a loose joint is a thick layer of flexible plastic, which is weaker than air.
Cold shop note: PVA open time compresses below 15°C, and the apparent set in cold conditions comes before actual bond strength. Warm the joint components to room temperature for 30 minutes before you glue. Clamp in a heated space. Leave clamped for 24 hours before you stress it.
Clamp pressure: contact, not force. A squeeze-out bead along the full joint length means glue is distributed. More pressure than that forces glue out of the cell walls before it sets. The joint you're building is between wood and wood, not wood and glue.
One call worth making explicitly.
There's a good argument that all furniture repair should use hide glue because it preserves the option to redo the work without damage.
A 200-year-old Windsor chair and a $200 side table don't have the same requirements. The reversibility argument is strongest where the original construction has value that exceeds the cost of the repair: old-growth wood, hand-cut joinery, one-of-a-kind pieces. On everyday furniture, durability is the priority.
Make that call for the piece you're working on. Not as a policy.
The joint failed. Whether what you're building is better than what was there before depends entirely on whether you cleaned the surfaces.
Do that right and the rest will hold.