Official Luthiers Forum!

I am starting a new series called Luthier Tips du Jour mailbag. These are questions that I get from viewers around the world. I decided to address these in the new series called mailbag. Here is the first one and it comes from a viewer in Spain that had some questions about humidity - the subject of my last Luthier Tips du Jour video.

Nifty little tool.

I like how readable this 'analog' device can be.

Very nice and simple. What is the purpose of the mahogany laminate? Does it matter what thickness the veneer is?

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Cool, cool, cool! I know what Im making this afternoon! Thanks for the video it was very informative.

The laminate gives a differential layer. They will both expand and contract at different moisture levels; the larger open grain wood will absorb or lose moisture much more than the laminate, so it will swell or contract more - fighting against the glue joint, thus causing the lamination to bend. Technically, this is also true for changes in heat (that's how the old style blinkers work in your car turn signal or Christmas tree lights) but heat will change the volume of the two different woods in an inconsequential amount, compared to humidity.

The thicker the veneer, the less it is going to be able to move, so you want it to be as thin as possible.

Some examples of these were in another thread a few months ago. I made one of these about 25 years ago, based on plans I saw in FINE WOODWORKING. Mine uses two strips of the same species--one is a cross cut slice, so it is ALL end grain. The other is long grain. I carried them around in the back seat of my car (parked outside) until the national weather service told me that the relative humidity in my town was 50%. Then I glued them with epoxy. I did build a stand--actually, a wall hanging fixture. My fixture has an arched cross beam. I put everything in a sealed container with a bowl of water and set it in a sunny window until the laminate flexed its farthest in one direction. I marked that on the cross beam. I then put everything in another sealed container with a lot of desiccant until the beam moved its farthest in the other direction. I marked the beam. Then I marked the center point of these two references as 50%, because I knew the laminate would be straight at 50%. I centered the 50% reference point in the fixture and glued the cross beam in. Then I used dividers to strike off and mark equal increments on either side of 50%. My unit is still in service, and still quite sensitive. My increments don't necessarily go from zero to 100, but they give me a real good indication of where things are through a range of excessively dry, through excessively wet, and I always know where 50% is. The photo was shot during humid months. The humidity in my shop this evening is actually about 1 increment to the left of 50%. Not bad. I think I'll close a box tonight.

Great video, thank you.

Brings up a good tip that you should try to glue these up when the RH is near the median of the range in where you live, so that you have an indication of what 'normal' might be for you. I would think you can probably also build mini versions of this that could be kept in each of your guitar cases (less width should allow the end grain to absorb more quickly, and thus bend more noticeably, and thus be more measurable in a smaller package? Neat little bonus to include with each guitar you make.