Joint Tightness and Glue


At the 2024 Working Wood in the 18th Century conference, some questions were posed by the audience regarding joint tightness and glue. Certainly timeless questions. In response, two useful references are cited below that will give testing results and recommendations:
  • Tips for Stronger Tenons; Fine Woodworking, January/February 2017 by Dan Bollock - Purdue University
  • Your Glue?; Fine Woodworking, July/August 2007 by Mark Schofield - managing editor. Testing was conducted at Case Western Reserve University
In addition, W. Patrick Edwards (2014 Cartouche recipient) provides two articles on period glue worth reading/study:
  • Understanding Traditional Protein Wood Glues; Spring, 2015
  • Why Not Period Glue? - SAPFM Journal; November, 2001
Both of these articles can be found on his website.
We all like joint tightness, but wood moves, and yet modern glues work fairly well. Of course, good clamping can draw a joint very tight and the rest is dependent on the glue. With mortise and tenon joints, gluing is a secondary action, that is not under pressure but by contact only. It's hard to beat "draw-bore" mortise and tenon joints. They've been known to hold tight for 300 years in "pilgrim period" furniture.
How does glue work ? The short explanation ! Glue works thru molecular bonding. The smoother the surfaces are the better the surface contact, and that is smooth planning of the surface not sanding. Spread the glue evenly and clamp tight. It is the clamping that produces molecular bonding with good surface contact. A rough surface produces secondary bonding, that is glue grabbing rough particles and sticking to it. Molecular contact requires a very smooth surface producing a lot of surface touching and contact. Water is the cheapest glue, but it only works below 32 degrees. Try separating two frozen together pieces of wood, good luck ! Water has soaked into the wood and frozen with the wood molecules, result molecular bonding. An attempt to separate usually yields splitting the wood, not the water glue joint.