Glue and adhesives in carpentry — from PVA to bone glue
How you choose the right glue for the task and why old hide glue is still used in restoration
A joint is only as strong as the glue it is joined with and different tasks require different properties. Water resistance strength open time and — in restoration work — the ability to take the joint apart again are all factors that determine which glue is the right one.
§PVA — the classic wood glue
PVA glue, often called ordinary white or yellow wood glue, is the most used glue in carpentry. It is water-based, easy to clean with a damp cloth before it dries, and gives a very strong joint on tight wood joints. It requires even pressure while it cures and is therefore used together with clamps or a press.
§EN 204 and the D-classes
Wood glue is classified according to the European standard EN 204, which divides glues into classes based on how well they withstand water and moisture. A D3 glue is water-resistant and can be used indoors in damp rooms such as kitchens and bathrooms or outdoors where the furniture is protected from direct weather. A D4 glue is waterproof and designed for full outdoor exposure; to achieve this class the glue line must be able to withstand six hours in boiling water followed by two hours of cooling in cold water and still deliver at least 4 N/mm² in shear strength.
| D-class | Resistance | Usually used for |
|---|---|---|
| D2 | Dry, indoor use | Furniture in normal room climate |
| D3 | Water-resistant | Damp rooms or sheltered outdoors |
| D4 | Waterproof | Full outdoor exposure |
§Contact adhesive for veneering
Contact adhesive plays a special role in veneer work. It is applied to both surfaces and binds the moment they meet, which makes it ideal where it is difficult or impossible to set clamps — for example when veneering curved surfaces. On the other hand, it leaves no room for moving veneer once the surfaces are joined, so dry adjustment in advance is crucial.
§Construction adhesive and epoxy
For tasks where two materials must be joined or where the joint must withstand high loads and outdoor weather, polyurethane glue (PU glue) is often used, which expands slightly as it cures and fills minor inaccuracies. Epoxy is typically used for repairs, filling holes and cracks, or where wood must be glued to other materials such as metal.
§Bone glue and skin glue — restoration adhesives
Bone glue and skin glue are traditional animal glues that are heated and used warm. They are weaker than modern synthetic glues but have a property that makes them indispensable in restoration: they can be dissolved again with heat and moisture. This means that an old joint glued with bone glue can be carefully disassembled and glued again without damaging the original wood — a possibility that modern permanent glues do not offer.
§How to choose glue
Ask first what the task requires: does the furniture need to withstand moisture or weather, should it be glued quickly without clamps, or should the joint be separable again in decades? The answer almost always points unambiguously to the right glue type – and a wrong choice here can cost both time and an otherwise beautiful piece of work.
“The best glue is not always the strongest — it is the one that suits what the item needs to hold and how long it needs to hold.”