Official Luthiers Forum!

I fancy Mr. Hall's brevity it these discussions - very Joe Friday...just the facts and do with them what you will.

We rehydrate at a target of 55% -60% RH for 4-5 days (prevents over-humidification), then move to shop humidity (42% target) for a day or two to see where the crack goes. If still closed and not badly contaminated following humidification, we glue and cleat with something close to Mr. T.J. Thompson's radiused cleats for linear top cracks, then do the finish touch-up work. If not closed after humidifying and stress-testing, or if the joint is contaminated with dirt, grime, Titebond, etc., we compression-spline the crack and cleat. Like Mr. Franklin, we believe that, while a good repair will help resist opening of that same crack with reduced RH, the owner will likely as not repeat the abuse. Thus, the only repairs we do not routinely warranty are low RH-related damage.

For a time, my boss insisted on baking a quality guitar humidification system into the cost of low RH crack repairs, but saw no correlation between that practice and follow-on low RH abuse by the same customers, so now provides just a detailed tear sheet on low RH damage and how to prevent it with the paid repair ticket, and the advice that re-repairs due to continued abuse are usually 2x or 3x the original cost.

In a nutshell, the saying in the shop is:

"If they did it once, they will do it again: repair, educate, and double the estimate for repeat offenders."

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A constellation only takes shape when one maps the whole.
- Beth Brower

Hesh, I don't think that I told an entire industry that rehydration flat out doesn't work. If you'll reread my post you will see that I have said that it could be useful under some circumstances. I don't think this is one of them. You say you were on the fence about it but you have given the OP instructions for rehydrating but no instructions for filling the crack. Here's what worries me. It's a gift. It's going to someone he knows. Would you want a new guitar that you have given to a friend to possibly come back with a crack in the back. You say you have seen guitars you repaired by rehydrating after a number of years. Those worked, but what about the guitars you haven't seen after a number of years?

I generally enjoy making guitars and other instruments and have been invited to exhibit at national and international guitar festivals such as Healdsburg, Montreal, Memphis and the upcoming Santa Barbara celebration for more than fifteen years, but that doesn't keep local players of our rural community from knocking on my door in need of instrument repair which I have also been doing for the same length of time. While our area doesn't generate the amount of work that you do, I'm well versed in all manner of repairs, lifted bridges, neck rests, loose braces, crack repairs, setups, refrets, etc. Though I'd rather be building, I do a t least 2-3 repairs a week. The important part about these repairs is that I know these people. I will see these instruments again and I don't want to see the same problems return. We have our share of low humidity in the 20-30% range but it happens in the summer and it's exacerbated by air conditioning rather than heating, so I see my share of RH problems. I don't feel like I have to justify what I do to you but I think that it's important for others to know about my skill and experience so they aren't influenced by your insinuation.

I'm glad that this discussion has encouraged others to join in. David, you asked what I would do if an instrument gets too dry or too wet. If it's dry and has a crack I fill it with a sliver (or spline) of similar wood. If it get's too wet I check to see that the glue joints have not been compromised and that no braces have come loose. The instruments that I have seen exposed to high humidity haven't usually suffered as much. I do occasionally have to steam out a dent but that is more of a cosmetic than a structural problem.

I like to build under similar conditions as John Hall describes with well seasoned wood and 38-42% RH. I I like to have some dome in the top and back. I don't think that the OP's crack is from being built in a dome because that outside area is being compressed.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Ken no insinuation was made. Instead I am asking questions in an effort to get a level set on your repair experience level. I also asked questions of the OP for the same reasons and to hopefully ferret out my suspicion that RH played a role in this back crack.

Why did I not provide a method to fill the crack? I wasn't asked AND in the absence of being able to look at the box first hand it's still very much a mystery why it happened in the first place but nonetheless important.

Anyway I suspect that we will have to agree to disagree and that's what I'm going to do here.

Peace

Ken, your absolutely right about the doming not being relevant to the crack. I failed to notice initially it was at the end. I had a kneejerk response when I heard 15' radius. I saw what I wanted because I feel builders are overly enamored with tightly domed backs in general. It's true you can hold your car up with a tightly domed body while you change a tire, but I do think tight doming puts some un-necessary strain on the wood.

Your advice about filling both sides so it looks naturally symmetrical is pretty sage and indicates a lot of experience and wisdom. But not cycling a crack through typical humidity swings to help figure out what the cause and best fix might be I found surprising and confusing.

My question about what would the fix be was rhetorical. If someone has let an instrument dry out to the point of cracking, the problem, at least could be, behavioral. Every repair person has struggled with the best course of action regarding cracks. I'm always trying to find a better way. Filling a crack with a splint is often necessary but if it facilitates bad care, I'm not sure it would make a repeat offence any less likely. Maybe it would.

With the crack in question I do suspect the sapwood is at least part of the problem. I think it's a strange modern aesthetic. What used to be considered inferior portions of the tree are now considered fancy. Go figure.

At this stage of construction, with an unbound body, I would be inclined to remove the back, remove the braces and glue the crack.
If the resultant repair is aesthetically acceptable proceed to rebrace and refit at a confirmed low humidity.

The wood does not lie about it's history of humidity exposure, you just have to understand what it is saying.

Here it is telling you that either
-it was braced/glued to the rims at a high RH and then exposed to low humidity
-it was braced/glued to the rim at acceptable RH and then exposed to VERY Low Humidity

The other thing to be aware of is the hysteresis effect. When wood is dried to a very low RH and then brought back to a normal level, it does not recover all the shrinkage which occurred. This is why rehumidification at high RH can be useful for an instrument exposed to abnormally low RH, as long as, after rehumidification, it is brought back to a normal level before repairs are carried out.

One addition to Jeff's list.
If wood is exposed to very high humidity or standing water, the cells get crushed in places where they are restricted. Along back braces and end blocks for example. Then when it dries, it shrinks to a smaller size than it was. A lot of people have seen instruments that were victims of flooding. When they dry out they often have huge cracks. The wood cells got crushed in the swelling and then the drying caused havoc.
There's a picture in Hoadely's "understanding Wood" book illustrating the effect. He calls it,"compression shrinkage". It's the reason I'm against extreme over humidification to close cracks. A little over normal humidity can overcome hysteresis, but too far causes revenge effects.
I can't prove it but I think some permanently sunken tops are the result of an instrument spending time "over puffed" in a very humid climate. When they return to a dryer climate they paradoxically show signs of excessive drying.

Using excessive moisture when cleaning squeezout along braces I think can contribute to this kind of invisible stress.

I hate two things, coming late to a party (fashionably late is ok) and disagreeing with Hesh. Mind you, it is not to do with wood or guitars so I figure my two cents will be viewed as just wanting to inform.



Not quite. Most general purpose modern humidity sensors are of the capacitive type, a piece of hygroscopic material (plastic) is sandwiched between two plates and the capacitance between these plates are measured. I will leave it to others to explain the capacitive sensor faults and just mention that accurate temperature measurement is important in a sensor and if it is off the humidity reading (which is a calculated value) will also be off. Just as the relative humidity in a measured area will be different than the average humidity of the space if its temperature is different.

http://www.vaisala.com/en/lifescience/blog/Lists/Posts/Post.aspx?ID=40

(Well respected company in temperature and humidity measurements)

Fred great post and many thanks for making it! You are edumacating... me

Great resource on how they operate - thanks Fred!

Over the last 20 years I've gone through a lot of digital hygrometers, many lasting only a few years before they begin to read innacurately. I've torn many apart, and seen the capacitive sensors just as shown in your reference. My assumption was that in a dusty shop the sensors were either being contaminated, or breaking down in some way.

Reasoning why they fail has never been more than a casual hypothesis for me, made without much thought or research. We just check the shop hygrometers periodically against our psychro-dynes (love the University of Michigan Property Surplus), and when they cease to function for reasonably accurate at-a-glance observations, I just pitch and replace them.

That's a great resource for a better view of how they work and how they can fail.

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Eschew obfuscation, espouse elucidation.