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This partially completed cocobolo box has been hanging in my shop for a year or so. Was getting ready to add the end graft and binding channels when I noticed a crack in the spalted sap line. The crack is about 4 1/2 inches in length and has opened enough to be a through and through crack. This guitar is not a commission but will be a gift. Looking for best practice advice before I proceed. Thanks much.
Tom

Hey Tom - What's the RH control situation in your shop? I have to ask because how this is approached will depend or should depend to a large degree on why it happened. Also how well seasoned was the wood?

We can often force cracks closed with RH manipulation but it's not always the right approach in the presence of dimensional instability resulting from green or less than well seasoned wood. When we do force a crack closed, glue and cleat, the likelihood of another crack somewhere else is increased if the root cause is dimensional instability of the wood.

Let me know the answers to these two questions and it will help to suggest a way forward that will work best for you.

Central heat and air year round maintains a RH between 35 and 45 percent.

Wood came from a trusted vendor at .16 and acclimated about a year before thinning to about .095.

How is the curvature of the back right now? Convex, concave, flat? And has it been hanging near the ceiling by the endpin hole? Maybe just the tail area got dried out by the hot air layer at the top of the room. I've measured some pretty extreme temperature/RH differentials between the floor and ceiling in the winter.

Back was set at a 15ft dome and looks stable at that shape. It was hanging from a rafter at 8 ft. I have a two story shop that has an open ceiling for 1/2 the shop, sort of like a loft. I use the 2nd floor for wood storage. The box was hanging from a wire looped through the neck block holes. Ive probably built 10-15 instruments in this shop with this HVAC system moderate success. I have a hygrometer hanging inside next to the shop entrance and I check it almost daily.
if the crack was on a spruce or cedar top I know how to proceed with rehydrating, glueing and cleating. Just need some sage advice for a cocobolo back.
Tom

Puzzling. Why would such a large crack open up for no reason? Sounds like it will need to be filled rather than closed, due to the problem Hesh mentioned of being likely to cause another crack somewhere else.

That's exactly the rub that Dennis is speaking of and that is when to fill and when to force closed.

Lately we've had our own methods for fixing cracks evolving a bit. These days we rehumidify for up to three days as needed and then let the thing stand in 45 - 50 humidity for a couple of days before deciding. If the crack closes and stays closed we glue and cleat if not we fill. Filling will likely show now matter how you do it especially in sap wood.

Some other things to keep in mind are that RH is not a static thing. Our hygrometers may read a steady reading but if someone breaths or walks across the room if we had hygrometers that sampled at a very fast rate you would see what I mean. It's possible as mentioned that where the hygrometer is is fine but it's drafty somewhere else in the shop where the instrument may have been.

There is also the possibility that your hygrometer is not reading accurately. There is not a digital on the market until you get into the high priced industrial systems that is reliably accurate. Every year I check out the several digitals I use at my home against a lab standard wet bulb test and I see drift, commonly, of 9% or better on the digitals. They rely on an electro-chemical reaction and the chemicals break down in only several years making the digitals inherently inaccurate.

At this point it's a tough call but my suggestion would be to do the following:

1) Be absolutely sure that your hygrometer is accurate and where you had the guitar is the same RH as what the hygrometer is reading if it's in a suitable range. Makes no sense if this guitar is a tell tale of an RH problem in your shop to fix the symptom and ignore the root problem.

2) Bag the thing with a car wash type sponge acting as a wick and water (8 ounces or so) in the bottom of the bag and hang it for up to three days. Inside the bag will be in the neighborhood of 75% RH. You can briefly look at the crack's progress in closing each of the three days if you wish. After it closes completely let it sit in your RH controlled environment for two more days and observe periodically and see if the crack is opening back up and to what degree.

3) If when all is said and done the crack wants to remain open and open a goodly amount I would fill it. If the crack does not reopen or only reopens a very little bit I would rehumidify in the bag for perhaps only one day until it's closed, glue and cleat.

There are lots of ways to approach this, obviously and I'm only describing what I might do. I'm always keen to understand what went wrong and not just put things back together to be sure that I'm really addressing the root issue(s) and not just the symptoms.

There are other possibilities here as well and wood is by no means a homogenous material and that's the challenge sometimes and maybe what's fun about working with it too. It can make you think.

One last thing. When the crack is closed and glued use a feeler gauge under the braces in the proximity of the crack to be sure that they are glued down well. If not push, inject, blow, some glue under the braces in that location and clamp.

Thanks for the input.
I have the sister set of coco to the one that cracked. I checked it and all is ok with it in its unconstrained state. Am just guessing here but when putting the back panels under stress and then gluing to conform to the 15' dome, the weakness in sap line became apparent. Some shrinkage may have occurred but my gut tells me there was some compression stress inherent in the wood and putting it under further stress made it reveal.
I'll try the rehydration method and see if that does any good. If not I'll fill and try to hide it as best I can.
Once again....thanks.
Tom

It might be worth checking the RH at the level in your shop where the guitar box was hung. I found out the hard way that the spaces between the floor joists that are the ceiling in my basement shop are a lot drier than the air in the rest of the room. I think they collected hot air from the furnace.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

I wonder, if RH was not an overwhelming issue, if the build itself could have been a problem? I can imagine lots of scenarios where the box is forced to shape inside a mold and held static there until everything's glued up, then when released lots of different localized stresses and strains show up. Maybe a long shot but worth considering...

I agree w/ James's concerns. Because it's not a completed instrument yet, I would think about long term durability. The combination of tight back radius and Spalted sapwood may be repairable now but not bode well for the future when much more time/ effort are invested. Spalting and sapwood are certainly striking visually but............
A strip of it that is entirely under the reinforcing strip might be ok.
If the back is struggling to survive your shops controlled humidity, the outside world could be a challenge. Might be better to bite the bullet now.

James and David....your thoughts appreciated and somewhat mirror my own. I'll try Heshs rehydration tip and if that doesn't do the trick it must be from some other cause of stress, whether inherent in the wood or caused by me when building. That said, I remember the bending process because cocobolo is my favorite wood to bend. There was virtually no spring-back in the sides so I'm thinking they must have conformed to the mold so it's hard to understand why an abnormal building stress.

It's not abnormal stress but wood really doesn't want to be bent around a basket ball. I'm sure I'm not the only nut case who has thought about jointing and gluing backs in a 1/2 hollow form.

I've never been a rehydrating fan because people don't keep their instruments in a controlled environment. My thinking is that if it happened once it will happen again. So I would fill it with a thin sliver of wood. Instead of rehydrating it I would dry it out as much as possible to make it open up before I repaired it. Now if you want it to look like it wasn't repaired, then make a slice on the other side to mirror the crack you have. Then fill both sides. It will look like like nature did it. Heck, where's Padma when you need him. You could even use Bondo. I thought you were doing some Bondo repairs, Hesh? It's sapwood. You expect irregularities. I'm guessing there was a hairline crack to begin with. You can see on the opposite side where there might be a weak area. Just kidding about the Bondo. Sliver and hot hide glue.

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

Who sells "tone Bondo?"

Rehydration is so very useful that at this very moment I have two instruments in the bags that would have to be scrapped if we could not rehydrate them. They are both unplayable, strings on the frets, cracks, necks in back bow, etc. Without rehydration both clients would be SOL.

Now you are right, Ken, it's certainly not for every situation and I was always on the fence with this situation if RH is a contributor or if it's something else inherent in this specific wood. The cool thing about rehydrating and then letting it reacclimate to normal (appropriate) RH for a wooden instrument is the OP will be able to watch how the rehydration closes the crack, or not, and what a couple of days at normal RH gets him too. If that does not close things all the way or most of the way I agree that filling is the next approach. I did bring up the notion of closing some cracks can make new cracks when forcing closed is not appropriate.

It most certainly could be a flaw in the wood and that's what we hope to find out one way or another.

Regarding tension in the structure here's an interesting thought to explore - quantum physics. Does wood seek it's lowest energy state in a structure? Have fun.

The only time I might concede that rehydration could possibly be a useful procedure would be in the case of an instrument being suddenly exposed to a heat or humidity change such as being left in a car or next to a stove. Even then I don't know if it would be a long term solution. I find it hard to believe that rehydration is a long term solution for a back bow problem or frets on strings. (Are those different problems?) I can see that it could be an inexpensive short term solution in a situation where the instrument might not warrant a more expensive solution. Back bow, frets on strings and cracks can all be solved more permanently without rehydration.

As for this repair I might ask if the guitar is for personal use or for sale. If it's for sale, I would put another back on it.

Has anyone done any tests on the relative shrinkage rates for sapwood versus heartwood? I suspect they could be different for some species.

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

Oops, I reread the original post and see that it's a gift. So I might repair it depending on how close the friend or relative is. If it's a mother-in-law I would rehydrate.

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

Ken manipulating RH is a staple of the repair world and done every single day. It's more prevalent in colder climates where forced air heating systems and a lack of humidification can dry out wooden instruments.

If an instrument is rehydrated before too much time passes it often can completely recover and the fix is long term provided that it's not allowed to dry out again.

This morning I will take a nice acoustic bass out of a bag that it's been in the bag for two days and it will have it's top dome restored to the original shape. The back bowed neck and sharp fret ends will to a lesser degree be corrected too since the neck was in our normal 50ish% RH shop this time of year.

Rehumidifying wooden instruments will often, not always, save the day.

The technique is also used for a number of other repairs such as replacing curled pick guards on Martins and others. The guard curls over time and so too does the wood under it. By applying water to the top side of the guard patch and heating the underside the curl is eliminated and then the patch can be finished if finish was not present and a new guard installed.

Manipulating humidity can work against us or for us and it basically works the very same way in either respect in terms of what happens to the wood making it expand or contract.

Necks when they get dry can back bow likely because of the differential expansion/contraction rates of the dissimilar materials of the fret board and neck. Exposing a back bowed neck that is in back bow because of being dry to high humidity for a controlled period of time can straighten the neck by expanding the neck wood at a faster rate than the fret board wood.

A double action rod helps too... Sharp fret ends that have not gone all the back in can be filed flush and reshaped for comfort.

Rehumidifying instruments is very much a long term AND economical repair that is done countless times every day somewhere and very useful too.

Where rehumidification does not work well is an instrument that has been permitted to dry out and left that way for months or years. That's when the dimensional instability inherent in these structures can take a permanent set and all bets are off.

It's one of those 14 clubs in the bag so-to-speak and when deployed appropriately can provide clients with great value at very little cost and even save an instrument from the burn pile.

It's a long response but I still don't buy it, especially in this situation. If someone takes a guitar into a shop and the shop does a repair that doesn't last more than a couple of years do you think they will bring it back and say it wasn't a good repair? No they will try someplace else. Or more likely the guitar will have moved on to a new owner. Have you tracked all of the instruments you have rehydrated for more than two years? Chances are that what caused the problem will cause the problem again. Why not just fix it? What is happening to all of the guitars on the East Coast that aren't getting rehydrated. Do they all eventually end up in David's shop? It's a shame that rehydration is a staple of the repair world. From the OP's comments it doesn't sound like this is a case of prolonged high humidity.

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

Ken perhaps check out Frets.net where repair guys, professional repair guys gather and inquire as to who uses rehumidification to deal with dried out guitars - it's very common and again in climates where guitars can temporarily dry out and it works more often than not.

As for tracking rehumidified guitars not every one but dozens and dozens that we have been keeping going for 8 years now or so. Cracks closed and cleated after being rehumidified stay closed and new cracks don't appear.... provided that the instrument is not permitted to dry out again.

Now regarding the OP's original question I again... will say that I was on the fence as to if this one is better filled or rehumidified closed and cleated. That's why I asked questions regarding RH, how well seasoned the wood was, etc. It could be that filling was more appropriate for this one, I can't know because I don't know all the particulars.

I would ask you though Ken if you do repair work and if so how much? Telling an entire industry that a very common and benign practice flat out does not work is a real reach and our bank account and hundreds of happy clients would not agree with you. Rehumidification if applied correctly to the correct situations works great and works great every single day somewhere I am sure. Last winter we rehumidified a couple hundred guitars AND again, Ken, my climate is different from yours. Our RH can plunge below 20% in our cold winters and that cracks and caves in guitars.

Dave Collins and I are business partners in Ann Arbor Guitars. We are successful and did 1,100 repairs last year and lately have been turning away more instruments than we accept to work on. This is not a game or science class/project for us and the methods that we employ are what we believe to be best practices knowing as we do that we have the tremendous responsibility of the valuable personal property of others at stake here.

To be crystal clear since this discussion is surprisingly disappointing at least to me... I'm not advocating bagging and tagging every guitar but I am conveying our considerable experience with the technique that when applied to the correct situations, a recently dried out, caved in, sharp fret ends, strings on frets.... guitar bagging them can often bring them back as good as new. It works.

RH is indeed a viable concern. When gluing I do like to lower the shop RH to the 40% to 35% level. That allows that the body can be stressed a little in lower RH conditions unless I am building a guitar I know is going to the coast or gulf then I keep the 50% level. I also use wood that I have personally stored for 2 yr or know the suppler .
Learning about RH and what it can and can't do is as important as learning to make a joint. It is also the most over looked part of building until something pops. We all did it and all learned from 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

Ken your position is so unusual I feel I must be misunderstanding you. are you opposed to bringing an instrument back to the moisture level it had during assembly? Or just over humidification?

You say, "Chances are that what caused the problem will cause the problem again. Why not just fix it? " If the problem is someone let an instrument get too wet or dry, what would be the fix?

Woods behavior in a guitar structure is more complex than simply moving in a linier way with moisture. The presence of a crack does not necessarily mean there isn't enough wood.
I'm sure you have hydrated out a dent at some point. When the moisture returns to normal you have effected a more or less permanent change in the space the wood takes up at a given moisture level.

The state of wood in an instrument reflects not just the current conditions but also the history of extremes it has experienced.