What Happens Inside a Mortise and Tenon Joint?

This is an excerpt from “The Woodworker: The Charles H. Hayward Years: Volume III” published by Lost Art Press.

Readers will recall that in January WOODWORKER we gave on page 8 an article “Wedging Mortise and Tenon Joints.” The following letter is from a reader who does not agree with the view expressed in it, and we publish it here as the subject is of considerable interest. Possibly readers may have other opinions about it, and if so we should welcome correspondence.

If your contributor would conduct the following experiment, he might be induced to modify his views concerning the gluing of a mortise and tenon joint as described in his article in last month’s WOODWORKER. Cut two or three inches from the end of a wide board. Repeat this, so that there are two pieces of exactly the same width and of a similar texture. Mark the width exactly on a board and soak both pieces in water until saturated. Measure this against the previous width. The wood will have expanded to a degree depending on its original water content.

The-Woodworker-Vol-3

AN INTERESTING EXPERIMENT IN SHRINKAGE Piece A is cramped at ends only, centre remaining free. At B cramps are fixed along the width.

Fix piece A firmly down on a board with handscrews at each end so that, although the centre is loose, the extreme edges cannot move during drying. Fix piece B to a board with handscrews all along its width so that it cannont move at any point during drying. Place both pieces in a warm atmosphere and leave to dry. In the process of drying piece A will split, but piece B will dry out without shrinkage, and will retain its new width permanently. Contrary to what might be expected, it will also be largely unaffected by small atmospheric changes. The cells of the wood seem to be permanently stretched. This experiment proves that wood will be largely impervious to atmospheric changes and will lose its customary tendency to shrink or swell, if it is held at every point.

To turn to the mortise and tenon joint, it will now be appreciated that if the whole of the sides of the tenon and the sides of the mortise are in contact and are glued, no shrinkage can take place at these points. It also follows that if part of the tenon and mortise is unglued, shrinkage and consequent movement will take place in the unglued portion, while the glued portion will remain stable if it can withstand the pull of the unglued portion so close to it. So far as strength alone is concerned, it is obvious that a completely glued joint must be stronger than one partly glued.

The conclusion seem to be: That there would be a loss of strength in a joint only partly glued.

That the unglued portion puts an added strain on the glued portion.

That a joint properly fitted and glued will not move at the shoulder any more than any other part of the joint.

Meghan Bates

 

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8 Responses to What Happens Inside a Mortise and Tenon Joint?

  1. Richard Jones says:

    All assuming the glue never loses its ability to resist creep, or was never a creeper in the first place, e.g., PVA and other emulsions types that are well known creepers, and that the adhesive doesn’t fail through molecular or chemical deterioration. I don’t buy the conclusion, in part because I’ve witnessed too many M&T failures affected by time dependent stresses of one sort or another.

    • Jeff Faulk says:

      Bearing in mind that he is publishing this with, at the very latest IIRC, 1950s technology. He did publish some fairly sophisticated experiments in The Woodworker, but most of them were about progress in new timber technology. Unless he cut apart several M&T joints over time, he cannot know what was going on in there because I doubt they were much in the habit of hoisting a pile of experimental joints into the local dental office for X-rays!

      • Richard Jones says:

        Jeff, that suggests he was speculating doesn’t it? In his experiment he’s locking two pieces of wood together with the grain set at right angles to each other using cramps. Assembling an M&T joint doesn’t generally involve cramping pressure applied to the mortised member that would press the cheeks of the mortise firmly to the faces of the tenon – the pressure is in line with the joint’s assembly direction and merely closes up the shoulder line. Therefore the intimacy of the tenon faces to the mortice cheeks is entirely dependent upon how well the joint was cut, plus whatever locking qualities are added by the adhesive, including its creep characteristics and gap filling abilities. If his experiment used hide or Scotch glue, for example, it does have gap filling abilities, but my experience with this adhesive type is that over time (decades and centuries through repairing antiques, for instance) it morphs into a relatively fragile crystalline like structure. Related discussion might be raised with regard to alternative adhesive types, but in the end, the question in my mind will always be “How well executed is the M&T?”

        And then there’s the issue of partially gluing the whole joint raised in the blog post. There are M&T forms where it would generally be considered less than ideal to glue the full tenon width. For instance, there’s a good reason to glue only one fork of the forked M&Ts at each end of the wide mid-rail (perhaps up 320 mm wide, for instance) of a four panel architectural door.

        • Hugo says:

          Richard,
          thank you for the analyse. I am often wondering the location of the glue (beginning, end) and you did point out a good question concerning the fact of gluing only one side of the tenon.

  2. julienhardy says:

    Interesting. So I’d like to understand why some recent articles tell us the advantages of only glueing one side of a wedge in a wedged through tenon joint. What am I not catching?

    • Patrick Harrington says:

      The idea behind gluing only one side of a wedge in a through tenon is so that any opening of the joint occurs at the unglued face of the tenon, rather than on one of mortise/tenon interfaces. In windsor chairs, for example, if the tenon were to shrink enough relative to the mortise, you would rather the glue joint all around the tenon remain intact, and let the stress be relieved between one side of the wedge and the mating side of the tenon.

      For frame-type joinery, the width of the tenon has to get pretty wide before I worry about the joint, and at that point I usually divide the tenon into a twin tenon (at the bottom rail of a door, for example).

  3. DAVID G ELDRIDGE says:

    How do you cramp a board? lol
    “AN INTERESTING EXPERIMENT IN SHRINKAGE Piece A is cramped at ends only, centre remaining free. At B cramps are fixed along the width.”

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