More on Forensics and M&T Joinery

I dismantled another piece this week and that was an oak fall-front writing desk I bought a few weeks ago. The desk is simple enough and made from solid oak and that was why I bought it. If I don’t like the desk or the style then the oak was worth the price I paid and more. I felt the desk worthy of saving and restoring and the top portion of you recall was made with mitred dovetails that hid the dovetails completely within the mitres. This aspect of the work was very neat and precise. I had however been a little troubled with the mortise and tenoned framework of the stand the desk was anchored to because two or three joints were loose and the desk’s upper weight seemed to readily part the shoulders. As I decided to take the frame apart my concerns were confirmed and suspicions verified.DSC_0131
Parting the joints I heard a sort of crunching crumbly sound not dissimilar to animal glue but I also knew the colour to be another glue I had used 50 years ago as a boy. Those I worked with touted this as one of the best glues and I accepted what they said to be true. Looking at it now I have shifted my opinion in some measure.
The glue he used
Cascamite is a synthetic resin used as an adhesive for general joinery usually for joints that are exposed to outdoors; windows and doors, door frames and so on. It was used on glue lines too where long edges were jointed for laminations. Cascamite is generally sold as a powder you mixed with water to a consistent thickness or viscosity and sets up during the cure which gives it its gap filling properties. It’s said to be suitable for load bearing and laminating. It leaves a clear glue line and will not discolour hardwood, its also mould resistant. After the glue is mixed you need to work with it quickly as it can quite rapidly change from thick liquid to a solid jelly and in about 6 hours (temperature dependent) sets rock hard. It’s touted as having excellent well proven adhesive for joinery, cabinet work or boat building and I know that to has its place, but seeing this piece 70 years on I feel questioning as to its long term efficacy.

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Here you can see how much the tenon was undersized in thickness. very poor work here and a disqualification really.

DSC_0033 Because the tenon thickness was undersized according to the mortise, a full width gap was evident on almost every one of the wide faces of the tenons. This surprised me because the overall piece was well executed. It shows the significant impact good joinery has on longevity and that glue generally cannot substitute for the levels of accuracy it demands. This for me is where harmony becomes evident in the word joinery which has its root in the word harmos. Now the tenons were all dead to size or slightly over width. Very different than thickness. You can use the extra width for tightness here but it substitutes for the real art of joinery. many joinery companies (or the staff machinists and assemblers) use foaming adhesives such as polyurethane glues to expand around the joints. This is only a temporary gap-filling fix (pun intended) for poor workmanship and the missing craftsmanship that’s become standard in the industry. It lasts long enough for the guarantees of a year or so but not really good practice.

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I glued on the thin slither, jokingly called “special shim joinery” when I was a boy who made a bad joint, which pushed the other face to fully engage with the wall of the mortise too.
Inside the holes the glue was both brittle and fractured. it flicked of readily and was easily removed with the corner of an old chisel. So to chiselling off the glue from the faces of the tenon. I decided to thicken the tenons by adding thin veneers to the face of the tenons and then fitting the tenons back to the relevant mortise holes. I am also considering adding a mid rail along the long length to increase stability. I think wider top rails was an answer to the problem too. The weight of the top box is to much for flex in the legs. An inch extra and rightly fitted tenons would have meant no joint failure.

6 Comments

  1. Jon on 1 September 2014 at 1:04 pm

    Paul,

    Out of interest, I’ve used cascamite on a couple of projects in the past, none of which are 70 years old, obviously. The adhesive is known for setting glass hard, and glass brittle, hence in the boat building trade it’s only used where it can be backed up with a secondary mechanical fix, such as a screw. Otherwise it’s epoxy resin and not much else.

    In your experience is this failure more to do with poor joinery or would the glue have failed in this manner over time regardless?

    Thanks,

    Jon



    • Paul Sellers on 1 September 2014 at 5:35 pm

      I noticed that all of the joints had failed in some measure. That’s 14 mortise and tenons. The reasons may vary but one thing I did feel was that shrinkage in a tenon can split along the glue line if the mortise wood is not moving or shrinkage at a parallel rate. In other words bot woods should be dry to the same level at the point of union. If tenon wood is drier than mortise wood that would be fine, but we furniture makers generally use wood from the same batch and uniform in moisture levels. These joints were surprisingly poor even though the actual forming of them was very good. I repaired and glued up the whole framework this evening and was very happy with the eventual outcome as I left for home.
      All in all it was the joinery I felt contributed most to the failure, and there is something of value in using glues that allow splitting on the glue line for repair work to take place, for furniture I will continue with PVA and/or animal glues and perhaps use the Cascamite for outdoor work but then again I think that the newer PVA waterproofs seem to be holding their own these days..



  2. Thomas Tieffenbacher/aka DocSavage45 on 1 September 2014 at 7:27 pm

    Paul,

    Great investigation and repair advice. Do you think more than one woodworker may have put this piece together? Hence different quality joinery? What do you think about using epoxy’s in adding strength to the joinery?



    • Paul Sellers on 1 September 2014 at 7:58 pm

      I think, yes, on the mahogany table, as there are discrepancies in standards of workmanship at different parts and, no, on the oak desk as the standards are equal but the mistake on tenon thickness didn’t reflect poor work so much as poor judgement of thickness.
      It’s not good practice to use epoxy because of its irreversibility as a glue. Most other glues can be reversed or parted but not most epoxies.



  3. Salko Safic on 3 September 2014 at 1:51 pm

    I believe that this is mass produced furniture and they most probably had the machine set to cut undersized hence why they used that glue to pack it in tight. I could be wrong but that’s what it seems to me



    • Paul Sellers on 3 September 2014 at 4:12 pm

      Then you are wrong I’m afraid. There are tenon saw kerfs in tenon faces and too much effort elsewhere. No way anything but totally hand made with no machine work anywhere.