WORKBENCH GUIDE
Wood glue basics: open time, clamping, moisture
Good joints depend as much on timing, pressure and wood moisture as on the glue. What the Wood Handbook says, and what to take from the label.
The times on the label
The Wood Handbook defines four times. Pot life is how long a mixed batch stays usable. Open time runs from spreading the adhesive to joining the parts, closed time from joining to applying pressure, and clamp time is how long the assembly stays under pressure. Full strength can take hours or weeks after pressing.
Higher temperature shortens all of these times. While the surfaces are open, a waterborne glue loses water to the air and to the wood and can become too dry to bond. Polyvinyl acetate (PVA), epoxy, polyurethane, resorcinol and many isocyanate adhesives cure at room temperature; phenol-, melamine- and urea-formaldehyde adhesives need heated presses. Times differ by product, so use the data sheet of the glue you have.
Moisture content
Adhesives for wood are generally designed for wood at 6% to 14% moisture content, and 12% is a typical specification. Below about 6%, wood absorbs water from aqueous glues so quickly that flow and penetration suffer even under high pressure. Isocyanates and polyurethanes, on the other hand, need some water to cure.
Glue parts at similar moisture contents: the Handbook recommends keeping differences below about 5 percentage points for lower-density species and 2 for high-density species. Bonded joints keep their strength indefinitely if the wood stays below about 15% moisture content and at temperatures within human comfort; some adhesives lose strength under long exposure above 38 °C.
Pressure and density
Clamping brings the surfaces into close contact and pushes adhesive into the wood. Too much pressure squeezes the adhesive out and starves the joint. Overpenetration is common in low-density woods and excessive squeeze-out in high-density woods; the strongest joints use moderately high pressure for the density of the wood.
Joint strength rises with wood density up to about 700–800 kg/m³ at 12% moisture content. Above that, strong joints with high wood failure are harder to obtain consistently, so very dense species need closer attention to surface preparation and to the adhesive maker’s instructions. These species in this database exceed 750 kg/m³:
| Species | Density at 12% |
|---|---|
| Cumaru | 1,070 kg/m³ |
| Ipê | 1,040 kg/m³ |
| Jatobá | 940 kg/m³ |
| Garapa | 790 kg/m³ |
| White oak | 761.6 kg/m³ |
Surfaces and joints
Freshly planed or sawn surfaces bond best. The Handbook shows that sanding with new 36-grit paper crushes several layers of surface cells, and water-repellent surfaces make adhesives and finishes struggle to stick. Edge-grain joints can be as strong as the wood; shaped joints such as tongue and groove mainly help alignment, not strength.
Edge-gluing jatobá (940 kg/m³) for a tabletop: check both boards with a moisture meter and keep them within about 2 points of each other, plane the edges just before gluing and follow the glue maker’s open and clamp times for the room temperature.