Building Relative Humidity

Construction and repair of Classical Guitar and related instruments
Alan Carruth
Luthier
Posts: 2663
Joined: Mon Dec 24, 2007 6:56 pm

Re: Building Relative Humidity

Post by Alan Carruth » Mon Jul 03, 2017 4:42 pm

Jim Frieson wrote:
"Varnish the braces and the top won't move ."

I doubt that. Varnishing the braces might help keep them from changing length, but that's not the problem: the problem is that the top changes width. Although putting a finish on the inside of the top might, in theory, slow down the changes, it won't help over the long term. The U.S. Navy put a lot of effort into research on ways to stop wood from absorbing moisture, and nothing they tried, including 1/4" of epoxy, worked. That's when they switched to making small boats out of fiberglass. If there were a solution as simple as varnishing the braces everybody would be doing it. I know it's been tried in the past, and such innovations that actually work spread fast.

Jim Frieson
Posts: 508
Joined: Mon Jan 24, 2011 12:48 pm

Re: Building Relative Humidity

Post by Jim Frieson » Mon Jul 03, 2017 6:11 pm

Alan Carruth wrote:
Mon Jul 03, 2017 4:42 pm
Jim Frieson wrote:
"Varnish the braces and the top won't move ."

I doubt that. Varnishing the braces might help keep them from changing length, but that's not the problem: the problem is that the top changes width. Although putting a finish on the inside of the top might, in theory, slow down the changes, it won't help over the long term. The U.S. Navy put a lot of effort into research on ways to stop wood from absorbing moisture, and nothing they tried, including 1/4" of epoxy, worked. That's when they switched to making small boats out of fiberglass. If there were a solution as simple as varnishing the braces everybody would be doing it. I know it's been tried in the past, and such innovations that actually work spread fast.
It was a joke . Followed by another joke .
Not that one would be required because varnish the braces to keep the top from moving is funny by itself .

Grooveman JS
Posts: 212
Joined: Tue Jan 03, 2017 2:56 am

Re: Building Relative Humidity

Post by Grooveman JS » Tue Jul 04, 2017 5:05 am

Jim Frieson wrote:
Sat Jul 01, 2017 10:19 am
The top is braced on the inside , during past winter , at 35% relative humidity .
Now it is 60 % relative humidity .
If I had braced it at 60% and exposed it to 35% , the effect , opposite .
Illustration of the effect of humidity on the broad expanse of tops and backs .
DSCF6498.jpg
wowww :shock: I know humidity & temperature causes expansion & contraction in woods; but never really saw the physical effects of it until this picture.......
Masaki Sakurai MA-RF
Antonio Picado Concierto DT

Grooveman JS
Posts: 212
Joined: Tue Jan 03, 2017 2:56 am

Re: Building Relative Humidity

Post by Grooveman JS » Tue Jul 04, 2017 5:13 am

As musicians are travelling more often because the world is getting more inter connected....I guess it'd make sense to build an instrument to cater to a range of humidity conditions while accepting the general fact that most guitars would do well with a RH environment of between 50-60% (or 45-55% depending on which part of the world) for maintenance purposes.
Masaki Sakurai MA-RF
Antonio Picado Concierto DT

SteveL123
Posts: 423
Joined: Thu Feb 09, 2017 5:05 pm

Re: Building Relative Humidity

Post by SteveL123 » Tue Jul 04, 2017 3:15 pm

Alan Carruth wrote:
Sun Jul 02, 2017 8:12 pm
.............
One issue with this is the one I ran into with my relay trigger: when the humidity goes much lower than what it was when the strip was glued on the shrinkage sets up a lot of stress in the cross grain strip. Eventually it cracks, and the thing doesn't work any more. .....................
An improvement over the aluminum strip is don't use aluminum. Put a small magnet at the tip of the Spruce needle to trigger a reed switch to turn the lamp on.

Alan Carruth
Luthier
Posts: 2663
Joined: Mon Dec 24, 2007 6:56 pm

Re: Building Relative Humidity

Post by Alan Carruth » Tue Jul 04, 2017 5:56 pm

SteveL123 wrote:
"An improvement over the aluminum strip is don't use aluminum. Put a small magnet at the tip of the Spruce needle to trigger a reed switch to turn the lamp on."

?

I used a strip of tinned brass stock, not aluminum, on that rig. At any rate, the material would not matter much: the problem is that you have a piece of cross grain stock, which changes width by a lot as the humidity changes, which is glued to something that doesn't. That's what causes the thing to bend around and make or break the contact. If you assemble it when the humidity is high shrinkage of the cross grain strip will probably destroy it fairly quickly. If you put it together when the humidity is very low to avoid that it might destroy itself anyway as the cross grain strip gets crushed over time. That's why I was so interested in the 'weather stick': maybe it would hold up better.

SteveL123
Posts: 423
Joined: Thu Feb 09, 2017 5:05 pm

Re: Building Relative Humidity

Post by SteveL123 » Tue Jul 04, 2017 6:06 pm

Alan Carruth wrote:
Tue Jul 04, 2017 5:56 pm
SteveL123 wrote:
"An improvement over the aluminum strip is don't use aluminum. Put a small magnet at the tip of the Spruce needle to trigger a reed switch to turn the lamp on."

?

I used a strip of tinned brass stock, not aluminum, on that rig. At any rate, the material would not matter much: the problem is that you have a piece of cross grain stock, which changes width by a lot as the humidity changes, which is glued to something that doesn't. That's what causes the thing to bend around and make or break the contact. If you assemble it when the humidity is high shrinkage of the cross grain strip will probably destroy it fairly quickly. If you put it together when the humidity is very low to avoid that it might destroy itself anyway as the cross grain strip gets crushed over time. That's why I was so interested in the 'weather stick': maybe it would hold up better.
What I meant was do not use metal strip on the stock for your switch, use a magnet at the tip of the hygrometer like this one viewtopic.php?f=11&t=107791&hilit=hygro ... 5#p1145948 by James Lister to trigger a reed relay to turn the lamp on/off.

SteveL123
Posts: 423
Joined: Thu Feb 09, 2017 5:05 pm

Re: Building Relative Humidity

Post by SteveL123 » Tue Jul 04, 2017 6:07 pm

Alan Carruth wrote:
Tue Jul 04, 2017 5:56 pm
SteveL123 wrote:
"An improvement over the aluminum strip is don't use aluminum. Put a small magnet at the tip of the Spruce needle to trigger a reed switch to turn the lamp on."

?

I used a strip of tinned brass stock, not aluminum, on that rig. At any rate, the material would not matter much: the problem is that you have a piece of cross grain stock, which changes width by a lot as the humidity changes, which is glued to something that doesn't. That's what causes the thing to bend around and make or break the contact. If you assemble it when the humidity is high shrinkage of the cross grain strip will probably destroy it fairly quickly. If you put it together when the humidity is very low to avoid that it might destroy itself anyway as the cross grain strip gets crushed over time. That's why I was so interested in the 'weather stick': maybe it would hold up better.
What I meant was do not use metal strip on the stick for your switch, use a magnet at the tip of the hygrometer like this one viewtopic.php?f=11&t=107791&hilit=hygro ... 5#p1145948 by James Lister to trigger a reed relay to turn the lamp on/off.

Jim Frieson
Posts: 508
Joined: Mon Jan 24, 2011 12:48 pm

Re: Building Relative Humidity

Post by Jim Frieson » Wed Jul 05, 2017 12:50 pm

Grooveman JS wrote:
Tue Jul 04, 2017 5:05 am
Jim Frieson wrote:
Sat Jul 01, 2017 10:19 am
The top is braced on the inside , during past winter , at 35% relative humidity .
Now it is 60 % relative humidity .
If I had braced it at 60% and exposed it to 35% , the effect , opposite .
Illustration of the effect of humidity on the broad expanse of tops and backs .
DSCF6498.jpg
wowww :shock: I know humidity & temperature causes expansion & contraction in woods; but never really saw the physical effects of it until this picture.......
Maybe I should have photographed the other top , a spruce top . It is much more cupped . Both glued at the same time under the same conditions .
I keep them clamped on the workboards untill I am ready to build with them

User avatar
Alexandru Marian
Luthier
Posts: 3178
Joined: Sat Dec 23, 2006 5:02 am
Location: Bucharest, Romania

Re: Building Relative Humidity

Post by Alexandru Marian » Wed Jul 05, 2017 1:11 pm

Yes i thought it looked scary for a cedar top but 35 to 60 is a huge gap. Spruce puffs as much from just a 10% increase.

Even scarier is when you brace high and it dries...

Alan Carruth
Luthier
Posts: 2663
Joined: Mon Dec 24, 2007 6:56 pm

Re: Building Relative Humidity

Post by Alan Carruth » Wed Jul 05, 2017 6:34 pm

Thanks SteveL123. I rigged that up about 35 years ago, using what I had. If I ever do it again I'll certainly do it differently.

Jim Frieson
Posts: 508
Joined: Mon Jan 24, 2011 12:48 pm

Re: Building Relative Humidity

Post by Jim Frieson » Sat Jul 08, 2017 2:32 am

Alexandru Marian wrote:
Wed Jul 05, 2017 1:11 pm
Yes i thought it looked scary for a cedar top but 35 to 60 is a huge gap. Spruce puffs as much from just a 10% increase.

Even scarier is when you brace high and it dries...
Alexandru , It is a redwood , but I have also a spruce top braced in the same way under the same conditions : it is much more cupped .
High humidity and it dries , do you mean ? But it gives me the idea :
When putting on drum heads I wet them , then they were tight and resonant when dry . I worked with a Japanese carpenter , and in making shoji screens and so on , the paper was dampened
before the frame was glued to it , and when it dried , it was tight and stiff . The usual desire for a guitar plate is you want it relaxed , not under tension but it is an interesting idea for grid-braced tops .

Grooveman JS
Posts: 212
Joined: Tue Jan 03, 2017 2:56 am

Re: Building Relative Humidity

Post by Grooveman JS » Sat Jul 08, 2017 8:31 am

While we're on this subject, I'd like to ask is there a product in the market to stabilise the wood on the instrument (something you can apply on the wood.......I do my part; as in keep room RH at about 50% or slightly below 47-49% (usually its about a range of 43-60% as the humidity changes with room temperature).....I live in a high humidity area & i control the humidity using the aircon setting temperature between 24-26 deg.
Masaki Sakurai MA-RF
Antonio Picado Concierto DT

User avatar
Alexandru Marian
Luthier
Posts: 3178
Joined: Sat Dec 23, 2006 5:02 am
Location: Bucharest, Romania

Re: Building Relative Humidity

Post by Alexandru Marian » Tue Jul 11, 2017 7:06 am

Jim Frieson wrote:
Sat Jul 08, 2017 2:32 am

When putting on drum heads I wet them , then they were tight and resonant when dry . I worked with a Japanese carpenter , and in making shoji screens and so on , the paper was dampened
before the frame was glued to it , and when it dried , it was tight and stiff . The usual desire for a guitar plate is you want it relaxed , not under tension but it is an interesting idea for grid-braced tops .
If you let the top dry after bracing it, it will go concave with an even scarier look than when it puffs. For example with the traditional 7+2 bracing, the tail part will go UU with the closing bar severally bent upside down. Even the stiff transverse bars go a little concave. And if you quickly assemble the guitar before the top distorts, it will likely crack badly the second it goes under 45%...

RedCliff
Posts: 139
Joined: Thu Aug 13, 2015 9:26 pm

Re: Building Relative Humidity

Post by RedCliff » Tue Jul 11, 2017 8:33 pm

Alan Carruth wrote:
Mon Jul 03, 2017 4:42 pm
Jim Frieson wrote:
"Varnish the braces and the top won't move ."

I doubt that. Varnishing the braces might help keep them from changing length, but that's not the problem: the problem is that the top changes width. Although putting a finish on the inside of the top might, in theory, slow down the changes, it won't help over the long term. The U.S. Navy put a lot of effort into research on ways to stop wood from absorbing moisture, and nothing they tried, including 1/4" of epoxy, worked. That's when they switched to making small boats out of fiberglass. If there were a solution as simple as varnishing the braces everybody would be doing it. I know it's been tried in the past, and such innovations that actually work spread fast.
This has always puzzled me. You coat wood in 1/4" epoxy - but it still absorbs water. Solution = coat glass fibre in epoxy. Anyone who has had a glass fibre boat knows what happens if you wear through the epoxy - the glass fibre absorbs water. So if epoxy isn't waterproof, glassfibre boats would very quickly be just as useless as a wet wooden boat. I have always figured it had nothing to do with the epoxy coated wood - that will be fine (i've built a few cedar strip kayak - no problems) it is the joints and fixtures that don't get sealed in the same way as they do on glass boats that allows the water to penetrate under the epoxy (or through abrasions). Could be wrong though. Tempted to do a test.
Giles Ratcliffe
Sheffield
England

Return to “Luthiers”