Messages Archive 2010

Subject:
Re: Joint life ( how long will glue last)

ThomD
That is probably true to a certain extent. In most structural applications creep is your big enemy. people seem to want to believe that flexibility is structures is as good as it can be in personalities, but the reality is that for most uses flexibility is a bad thing in glue, at least at the range we get. Possibly there is a glass like glue that would be too hard, but since it has never been sold, I haven't seen that as a problem. Titebond ORG seems to be low creep thus it's success holding bridges on against heavy string load. LMII makes a better lutherie glue, and guess what, it is noticeably harder. I have a hard time wrapping my way around the idea a glue is simultaneously more and less creep resistant at the same time.

I have seen failure in cabinetmaking due to wood creep. The most interesting example in my experience is the joints on my heavy trestle table. The top is nearly 1.5" thick, and about 5 pieces. These have a different expansion ratios in the minor axis. What happened was that the top was glued up and smoothed out perfectly, yet after a while one or two of the joints became noticeable after a year or two. If one runs ones hand over the surface, one feels sharp corners, just as if the surface had not been flattened. At first I was stunned thinking that I had missed finishing the surface, but then it became obvious that I could not have made such a mistake running my hands over the surface while finishing it. So the question is what would have happened with harder glue less likely to shift. Would it have split, or would it have controlled the wood surface. Don't know. The glue I used wasn't Titebond, by the way, at that point it wasn't sold locally, but I don't know whether it would have had the strength to hold up either.

One of the problems with modern glues is they are often combined in commercial work with modern joints, so you get joints that are a nightmare to fix, like finger joints or dowel joints. In my experience these have a 100 percent failure rate over 40 years. That isn't to say every joint will fail, but every piece I have seen with heavy use will have one joint that fails. I had to choose, I would do these new joints with old glue, and use the new glues on old joints.

While we are going through a renaissance of interest in hide glue, in the past, the few big name makers I ran into said they used new glues not because they were better to repair or over the long term, but because they were better in a single cycle. If you sell someone a 3K end table, you never want it back in the shop, and they felt they were more likely to get that service from modern materials, possibly both in the glues and finishes. Of course these guys were using the old joints.

My observation is that the hard glues are normally more expensive, if that tells you anything.

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