Richard Jernigan -> RE: Several things I am curious about the guitar. (Jun. 29 2018 3:31:44)
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Speaking of science, I googled around for maybe half an hour the other day, and didn't come up with anything immediately applicable to reasonable guitar environments. There were some formulas that could be worked out, but I didn't feel like it would be worthwhile. I did come across a comparison of untreated wood, shellac coated wood and polyurethane coated wood, in conditions that would be extreme for a guitar. The charts show what happens if you start off with dry wood, untreated or coated, and expose it to 90% relative humidity for more than a month. The top chart compares uncoated wood with shellacked wood. The x-axis is the number of days of exposure, the vertical axis is the percentage of water in the wood. That is, if you started off with a pound of dry wood, after x days the weight of the water in the wood is y% of one pound. The different curves are for uncoated wood, the top curve, and for 50 microns, 54 microns and 57 microns of shellac coating. This is roughly one, two or three coats. A pound uncoated wood would end up weighing nearly 1.09 pounds at the end of 36 days. The second chart is similar, but for polyurethane. [image] Wood water absorption by rnjernigan, on Flickr[/image] The data is from https://scielo.conicyt.cl/scielo.php?script=sci_arttext&pid=S0718-221X2016000300005 which contains some other stuff as well. Both coatings inhibit water uptake. Polyurethane is better than shellac, especially in the first couple of days, but it's not a very big difference in the long run. The 50-year old polyurethane on the Ramirez is both thinner and harder than it was when new. I don't know whether or how this would affect the results. I suspect that shellac changes significantly with time as well. But as I said, this is an extreme environment, soaking at 90% relative humidity for more than a month. The expansion across the grain of a spruce top would be several millimeters. The wood soaks up a lot of water, but it takes time. Over a day or two, with only 20% change in air relative humidity, the water uptake or loss would be a whole lot less. From the rough numbers I ran, I couldn't arrive at any conclusion one way or the other, whether a coating of shellac or polyurethane would significantly inhibit cracking under normal humidity swings. Julian Bream eventually had Hauser II replace the back of the famous Romanillos #501, which kept cracking. The guitar was French polished on the outside, but untreated on the inside. He didn't ask Romanillos to do it because Hauser had a better stock of seasoned Brazilian rosewood. If I remember correctly, the original back came for a dining table Romanillos came across at an estate auction. It was hard to get guitar wood in England in the 1970s. Bream never had any more trouble with the back, but he eventually had Romanillos replace the fan struts on the top, when he felt the guitar was losing some power and tone color after more than 15 years of world travel and hard playing. Bream traveled a lot. When I picked him up from the airport in Austin for a concert, he had just an ordinary case, not a humidity controlled one. He complained about the swings of humidity, from centrally heated rooms in winter in Sweden to the humid day we met in Austin. I saw the guitar up close that day. It looked like it had been rode hard and put up wet. My '73 Romanillos, the one just before Bream's, is Indian rosewood. Since 2000 when I bought it, it has lived in a Mark Leaf humidity controlled case, first in an apartment in the Marshall Islands, where the relative humidity often was in the high 50s%, then in Austin where it gets down around 35% in my house in the winter time. It has never cracked. I don't even see a change in the action between winter and summer, but the Mark Leaf case has the best humidity seal of any I have ever owned, judging from how long an Oasis soundhole humidifier lasts in it, compared to Karura and Visesnut cases, which have gaskets that are supposed to seal them. RNJ
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