Here at our shop we (Richard Raskob and Myself) have been discussing the proper glues to use when installing soundboard shims. I thought I'd post some of our ideas in the hopes that we could get some feedback from all of you, and maybe find someone who has done some real quantifiable testing along these lines. The standard wisdom for glues whan working with soundboards seems to be "the harder the better". When we glue maple to maple on bridges, or maple to spruce, the harder glue will more efficiently tranfer the energy to the soundboard where we want it. So far so good. For years we have applied the same concept to soundboard shimming. Hard glue = good transmission. Soft glue = bad sound transmission and should be avoided at all costs. We mostly have used quick set epoxy on our shims because it is very hard, and can be colored easily for a good match at the seam. We have also tried Garret Wade gap filling glue (good stuff) and traditional hide glue as well. The problem with these water based glues is that they tend to swell a well fit shim to the point where it may become impossible to install after the glue is applied. The fact that epoxy doesn't effect the shim much dimensionally is a big plus for us. The big minus with the quick set stuff is that it gives you a very limited amount of time for glue and clean-up during a job that calls for great care. I would like the luxury of more time. So... We have been trying a 2 hour epoxy lately and have been very happy with the results, *except* it ends up being significantly softer when it sets. This brings us to the point, and the question for discussion: Is soft glue in soundboard work such a bad thing? Our conlusion at this point is that it is in fact a GOOD thing. OK class, here are the discussion points for the next time we meet... 1. Isn't the best glue to use one that is of similar hardness to the soft spruce that it is gluing? wouldn't a soft glued board react more like the original board? 2. Wouldn't a soft glue be more likely shift and flex with the board with changes in humidity, thus being less likely to crack over time? 3. Isn't a hard glue joint a high impedence structure, more likely to reflect sound back, than to pass it on to the rest of the board 4. doesn't most of the sound get from the bridges to the outside of the board through the ribs anyway? This is where we're at... Comments please John McKone, RPT Operations Manager Haugen's Pianos Twin Cities
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