Glue question

What kind of glue would you recommend for shims in a soundboard (crack)?

A glue-related topic! I missed that.
I only use hide glue, so that’s what I’d recommend: fast, effective, clean.


In principle I agree, Domenico, but …
If you are not (or are no longer) involved in harpsichord building, you need to procure hide glue, and have the skills to use it properly: this may be particularly tricky for the uninitiated in the case of a precision-work such as shimming.

If for general harpsichord building hide glue is recommended for different reasons (mainly for being easy to remove if needed), I do recommend titebond/vinyl glue for the occasional amateur use for repairs such as shimming (where you are highly unlikely to later try to remove the shim). This type of glue is easy to dilute with water, very easy to apply, and before it dries you have plenty of time to remove the excess glue with a wet towel. No special skills or processes are needed.

The great glue debate, my favourite, and the archetypal harpsichord list topic!

Titebond is fine for shimming soundboard cracks. I would not be diluting it. What for? As far as I recall Franklin recommends diluting with no more than 5% water but I cannot find the reference for that just now.

As to hide glue. Franklin makes a very good liquid hide glue, which experience shows is excellent. Less messy to clean up than Titebond Original.

More important than the glue used are the ambient conditions in which the shim is inserted. Where the shim is needed to compensate for long-term shrinkage of the soundboard, it’s desirable for the shim, at rest in the lowest likely humidity, to be lightly compressed by the surrounding wood. To achieve this, glue the shim when the humidity is low, if necessary shrinking the soundboard slightly by artifice. In this way, the soundboard is unlikely to rupture again under tension.

@alvisezuani I totally agree with this.

Also worth mentioning that generally a soundboard crack has just spilt along the grain and is not a huge gap. You don’t want to be cutting out a great triangular groove with a chisel to repair it. What we do is make a very thin plane shaving of soundboard wood, really thin, use a thin layer of Titebond on both sides and insert. Trim the excess wood when dry.

Something else worth saying is that treble cracks hardly affect the sound, if at all. The only thing is that they look unsightly. But they don’t necessarily have to be repaired. If you have major splits other than the treble region then you have much bigger structural issues.

I do have two minor cracks in the treble that don’t seem to affect the sound. The instrument was in a low humidity environment. With 55% humidity the cracks have closed up and I am tempted to try low viscosity cyanoacrylate.

But then if the humidity drops, the instrument might acquire a new crack alongside the mended one.

I have never had a crack open so wide that it needed shimming: such a crack must indicate serious movement. The small cracks that I have had have been repaired with the much-abused Titebond type glue, and have never opened up again. I also have a small treble crack that I have not repaired, preferring to use it as an indicator of relative humidity.