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Advice needed - lost the radius on back due to humidity drop
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=29449		 Page 1 of 1
Author:  Dave Olds [ Mon Oct 11, 2010 10:28 pm ]
Post subject:  Advice needed - lost the radius on back due to humidity drop
Hi all,

I braced the back of my current build a month or so ago ( during the winter down here in NZ ) before I noticed that the humidity was pretty high - in the early 70s. duh . Unfortunately I do not have humidity control.

I decided to hold off bracing the top and gluing the back to the sides until the humidity came down to a more manageable level which it has now done - it is currently 53% odd. That is still a little higher than most Nth American builders would find acceptable, but here in Auckland the humidity never drops below 30-35 % and during summer can be in the 80s and 90s for long periods - so I have found building in the 50s is OK, as long as the guitar is staying here.

I have kept an eye on the back over the last month, and while the humidity remained high, it kept its 15ft radius nicely. However, in the last week or so with the sudden drop in humidity, the radius has pretty much disappeared. If I lie the back flat on a workboard it is close to dead flat - in fact it almost looks to be slightly concave. ( It's pretty amazing how the curve in the braces can be pulled straight like that )

My question - should I carry on with this back as it is, or should I attempt to remove the braces and redo them. I am unsure what impact the flat back would have - or whether there would be a danger of it coming apart if the humidity drops too much further.

Any advice appreciated.

Dave
Author:  David LaPlante [ Mon Oct 11, 2010 11:09 pm ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
Put it in a plastic bag with a wet sponge in an open dish or container to keep the back from actually getting wet. It should go back into it's original shape once rehumidified.
Get it on the rims once it's as it was. Hopefully it wll be ok as long as the guitar is not severely dried out in the future.
Author:  Dave Olds [ Mon Oct 11, 2010 11:21 pm ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
Hey David - thanks for the reply.

I had thought about re-humidifying the back, but thought it would just do the same thing again - but on the guitar this time. If I re-humidify, then straight away attach it to the sides, won't it just dry out again ? Being attached to the sides I thought that would increase the chances of it actually splitting.

( I am not questioning your advice here - I just want to understand how this works )

Dave
Author:  Jeff Highland [ Mon Oct 11, 2010 11:44 pm ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
I would pull it off and rebrace it.
You are right to worry about it splitting if you rehumidify it and install it.
Author:  Rod True [ Mon Oct 11, 2010 11:48 pm ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
Shave the braces off, make new ones and glue them on at a more acceptable humidity level. Then attach it to the rims. Brace wood is cheap versus the time it takes to fix a cracked back on a finished guitar.
Author:  Tim Mullin [ Tue Oct 12, 2010 12:55 am ]
Post subject:  Advice needed - lost the radius on back due to humidity drop
Like others here, I'd recommend shaving the braces off and rebrace under better conditions -- easy to do and not expensive in terms of material.
As a fellow builder here in NZ, I sympathize with your humidity woes. While the conditions in your shop are an issue, the bigger problem is the conditions where you're acclimatizing your wood. If the humidity there is all over the map, the ambient condition during glue up is secondary. Here in the lower North Island, I store all my wood and guitars in-progress in a large closet with a dehumidifier maintaining RH below 40 percent. Parts might be in my uncontrolled shop area for a few hours, but they go right back to the closet between work sessions.
Author:  Tom West [ Tue Oct 12, 2010 4:59 am ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
Remove the braces and rebrace at the correct humidity for your area.
Tom
Author:  David LaPlante [ Tue Oct 12, 2010 8:41 am ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
I have successfully done what I described with no ill effects, the guitar, almost 30 years old now, was fine.
The difference for you may be your ambient humidity and the level at which you braced the back.
My experience was with bracing a back in our relatively dry fall here in NY and having it dish during the extremely dry winter season. Rehumidifying it brought it back to the average value.
It was also Indian rosewood which is about as stable as one could hope for. In general I always try to brace a top or back plate at the lower value (about 40%) of the humidity ideal.

What is your potential liability with this guitar? If for a customer, the advice given above would be prudent.
Author:  Dave Olds [ Tue Oct 12, 2010 1:25 pm ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
Thank you all for your advice.

I decided to err on the side of caution, so I have removed the braces and will rebrace at current humidity. This is a good reminder to me to pay more attention to the humidity, and plan my work accordingly.

Dave
Author:  alan stassforth [ Tue Oct 12, 2010 8:20 pm ]
Post subject:  Re: Advice needed - lost the radius on back due to humidity drop
The same thing happened to me, but with a top.
RH was really high, in the 99% percent range.
Before I glued the top on, which was months later,
the top had a reverse arc to it.
I didn't care, because it is for me.
It sounds great.
It is a dead flat top, sort of, but not now, ha ha ha.
Anyway, my point is that I now take RH into account more.
Also, I put an arc to the top now.
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