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My new dehumidifier can either maintain my shop at 38% or 45% r.h. (sling psychrometer readings) using the 5%-increment humidity control. Is there any good reason to choose one of the settings over the other? Which setting should I choose to build? I would prefer to build between 40% & 43%. The finished guitar will live in central Pennsylvania where summer r.h. can get quite high & winter r.h. quite low. Of course, the guitar will be humidified in the winter.

50% r.h. used to be a common recommendation, but it seems like 45% is now the usual number given. During times of high relative humidity the wood can swell and cause the action to raise if the guitar is built at too low of a relative humidity.

My shop stays around 50-52% and that seems fine. I'm in Tennessee.

you are good in both
I would rather set braces on a plate at 38%
you are good as long as you don't go below the 38 or above 55

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I build at around 40%. 38 is okeydokey for me, but no lower. I will do no glue ups above 43%...

Point of reference:

Martin


[url]
http://www.martinguitar.com/guitars/gui ... idity.html[/url]

Gibson


http://www2.gibson.com/News-Lifestyle/Features/en-us/309-gibson-usa.aspx


Since they ship guitars all over the world I would say if you keep your shop at 45% you should be fine.
I don't let my shop get over 50% or under 40% for much more than a day or so before I take action. A day or so under the extremes wouldn't cause much of an issue because wood does not fluctuate in humidity like air, it takes a while for wood to take up or expel moisture. But you do want to maintain a somewhat constant humidity level.

Bob

Bob,

I disagree. I've seen wood change shape within minutes, let alone hours at the thickness we use them at.

I have to agree with meddlingfool just yesterday while sanding the flat sides, I was outside and the wood changed the moisture content within 2 minutes of the wood to where it started to bow. It was too dry as too warm.

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美智え

Well maybe I am mistaken and need to re-think my ideas on humidity then. Being a new builder I have just started acquiring tops, backs and sides and it has been my understanding that the wood should be stored at 45% humidity and you should build between 40% and 50% but no higher or lower. I have been working with wood for a good part of my life but usually at thickness of 1/2" to 1" and have built things in all different conditions and never worried too much about humidity other than if it was very high or low. Maybe I am just lucky but I have never experienced any bad effects from humidity.

So you are saying with 1/4" wood I am going to have to maintain my shop at exactly 45% humidity, no lower, no higher if I am planning on building a guitar? Maybe I am not understanding this correctly but a 5% humidity change will warp my wood in just minutes?

So how are most of you controlling the humidity to that exacting measurements when you build?

I have a de-humidifier in my new shop and when it is finished I will also put in a humidifier but I am pretty sure it will have some fluctuation of at least 5% unless I spend thousands of dollars on a control unit that will maintain a specific level.

Any help you can give me would be very much appreciated since I thought I had a pretty good understanding of it.
I am not the type of guy that is NOT going to work because the humidity dropped a few points.

So what the heck did they do before humidifiers and dehumidifiers were invented just not build guitars?

Bob

Yep. IME about an hour or two is all it takes for plates to acclimate. I keep the show face shellacked throughout the build, and if I need to dry them out for bracing, I toss them in the oven with a hygrometer and periodically turn it on for a few seconds to keep it warm. Done when the plate flattens out.

As for construction RH, it depends on the woods used. Some woods have higher dimensional change for a given RH change. Ebony in particular. Flatsawn woods also move a lot more. But some species, like cedar, redwood, rosewood and mahogany, have low movement rates. So if you use those quartersawn, you can build in lower RH to tolerate extreme dryness, and then when taken into a humid environment, they won't poof up any more than a guitar built with high movement woods in higher RH.

It pains me when I see people build with perfectly quartered redwood and rosewood in 45%, because it's a waste of the low range tolerance you could have by building dryer. You only need to build wet if the wood is going to swell enough in high humidity to pop glue joints. And then you have to baby it in low humidity, which is annoying, and thus will probably not be kept up for every day of every winter for the next 50 years or however long you hope for it to survive.

I aim for 35%, but err on the lower side. Tolerates up to 70% for sure, possibly 80+%, although I've only had it get up that high for short periods. I've still had more trouble in the winter. No cracked plates on those instruments, but popped un-bound back seams and popped un-pocketed brace ends.

One interesting test subject of mine is a small guitar with flatsawn spalted maple back/sides, built in 35%. It has a small crack in one particularly punky spot of a side, and brace end popped up (poor workmanship on my part... brace didn't reach the pocket), but those happened in winter. Otherwise has been fine in <20% to >70%. Makes me wonder if there's ever any reason to build higher than 35%, since flatsawn maple has very high movement. Maybe for guitars that will live in tropical environments.

One problem with low humidity building, particularly with spruce soundboards, is the upper bout rising and creating a ski-jump fingerboard. Shaping more fall-away into the fingerboard extension is one way to deal with it. But I think an L-shaped headblock, glued to the upper transverse brace would help even more. Stiff enough that it probably won't rise as much in the first place, and may drag the neck angle along with the upper bout expansion, which would tend to cancel out some of the action rise from the bridge area doming up. I don't have a good test subject to verify that yet... need to build more spruce topped guitars.

I keep my shop at 45% I will set for 35 the night before to get the air down to about 38% I want the air dry for placing braces , then I heat my tops and brace . Since I have been doing this , I have not seen a joint separation. as soon as the braces are on I am set back to 45%. Lets face it , you can glue in dryer conditions and in the life of the guitar, this may be more helpful when a customer allows the guitar to dry down in 30% conditions.
I use a flat upper braces and have not seen them rise , they stay pretty stable. I also use the old style bracing 3/8 by 5/8 transverse brace.

RH is one of the conditions you need to control when building also know where it is going. A guitar to AZ may need different building conditions than one going to Florida

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

So I will ask the question again...

What did they do before humidifiers and dehumidifiers were invented?

Did they only build when the humidity was perfect? That would mean that they only built a few days out of the year and sat around waiting for the humidity to drop... or rise...

I know this may sound like a sarcastic reply but it is a serious and I would like to know why it is so important today when people didn't obsess over it in days of yore.

Bob

depends who you are talking about? CF Martin used the heat and air flow in the attic to control the wood . In the glue room it was heated and they used cast iron registers . The glue room was always hot for the HHG. They did have a way to control it.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Here's a total humidity controller that works great and won't break the bank. The controllers on most machines are lousy. I have had one 5 or 6 years and it has not missed a beat.

http://www.amazon.com/GATHC1-Total-Humi ... B001MD8XJE

I usually build at 40-45%.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

RusRob; I'm with you. Perhaps Martin and Gibson maybe tried to do something about it. But realistically I doubt they even knew what the humidity was back in the day. It wasn't even reported on the weather stations until relatively recent times. The factory workers froze in the winter and burned smack up in the summer.
The thicknesses we work at causes our wood to move around a bit. I remember building in Arizona. How a rain storm would come up and you could actually see the wood warping,etc. I imagine the real old timers just fought through it. Maybe if your grampa's rheumatiz started acting up that probably was a good barometer.


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I don't sweat it if it's between 35-50%. Outside that range I crank up the humidifier or find something to do that doesn't involve joining wood to wood.

The critical step is the RH (EMC) of the top and back when braces are placed. It's important to control that very precisely, as it's directly correlated to the dome radius at, say, 50% RH. I brace with the tops at 38-40% (controlled in a cabinet), using a flat-top design. Braces are placed quickly with CA. If I brace with the tops at 32%, the guitar sounds tight.

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David Malicky

I sure am no expert, so i asked google some questions about it. It is speculated that Da Vinci was the first to invent a hygrometer. 1664 by Francesco Folli, 1783 by Horace Bénédict de Saussure. and 1887-1892 Richard Aßmann, there is a big list for different types of hygrometers. Does this mean that Stradovari used one for his guitars? Probably not, but there is always a possibility some luthiers used these instruments if they had access to them. I have also read that historically (Please correct me if i am incorrect on this one) luthiers would air dry/kiln and season wood to get the moisture content as low as possible (i have read as low as 15-20%) So i would speculate that instrument builders knew about it, and used airflow, heat from attics, cool damp basements, water buckets, even fires to control humidity to some degree. While i can not give you a definitive answer on if they knew about it or how to control it. It is in my opinion that of course they knew how to work with it. Here where i live the air is always very dry. So i always have to add moisture, and when i take wood from the humidified environment to a dry one outside, the water tries to escape fast leading to warped wood.

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美智え

There is a tendency on this forum and others to make this subject way more complicated than it needs to be. There are also lots of great replies here already that I personally very much agree with some or all of.

In an attempt to summarize:

If your climate is similar in terms of RH to Martin's location (meaning not Arizona or Saudi Arabia...) maintaining a range of 40 - 50% especially when bracing as David said is all you really have to do. Mind you that may not be easy and usually requires some augmentation by some of our plastic, fantastic consumer appliances.

I like a temp of 72ishF for my own comfort.... and to ensure that glues, finishes, etc. cure in accordance with manufacturer's stated cure times.

Know too that if you have to dehumidify the machine will introduce heat into the shop which may be undesirable causing one to crank the AC and making your electric company happy as a clam all the while....

Drier is better and violin makers and Steinway have been known to brace with lower RH and benefit from same in that these instruments likely withstand the ravages of cold, dry winters better than an instrument braced at say 50%.

As for how quickly RH can make a difference two stories: Upon finishing the bracing of a top one evening in the summer I decided to get a drink and sand the thing on my back deck. In 20 minutes my top had flattened..... Not good. It recovered in the dish in the RH controlled shop weighted and overnight.

Second story - there once was an OLFer (true story) who reported that on his very first guitar all went very well, he had sprayed 12 coats of nitro and was waiting the month for the finish to harden and at 2:00 AM he and his wife heard a loud "crack" coming from the basement. His first build cracked the top because the RH fell as the seasons got colder and drier. He was devastated....

Here in Michigan when I am building in the summer I have to dehumidify and crank the AC too to maintain 42 - 48% (the range I like) and 72F. In the winter I have to humidify but barely with less than 1/4 gallon a day needed in my nearly 400 square foot shop. I do humidify the entire house though with 4 humidifiers and as such am schlepping water far more than I wish to.

There is a belief that is incorrect that once you brace your top you have to move at light speed to attach it to the rim before something changes. This is BS if you control the RH in your shop well. Lots of factories including small factories (er I mean f*ctories...) may have braced tops hanging around for weeks or more waiting for a rim. Again so long as the RH is under control this should not be a concern.

For many of us controlling RH is not always easy but the consequences of not doing so can be catastrophic. If you ever intend to sell your creations please, please control the RH or your value proposition may be short lived indeed.

True since many of you have lots of experience with wood working that you have not had to pay attention to RH to this degree prior. It's also true that some of our plates are very thin, .090" or so and will react to RH changed quickly and sometimes in a manner that we do not wish.

Regarding what they used to do back in the days of the solitary Luthier toiling endlessly by candle light with sharp chisel in hand when they did not have Hammacher Schlemmer and a 50 mips Internet connection for the IPad to order a cheap, plastic digital hygrometer that reads inaccurately anyway... although we can't know what they did we can know what they produced and what often still exists today.

As such it's likely that they did seasonal work understanding more about what to expect from the seasonal changes than we may know these days. This information may have also been part of what is transferred from master to apprentice as well. In the rainy season they may have cut pearl or dimensional lumber and in the dry season they might be bracing away. They also had human hair and wood which is all that is required to make a hygrometer that actually works in a useful manner.

We obsess over a number, hence this thread, but perhaps they made and used shop-made hygrometers that were not unlike a traffic light signaling go, stop, and go faster....

We can't know but these were some pretty bright guys so my bet is that they had a method and used it to know when to do what. The proof is in the pudding as they say, I think....

common sense .
RH was controlled a number of ways and like Hesh points out this isn't electricity it doesn't happen in seconds. Salt was often used to help lower RH but I bet you knew that already.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Hesh is right. I can't control the humidity in my garage at all, so I store wood in a dry box that is somewhat controlled. Brace up on dry clear days.
I asked Cumpiano about this 15 years ago before I built number one. He said use a control box to store the wood if you can't control the shop, and avoid extremes - extremes in temperature and humidity when assembling. It has worked so far - I've never had a crack or warpage.


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Been at 45% for 30 years and brace at 35% although the high arch and laminated bracing seem to hold their arch. Still prefer to drop the humidity 10% for a day.