Back radius gone in one direction

[Matt Atkinson]

Took spool clamps off just now after gluing the top on last night. Thinking about setting up for binding channels and I see that the back radius is gone flat across the bouts. It’s still prominent end to and but the braces appear to have flattened out somehow. It was braced in a dish under a go bar deck with radiuses braces that fit. I’m getting ready to pull the back off and redo it. Thoughts?

[Simon Magennis]

My immediate feeling is that you have fallen foul of a humidity drop.

I am guessing that NH is cold right now and into the winter heating season. If so, humidity may have dropped quite a bit since the back braces were prepared - assuming you did them in advance. I reckon new back braces and careful check on humidity are order of the day. Lets see what others say.

[Bryan Bear]

I was going to say that I suspected humidity changes as well. Tell us about the timing and conditions of the bracing gluing and then gluing on the back. If the RH was high when the braces were glued on the back plate width would be x. If you let it set for a while before gluing it to the rim and the RH doped the back plate width would decrease to less than x and tend to pull the braces flatter. The length of the back plate wouldn't really change due to humidity changes so the dome on the rim would tend to keep the longitudinal arch relatively unchanged.

I'm going to guess that you braced the back in the dish then carved the braces. Some time passed before you glued the back on. In that time the weather changed and you started heating the house which lowered the RH.

[Matt Atkinson]

Yes. It got unseasonably cold and all hell broke loose. I just got the back off and it has reached its full potential in the wrong direction. Quite spectacularly. My hygrometer has been reading 40-60 with a lot of swings. I did use a dehumidifier until a couple weeks ago. Then it got very cold very fast. A couple days ago I put a bucket of water on top of my little electric radiator (the insulated part of my shop is very small 100 sqft) but to no avail. I did glue the braces in the early fall when things were nice though I always store it braced in the dish. Alas.
Well. Clean this up and try again. Suggestions for adding controlled humidity in my small space?

[Barry Daniels]

There are numerous discussions on the forum about humidity control. You might want to do some reading there and then come back to this discussion if you have any specific questions.

[John Clifford]

I live in Minnesota and build guitars in the basement of my house. During the summer, a dehumidifier does the trick. During the winter - you're not going to like this - I just don't heat the basement, and that keeps the RH pretty consistent. Yes, it gets cold down there so I have to wear multiple layers, but I find that easier to deal with than heating and then trying to humidify my basement, which would lead to all kinds of condensation problems.

[Matt Atkinson]

I seem to have got the humidity under control. I’m running a small humidifier now. Thinking about buying a plug and play humidistat. In any case, this back that started out flat has a obvious desire to be curved in one direction not the one I wanted. It was all nice and flat all summer, stickered and happy at 55% rh. I could install new back strips on the opposite face but I don’t want to. Would I be crazy to try it again as before? Will it eventually collapse? I have enough material to make a new back I think but I also don’t want to do that. Thoughts?

[Bob Gramann]

Usually, you plane the braces off (carefully) and rebrace it at 40% humidity.

[Barry Daniels]

What Bob said. It is the braces that are warping the back. The back is flexible enough to bend either ways.

[Matt Atkinson]

Thanks, all. I'm going to work on stabilizing the humidity and let it all rest for a good bit.

I found this thing

Might spring for it. I also need to empty out the shop, line the walls with poly and put sheathing over it. It's just studs and insulation now. Rigid foam on the ceiling. First things first.

I remember years ago someone, maybe Mario Proulx, saying he laminated his braces with carbon fiber. Is this a good thing? Seems like a lot of work.

[Clay Schaeffer]

One reason to arch the back and braces at 40% RH is so they can "flatten out" at the low indoor humidity levels encountered in the winter. Having the back flatten out is not a bad thing per se - it is only if it creates other problems that it matters. If you brace the back under lower humidity conditions then the humid summer conditions can cause the wood to swell and distort the guitar as well.

[Peter Wilcox]

So I guess the takeaway is to avoid using wood for guitars.

[Bryan Bear]

If your shop is around 55% in the summer and dries out in the winter months I think you would be better off working out a way to dehumidify in the summer so you can brace and attache plates in the summer months, rather than humidifying in the winter months.

Ideally you would brace and attach the plates in lower humidity some say 45%, many like to go lower (35% to 43% range. Doing these steps at lower RH means the plates can swell when humidity rises but won't shrink too much in the low humidity conditions it is likely to see. When the RH increases and a plate swells it will tend to make the dome larger (as long as we are not talking about drastically high humidity/swelling). Wen the RH decreases and the plate shrinks it will reduce the dome. If it shrinks too much because it was braced when the RH was a lot higher than normal, it can cause the dome to reverse as you saw in the free plate, or if it is attached, it can cause the wood to split apart because the rim prevents it from getting as narrow as it wants).

[David Boehnker]

I think it might have been David Russell Young back in the day who suggested using a hair dryer to heat the inside of the back prior to gluing the braces on to remove some of the water from the inside and
"pre-stress" it against a humidity rise. Back in the 80s I had a humidity cave-in with a top and called Don Teeter about it - he told me it happened to him all the time in his wood heated shop, and that if not too severe the tops usually pulled up when strung to tension. It actually worked that once, and I have much better humidity control now.