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I'm curious about one aspect of Hutch's question, which is where 43 came from? Or at least - where did 40-45 come from, which is what I often see. I assume it's just a happy medium that puts the instrument in a position to handle most moderate RH swings?

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I think that's all it is. The idea is that, if you build at that level of RH, the guitar is best equipped to handle both extremes.

I'm not sure the brace angle, or even the presence of braces, makes all that much difference in the rate of cracking in low humidity. The braces do, of course, prevent the top from being able to pull the edge of the rim inward, but I'm not sure it could do that to any useful degree anyway, braces or not. Old violins and mandolins often end up with reduced overhang (on violins) or protruding edges (mandos) due to 'shrinkage hysteresis' of the top and back plates across the grain.

What really helps reduce top cracking IMO is doming the top. This gives it someplace to go as it shrinks before the stress really starts to rise. I have seen guitars in very dry conditions that started out domed, and became flat, or even dished, on both the top and back surfaces. Then they crack. One of those was a brand new guitar I had put in an exhibition during the winter. I had a very pointed conversation with the curator about humidity when I dropped it off. When I went back for the opening of the show two weeks later the 15' radius of the IRW back had gone flat,and the top looked like corduroy. The R.H. was about 15%, and she admitted that it had been very uncomfortable. I gave her the money to buy a humidifier on the spot, but it was too late: the top cracked three days later when nobody was there to fill the humidifier during a long holiday weekend. When i went in to pick it up I pointed out the distress in other wooden objects in the collection: one fancy veneered cabinet almost had to be re-done. The replacement top sounded even better than the original, but.....

"I'm not sure the brace angle, or even the presence of braces, makes all that much difference in the rate of cracking in low humidity."

I'm pretty sure it does. Unbraced book matches sitting in an attic undergo seasonal humidity swings without cracking. Even baking them in an oven for a couple of hours doesn't seem to cause cracks. I think as the bracing becomes less cross grained and more parallel with the top's grain there is less tendency for the top to crack from low humidity. I'm sure doming also helps as long as the braces are not so strong as to keep the top from bending them.

We try to brace at 35-40 percent RH, with target RH at 37% (during summer, that takes A/C on full and two dehumidifiers running at max; winter is just cutting back to one humidifier).

I doubt we'd see quite so much in the way of cracks if the plates were not glued to the rim. Just that change would take 4" or so of top out of play.

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"I'm curious about one aspect of Hutch's question, which is where 43 came from? Or at least - where did 40-45 come from, which is what I often see. I assume it's just a happy medium that puts the instrument in a position to handle most moderate RH swings?"

Several years ago you would see people recommending building at 50% R.H. Eventually that recommendation was reduced to 45%. With more people air conditioning their houses the indoor humidity may not swing "up" as much, and being better insulated, indoor winter R.H. levels may go lower. Woodie may be ahead of the curve.

Looks like I'll be buying a better, bigger dehumidifier. The one I bought just wont keep up in the basement and my wife doesn't want it in our spare room so I'm going shopping. Once I get all my linings in and I'm ready to add bracing, I'll move my assemblies into the spare room to dry out before bracing and closing the box.

It's seems to be constantly raining or 70 to 90% percent humidity here. I'm curious to see what my Taylor sounds like after spending some time in the new dry room. I know it sounded better when I bought it.

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Hutch

Get the heck off the couch and go build a guitar!!!!

Clay S. wrote:
"Unbraced book matches sitting in an attic undergo seasonal humidity swings without cracking."

I was talking about assembled guitars. Of course a plate with braces going across it at any angle will be more prone to cracking than an unbraced one: the braces essentially don't change in length with changes in humidity, but the width of the plate does change. Similarly, an unbraced plate glued to a rim is more likely to crack than one that is not glued down, since the length of the rim also doesn't change to speak of, which more or less fixes the perimeter. Braced plates that undergo any sort of humidity swing move in all sorts of unwelcome ways, and can be very hard to get back to the shape you wanted. That's why I don't generally allow students to work on their guitars at home until the box has been closed up.

I guess we are arguing semantics, but from your statement:

"I'm not sure the brace angle, or even the presence of braces, makes all that much difference in the rate of cracking in low humidity."

gives the implication that with or without braces the rate of cracking will be about the same.

But let's not argue. For the most part I agree with what you are saying, except that I think (all other things being equal) brace angle has some effect on the rate of cracking from low humidity.