caul

A caul is a piece of wood placed between a clamp and the workpiece to distribute clamping pressure evenly across a wider area. It prevents the clamp jaws or bar from leaving indentations, bruises, or uneven compression marks on the surface being glued or shaped. The caul sits as an intermediary: the clamp grips the caul, and the caul transfers that force to the work beneath it.

Cauls range from simple softwood blocks to shaped or contoured pieces designed for specific jobs. In veneer work and marquetry, cauls are often made as flat boards slightly larger than the panel being veneered. In curved work, the caul itself is shaped to match the workpiece contour, ensuring perpendicular pressure across a bent edge or contoured surface. For edge-gluing boards, a caul might be a simple strip of pine or poplar placed across the joint line. The material matters: soft woods like pine compress slightly and conform better than hardwoods, which stay rigid and can cause uneven pressure distribution if the surfaces are not perfectly flush.

Repeated clamping bruises cauls over time. Glue squeeze-out bonds the caul to the workpiece if a barrier is not used, so waxed paper or plastic sheeting is typically placed between the caul and the fresh glue line. Some woodworkers keep dedicated cauls for different jobs: thin cauls for light veneering pressure, thicker ones for panel construction. A damaged or unevenly worn caul can introduce the very problem it was meant to prevent, so replacement is cheaper than the work ruined by uneven clamping.

The name derives from the membrane or covering sense of the word in other trades, though the connection is weak in modern woodworking. The term has been in use for centuries; old texts sometimes refer to a caul board or caul piece. In some regions and older references, the word appears as a caulk or calk, though those spellings now more commonly refer to caulking compound.