The Acoustic Guitar Forum

Go Back   The Acoustic Guitar Forum > General Acoustic Guitar and Amplification Discussion > Build and Repair

Reply
 
Thread Tools
  #16  
Old 09-03-2019, 08:54 AM
charles Tauber charles Tauber is offline
Registered User
 
Join Date: Feb 2011
Posts: 6,487
Default

Quote:
Originally Posted by Jared Purdy View Post
This should not be happening on a $4000 guitar!
Guitars are made of wood, a material that naturally expands and contracts in response to changes in its environment. Nitrocellulose lacquer is a material that is brittle and gradually shrinks. Neither are "stable" materials.

You don't mention a hygrometer. Do you have one and use it? A hygrometer lets you know what humidity you have, rather than guessing.

If wood contracts sufficiently in response to a dry environment, it cracks to relieve stresses. When it cracks, it usually cracks the finish attached to it, if it is a surface/film finish. Sometimes, however, the finish, itself, will crack as it shrinks: other times, due to rapid temperature swings, the finish will crack. Sometimes, finishes will crack due to stresses in that part of the instrument, such as near the bridge or adjacent to the fingerboard. Many instrument makers have discontinued using nitrocellulose lacquer in favour of "tougher" finishes that are more stable and less likely to crack (craze).

The cracks that you see in your finish could be due to any of those: the top, itself, cracking; rapid changes in temperature; the finish naturally shrinking or in an area of high stress. None of those have much to do with the selling price of the instrument. Some of them are "care" issues, others are the type of finish used.
Reply With Quote
  #17  
Old 09-03-2019, 03:12 PM
Jared Purdy Jared Purdy is offline
Registered User
 
Join Date: Mar 2012
Posts: 80
Default

"You don't mention a hygrometer. Do you have one and use it? A hygrometer lets you know what humidity you have, rather than guessing. ". It's not a guess, but thanks for reminding me that guitars are made of wood. That's a good one!
Reply With Quote
  #18  
Old 09-03-2019, 04:06 PM
charles Tauber charles Tauber is offline
Registered User
 
Join Date: Feb 2011
Posts: 6,487
Default

Quote:
Originally Posted by Jared Purdy View Post
...thanks for reminding me that guitars are made of wood. That's a good one!
It seemed appropriate since you stated a belief that expensive lacquered wooden instruments somehow, by virtue of their price, ought to be immune from the natures of wood and lacquer.
Reply With Quote
  #19  
Old 09-03-2019, 05:12 PM
Wade Hampton Wade Hampton is offline
Registered User
 
Join Date: Sep 2008
Location: Chugiak, Alaska
Posts: 25,415
Default

Jared, once I moved to Alaska I learned that, despite my best efforts and taking every precaution, lacquer cracks sometimes occur. You and I both live in northern climates and that sometimes means that the lacquer won't stay pristine. As Charles noted, this can occur regardless of the initial purchase price of the instrument.

It's mostly because of the inherent properties of nitrocellulose lacquer, which is not a particularly robust or chemically stable finish. Add to that the naturally occurring expansion and contraction of the thin, solid wood plates used for the top, back and sides of a professional quality acoustic guitar, and it means that these sorts of cracks can and often will occur.

Nitrocellulose continues to "off-gas" as the finish ages, as plasticizer chemical molecules migrate out. This is particularly noticeable when a nitro-finished guitar is new, as when you leave it in its case for a while, then open the lid and get that "new guitar smell." What you're smelling when you can smell the finish is those plasticizer chemical and other chemical compounds that have off-gassed.

While this process slows down as the years pass, it never really stops, and as those molecules leave, the finish quite literally shrinks. Which often causes finish cracking and checking, as in this "windowpane checking" in this photo:



˙˙˙

Or it can cause precisely the sort of finish crack you've described.

I should mention that windowpane cracking often occurs when an instrument is shocked by a sudden temperature change, as well, and those can be avoided if care is taken. But nitrocellulose finishes are still volatile, and its shrinking still occurs, regardless.

While I understand your indignation that this should happen to a well-maintained $4000 guitar, the truth of the matter is that more expensive guitars tend to be MORE sensitive to slight environmental changes, not less. It's ironic but true that an all-laminated wood guitar with a polyester finish that you buy in a box for $250 at Costco will be considerably more resistant to finish cracking than an instrument costing ten or twenty times more.

That's just part of the deal with a nitrocellulose finish, unfortunately.

Hope that makes more sense.


Wade Hampton Miller
Reply With Quote
  #20  
Old 09-03-2019, 07:43 PM
John Arnold John Arnold is offline
Registered User
 
Join Date: Jan 2009
Posts: 2,813
Default

Quote:
my understanding is that finish cracks come about because of temperature changes, not humidity.
Actually, high humidity can cause lacquer checks, particularly on the thicker lacquer that Gibson used. High humidity swells the wood much more than the lacquer. Low humidity can cause cracks in the wood due to shrinkage, but that will compress the lacquer.....not a scenario for lacquer cracking.
Most lacquer checking occurs because lacquer has more thermal expansion than wood. Usually the culprit is either extremely low temperature or a rapid temperature change.
Reply With Quote
  #21  
Old 09-04-2019, 05:20 PM
Jared Purdy Jared Purdy is offline
Registered User
 
Join Date: Mar 2012
Posts: 80
Default

Quote:
Originally Posted by John Arnold View Post
Actually, high humidity can cause lacquer checks, particularly on the thicker lacquer that Gibson used. High humidity swells the wood much more than the lacquer. Low humidity can cause cracks in the wood due to shrinkage, but that will compress the lacquer.....not a scenario for lacquer cracking.
Most lacquer checking occurs because lacquer has more thermal expansion than wood. Usually the culprit is either extremely low temperature or a rapid temperature change.
In this case I think it may be a result of having it exposed to the sun. The guitar is properly humidified, and the temperature differences between the inside of the house and the outside are significant, but not extreme as I don't use AC. However, playing in the sun in 30+Celsius is likely the culprit. It was a hot summer, and I played on my back porch where it routinely gets to way over 30+ C in mid day, and in direct sun light. Won't do that again. I had the guitar inspected, and the bridge and bracing are all as they should be, and there are no cracks in the sound board, so all is well.
Reply With Quote
Reply

  The Acoustic Guitar Forum > General Acoustic Guitar and Amplification Discussion > Build and Repair

Thread Tools



All times are GMT -6. The time now is 12:41 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2000 - 2018, The Acoustic Guitar Forum
vB Ad Management by =RedTyger=