D'oh!!!

[Dekka]

Where did I go wrong?
After building my first acoustic 000 guitar under the watchful eye of an experienced luthier, I have embarked on my second build of my own design at home. I have some very nice Silky Oak (Grevillea robusta) that has been drying for a couple of years and opted to make a back out of book-matched FLATSAWN silky because the figure looked so striking. At the same time I made a top from WRC which has been dished and braced with WRC. The Back was braced with very old California Redwood. Top and Back have been sitting together in the garage for a couple of weeks, however upon my return the top is as it was but the back has now reversed its dish-shape, i.e. it now has a concave to accommodate my ample tummy. Is this because:-
1. The Silky Oak wasn't aged enough?
2. The wood was not quartersawn?
3. It was braced at the wrong humidity?
4. It had a starkly different moisture content than the Redwood?
5. All of the above?

I'm going to discard the back and opt for some better seasoned quartersawn material but I do want to learn from this disappointing mistake. (It really is an impressive looking back....oh, well...sigh.)

[Nick]

Top and Back have been sitting together in the garage for a couple of weeks

I'd say that was your problem right there. Being flatsawn it's prone to cupping at the best of times but unless you have a climatically controlled garage any swings in humidity will make the back do whatever it likes. Flatsawn hardwood will pull the softer Redwood with it. If you can stabilise the environment to a stage where it was the same as when you braced it it may return to it's previously flat state. Some here have had more experience as this sort of thing as humidity swings seem to be more common place in Aussie and so can give better suggestions than I can! I am also extremely lucky & have the luxury of a constant environment in my shop so haven't had to face this problem.
Some timbers show really nice grain patterns but only when they are flatsawn so being quartersawn isn't essential but you have to have a stable or consistant environment to build with flatsawn.

[jeffhigh]

It's basically number 3.
The back is telling you that it was braced at a high humidity and is now at a low humidity.
but it is made worse by the flatsawn timber which will move more across the back width than quartersawn
Don't do anything until you at least get relative humidity measurement and preferably control.

You could plane off the redwood braces and rebrace the back with sturdier spruce braces at a low humidity and it will probably be Ok, whilst tending to move more than QS.

Most builders like to brace at 45%RH, i prefer 40-45

[Alastair]

Hi Dekka,

Quick comment:

Silky Oak, (either Grevillia or the northern Cardwellia) generally gets its spectacular figure from the medullary rays, which only show on the quarter.

If your timber looks like this:

or the centre panel here:

it is likely your back is q/s. Flatsawn is more like the edge of the lower bout...... very little figure.

Post a pic for confirmation, as these are Cardwellia, not Grevillia.


So I would go with 3 and 4 above.

Also would suggest that discarding is not necessary.

If the figure is good, (and hence near or on the quarter), you can carve off the braces, allow to flatten out, equilibrate your mc at a suitable RH, ad then rebrace.

[matthew]

Can you just turn it over and brace the other side? Then if it wants to move a bit, it will do so in the "right" direction.

[jeffhigh]

Can you just turn it over and brace the other side? Then if it wants to move a bit, it will do so in the "right" direction.

It's not that the wood itself wants to cup or warp, It's the interaction between the panel and the transverse braces
On guitar backs and tops we do what is avoided in cabinetmaking and glue long grain braces across the panel.
The different expansion and contraction rates change the panel curvature, and if excessive can cause cracking.

[matthew]

I only suggested that because I've had quite a few flatbacks that want to bend/cup and in one direction only. Doesn't usually happen on well quartersawn wood, but if there's slab or runout it will always tend to go one way not the other. I find it better to work with what the wood wants to do than to fight it.

[Dekka]

Thanks very much for the replies.
I 'm going to plane off the braces and set it aside for a later project.
Matthew, I can't flip it unless I build a Lefty.
Alistair, All my Grevillea robusta is showing rays only if Flatsawn. When cut on the quarter it shows only little dots. So I'm confused by your post??!!
Jeffhigh, I'll rebrace it only when the RH is at or less than 45%. I think it was between 50 & 60% when I did it.
Nick, I'm hearing you about the controlled environment but what happens when the finished instrument leaves the workshop?... Is it OK because everything is sealed?
All the advice is much appreciated. Cheers!

[Kim]

I am not saying this is the case in this instance, but for the uninitiated, woods like oak which display prominent medullary rays can be deceiving when looking at the end grain to determine whether or not it is flatsawn or on the quarter. This is because the medullarys can 'look' like grain, but they actually run 'opposite' to the grain. Therefore if the billet has been resawn with the blade cutting in the same direction as the medullary rays, it will produce quarter sawn wood with the rays being more broadly exposed upon the surface to produce the figure which Alistair has shown in his image.

I know that most would be aware of this but the hope is that the following will help anyone who is not so experienced with wood working to understand what I am suggesting above.

Here we have an image showing the endgrain of some well quartered WA Sheoak. The prominent 'light coloured' markings running left to right, or horizontally across the end of the board in this image are the medullary rays. Medullary rays radiate out from the 'centre' of a round log to its outer edge. If you look more carefully at this image, you will also see much finer markings running in a vertical direction, these markings are the actual grain of the wood.

If we now look at the 'face' of the same board, that is the widest surface, we see large 'darker' blotches. These darker markings are the surfaces of the medullary rays which have been exposed by the saw blade cutting 'across' them, in their same direction, to reveal more of them in the surface of the board to indicate it is 'quarter sawn'.

The same board turned 90 degrees to view the 'flat sawn' side shows none of the large darker blotches. This is because the saw blade had sliced straight 'through' the medullary rays, so whilst they are still darker than the surrounding wood, the rays now appear much narrower and more uniform as they run the length of the board.

Cheers

Kim

[Dekka]

Apologies, Alistair...My bad... It is quartersawn. Looking at Kim's She Oak I have taken the rays to be the growth rings when looking at the end grain.
The photo doesn't show it very well but with the right finish I think it'll look amazing. Thanks again.

[Allen]

Your problem is most certainly bracing up at a different RH than what you then found the back turned inside out. Don't fret, it's more common than you would believe amongst new and even seasoned builders who don't have a means of controlling the RH and get too keen wanting to brace up a plate.

Pop those old braces off. Stabalize the RH and wait for a couple days at the point, then brace again.

When I've got my rims ready to go is when I brace my tops and backs, as I like to be able to close the box up over 2 days at the most. Far too many things can go wrong if only one plate is attached and the box sits for a week or two waiting for the other.

[Mark McLean]

Hi Dekka
There was a good thread on humidity issues a few months ago and you might want to have a look through it here.
viewtopic.php?f=1&t=3749&hilit=+humidity

Towards the bottom of it I relate an identical experience that I had recently, and that was with flatsawn timber (the blackheart sassafras). My problem was worse than yours in as much as I had already closed the box and the plates then went concave. At least yours are still separate plates. I took it apart and rebraced it and now it seems OK.

You can definitely remedy it if you remove the bracing (plane them off) and then rebrace them under conditions of low relative humidity (<50%). Like Allen says, you are looking for trouble if you leave a braced top or back lying around for a long time to go through changes in RH. Best to get them done and glued to the rims in a short space of time, under stable low RH. That is fine if you have humidity control in your space, but a bit hit and miss if you are just waiting for good weather and hoping it will last (might get away with that if you live inland).

After my bad experience I bit the bullet and bought a dehumidifier on EBay and spent a bit of time sealing up the gaps in my shed and now I am able to keep it at pretty constant RH, even with the wet summer we are having on the east coast. Of course, the guitar is going to experience higher humidity later in its life and the timber will swell a bit (even after it is finished). But that will just make the top and back dome a bit more - the worst that does is raise the action a bit. The disasters happen when an instrument built in the wet later dries out (flat plates, cracks, loose braces and frets). Somebody composed a ditty:
"Dry-to-wet, it will live yet. Wet-to-dry, it will die"

I keep discovering that there is a lot more to guitar making than just the cutting and gluing.
cheers
Mark

[Mark McLean]

And Kim - thanks for that great pictorial lesson. I hadn't really cottoned on to that before but now it is crystal clear. Brilliant
mm

[Kim]

Another thing you may find helpful is to hang the top/back once braced until you can close the box. Doing this will at least allow even exposure to changes in humidity for both surfaces. Leaving any thin, wide, bit of wood laying flat on a work surface is asking for problems...or another hygrometer in your workshop

Cheers

Kim

[Dekka]

Thanks for the link, Mark. I've learned a valuable lesson. Had an early morning thunderstorm today and the back has now returned to its original shape. Unfortunately my work area is an open carport but I'll investigate building a dry-box and from now on I'll only glue when the RH is LOW.

[Chalks]

Hi Dekka.

I generally don't comment mainly because five is hardly enough to give credibility to what someone says. So I hope others with more knowledge back me up in this.

I hear what you are saying, 2nd build and all. The thing is what you have experienced is a leason. You really need to control the climate you are building in for more than the few hours to glue up. The plates are so thin and as you now know act as sponges. the days either side of gluing are important to control, especially the week before. Stabilising the moisture content of the timber is as important as the RH the minute you glue up. Stabilising takes time. You just can't wait for a dry day and expect the plates, braces, etcetera to all be at the same place if the days before have been wet then windy, hot then rainy and then hot again.

Even small things will help to control the environment. First thing to do is get a decent hygrometre. At least you will know exactly where you are at before you start and have good facts behind you when things go astray. They always do.

I know this may sound an overkill but it is important to the longevity of the instrument.

Chalks

[Corky Long]

I've found this to be a challenge, especially in my climate (New York) where there are pretty wide swings in temperature and humidity. I won't glue braces on in the fall, when the heat goes on and the humidity drops in the house. Frankly, I'm pretty careful not to glue the braces on until I'm able to close the box pretty soon thereafter. I've gotten into trouble with braced tops and backs even when I hang them to allow equal exposure to humidity. One way to avoid it all is to close the box and glue as soon as the glue is dry on the braces.