Implications of humidity ?

[Howard Williamson]

First timer here, getting towards the end of my first build.
Body and neck done but not glued together yet, fret board not glued on yet.
I'm located in the interior of British Columbia, R.H. in winter is 70 ~ 100%, Summer, 50 ~75%, occasionally dropping into the 40's.
I have no humidity controls in my home or shop.
Anything special I should be doing or taking into account here regarding my assembly or post assembly care ?
Cherry sides, back, neck, Cedar top if it matters -----all local wood.
Thanks

[Bryan Bear]

I your shop heated in the winter and/or air conditioned in the summer? Likely the RH in your shop will have some degree of humidity control; your shop RH is likely not the same as the RH outside.

[Alan Carruth]

Iirc, wood changes dimension least between about 40-60% R.H., and more above and below that. If the top was glued to the rim when the humidity was anywhere in that range it should be OK down to about 20% below the humidity it was glued at, if it's a normal domed top. Really flat tops might withstand a bit less change. Lower than that and you're risking cracks.

As Bryan says, it's the indoor humidity that counts. In these northern climes during heating season the humidity indoors will be much lower than it is outside: the cold outdoor air can't hold nearly as much moisture as it can when it gets warmed up. I had a new guitar top get cracked in a gallery sow in January because the folks running it did not understand about humidity and wood. During a cold snap the R.H. in the building got below 20% (and I think 'way below) and it cracked. Other wooden objects in their collection also suffered: the maker of one fine cabinet told me he needs to get it back and re-veneer it to make it presentable.

It's entirely possible that the R.H. was low when you closed the box, in which case you might find the instrument suffering in the other way: the top could swell and the action change. This won't usually damage the guitar directly, but it may make it hard to play, and it might not sound so good. This is why most working luthiers try to maintain a R.H. on the low side of 'normal': higher humidity is less of an issue than low. I keep my shop in the low 40% range year 'round. Sadly, it means I only get to open the windows for a few days once in a while in the spring and fall, the rest of the time I'm running humidifiers or dehumidifiers or air conditioning. The cost of doing business.

The thing you need to do is get a hygrometer of some sort. It does not have to be fancy: one of the coolest I've seen was a 1" wide strip cut along the edge of a sheet of plywood, that had one of the outer plies milled off. The difference in expansion of the two remaining layers of wood caused it to bend as the humidity changed. One end was clamped to a block nailed to the wall, and whenever the shop was open to the outside the owner would get the R.H. from the weather report, and mark in on the wall at the free end of the strip. It worked pretty well.

Alan Carruth / Luthier

[Bryan Bear]

Alan, I have been thinking of making something like this. Not so much for "measuring" RH but as a visual reminder of its importance for instrument makers. Plus, it would look cool and be an interesting conversation piece, It sounds like you are saying that it can be somewhat accurate too. Could one calibrate with a hygrometer and note "safe gluing range" on the scale for quick reference? I was envisioning a stick of spruce with one lam long grain and the other cross grain. Do I need to avoid water based glues? Do I need to glue at 50% humidity. I'm thinking that I could glue with hot hide glue and keep it clamped to the flat workbench for a few days until the wood strips reach equilibrium. Then straight would be whatever RH I unclamped them at. Sound reasonable?

[Chuck Tweedy]

If I were to make such a beast, I'd use a long grain strip and a cross grain strip.
I'd try to find the most unstable wood possible for the cross grain strip - I don't know what wood that would be.
I'd orient that cross grain strip in the least stable orientation possible (i.e. - not quartered)
I'd dry that cross grain strip to near 0% water content (bake it at 180F or so)
I'd glue it to the long grain strip right out of the oven with 5 min epoxy.
Then I'd sit back and watch it curl.

This is the equivalent of a bi-metal strip.
If you could glue it up like a clock string, you could even put a needle on the center piece and put it behind a gauge face.

[Charlie Schultz]

Here's another version (based on a Fine Woodworking tip). The beam (about 20" long) is anchored on the left end and the needle on the right-hand end will swing based on the contraction/expansion of the wood beam.

[Alan Carruth]

Chuck Tweedy wrote:
"If I were to make such a beast, I'd use a long grain strip and a cross grain strip."

That's essentially what you get when you mill off one ply of a piece of 3-ply stock, and it's all glued up for you.

I'd actually want to avoid too extreme a difference in expansion ratios. I made a 'humidistat' once by gluing a cross grain spruce strip to a piece of tinned brass shim stock, and used it to trigger a relay that switched a light on and off in the 'dry box'. After a while the wood delaminated from the metal. It worked great while it lasted.

Carleen's rig was surprisingly accurate: probably about as good as the little digital things. Because it was so long it swung through quite an arc: maybe a couple of feet all told, so it was easy to see changes.

I would not worry too much about glue type, or letting it dry perfectly before you unclamp it. I would make sure it was at equilibrium before I started to trust it, though. You can find that easily by weighing it.

The FWW rig looks cool, too. The free end could sandwich a pivot between the beam and the base, which would then rotate as the beam expanded and contracted, turning the needle. Maybe that's what they did: it's a little hard to see in the picture. You can get a pretty hefty amount of leverage that way, which would make the needle very sensitive, or reduce the length of the meter. It would also be easy to calibrate.

[Bryan Bear]

I love this place!

[Chuck Tweedy]

Alan, I guess the one thing I think would be important is to glue-up the strips when the cross grain strip's moisture content is low.
If it were at maximum moisture when glued up, it would need to pull the other strip in response to lower humidity. With a thin cross grain strip, there is no integrity there, so I would expect it to just split up into short pieces and have minimum action on bending it's long-grain mate.

[Michael Lewis]

I keep a couple $20 Radio Shack electronic hygrometers in my shop and glance at them a few times each day. They are probably not as precise as some better instruments but my main concern is to avoid the extremes, not to "know" the exact RH. People (and luthiers) a hundred years ago lived without such things for the most part, and learned how to deal with humidity issues after a fashion. There are still some fine examples of wood work and instruments from hundreds of years ago.

[Bryan Bear]

To Michael's point, the wooden "hygrometer" would be a neat addition to the shop. It can tell you if you are in a good range or not and have a cool woody vibe. Michael reminds me of my old wrestling days, guys wold always ask "how much do you bench?" I would tell them that I didn't know since I never add it up, but I get a good workout every time.