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RE: Several things I am curious about the guitar.
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Tom Blackshear
Posts: 2304
Joined: Apr. 15 2008
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 RE: Several things I am curious abou... (in reply to Stephen Eden)
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quote:
Perhaps, by Toms explanation, this is done to prevent the guitar from splitting under brief exposure to extreme low humidity. Still in the long term it hasn't helped
I think the obvious reasoning is that we don't leave a valued instrument or any guitar outside by the swimming pool over night, or drop it in the pool to see what happens.
My technique seems to work with building in 45% and following a very light shellac sealing. And to review an old instrument with cracks, first hand appearance, has very little real evidence to secure just what kind of treatment it received over the years.
Years ago, in terms I seem to remember, I read an article that stated shellac was a better sealant than lacquer or varnish.
For example: For about a hundred years, from the 1820s to the 1920s, shellac was the primary finish used (for all coats) by all small shops and factories. In the 1920s shellac was replaced in factories by lacquer for two primary reasons: shellac resin (from bug secretions) is a commodity product that was going up in price as demand increased, while lacquer was going down in price; and lacquer thinner (a blend of solvents) makes lacquer much more versatile in different weather conditions.
Shellac continued to be used by painters and floor finishers working inside buildings and by amateurs until the 1960s. Then three things happened that almost totally ended shellac being thought of as a complete finish:........
Shellac is much more difficult to use (see below) than these three finishes, so it almost disappeared as a finish except in a few niche markets such as French polishing and handmade reproductions of antique furniture............even though this article doesn't address the problem with moisture absorption rates it does address durability.
https://www.woodshopnews.com/columns-blogs/shellac-as-a-sealer-its-all-just-hype
He also goes on to say that Shellac has wonderful blocking properties, better than any other finish. It blocks silicone contamination, which causes fish eye, odors (for example, from smoke or animal urine), and residual wax extremely well.
I think shellac actually has the ability to permeate the wood better, as far as sealing the inner structure of the wood cells, due to its thin quality mixed with alcohol.
_____________________________
Tom Blackshear Guitar maker
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Date Jun. 28 2018 15:06:09
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Richard Jernigan
Posts: 3430
Joined: Jan. 20 2004
From: Austin, Texas USA
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RE: Several things I am curious abou... (in reply to estebanana)
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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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Date Jun. 29 2018 3:31:44
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Richard Jernigan
Posts: 3430
Joined: Jan. 20 2004
From: Austin, Texas USA
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RE: Several things I am curious abou... (in reply to estebanana)
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I’m not taking sides in the debate whether guitars ought to be finished on the inside. The only ones I have owned for several years have been unfinished inside. They have survived well, but they have mostly been kept at home in humidity controlled cases.
As I said, the data and formulas I turned up in a brief search didn’t settle the question for me. If I came across a guitar I would like to buy, it would make nearly no difference to me whether it was finished inside or not. The only one I have that is finished inside is loud, colorful and responsive. I’ve only had it for a year.
Which one is best? None of them. My opinion is that once a guitar rises to a certain level of quality, there is no more “better” or “worse”, there’s only “different.” One player may prefer certain qualities, a different player may prefer others.
The only reason I posted the data was to put some numbers into the discussion. Having spent a long career as engineer, physicist and mathematician I have a hard time thinking about questions like this with only qualitative statements to go on.
If I can’t arrive at a decision on my own, and experts disagree, the question remains open for me. Two of the main things a scientific career has taught me are how little I really know, and how many people “know” certain things that are wrong.
RNJ
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Date Jun. 29 2018 16:01:09
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