A hairline crack? (pics)

[Arash]

There's a hairline looking crack on my sound top. I am not sure whether it is a crack or not, I hope you could tell me that. I'm currently traveling and can't find a good guitar maker to have a look at it.

It starts from the lower bout below the bridge and continues as a straight line up to the rosette, between the 4th and 3rd strings, one wouldn't notice it by touch. It is in parallel with the joint line, more towards the 3rd string. It's a very thin line, almost invisible.

Here are some pics: https://files.fm/u/ws5egafe
https://files.fm/u/j6eusa4g

https://files.fm/u/zt2yfs5t

Zooming in would be necessary to see it. I should be able to take more photos.

Please share your thoughts.

[Jim Figurski]

It's hard to tell.  It looks like two different grains of wood coming together. Is there any kind of dip when you run your fingers over it?

[Arash]

Right, it's almost invisible and no dip can be felt when running finger over it. The latter pics are a bit clearer. It seems that there's an inward dip when I look closely.

[Greg Mirken]

A sure way to tell if it’s really a crack is to wet it with a drop or two of a mild solvent like paint thinner or naphtha. Then look for wetness inside with a mirror. The solvent will just evaporate and do no harm.

[Arash]

Thanks Greg, this guitar has an ebony center brace that makes it very hard to examine it like that. Is there any other possible method to make sure if it's a crack?

[Andrew]

You are currently travelling and you have a classical guitar, which could very well have been built at a much higher RH than it is experiencing now, so my first thought is that you need to humidify it. Check next to the fingerboard as well. Its hard to tell in the photo, but it may be that only the finish has broken at this point and the wood hasnt failed yet, but dont be performing tests on the go with no training, you may contaminate the crack. Just keep it clean/dont get anything in it, and humidify. If it seems to get tighter (or not) with a good case humidifier you should have some info to go forward with.

[Arash]

Thanks Andrew, I've checked next to the fingerboard and nothing is wrong there. My guitar has a sound monitor hole from where I could have a look at the back of the top and it seems OK. Though very hard to tell as the "crack" is a micro one.

[Mark McLean]

Yes - it is good advice to humidify the guitar.  And don't keep running your fingers over the "crack" as it will introduce oils and dirt into the crack which will cause some discoloration

[Arash]

Thanks Mark, any guesses on whether it's a crack?

[Dean Sibley]

Hi Arash,

have you considered putting a light inside the guitar at night ( use LED for low heat)? As mentioned I would be hesitant to apply anything directly to the surface.

Visit your local cigar shop & ask about RH beads - in a sealed environment they will maintain 70% humidity. They may help a little.

 When a curved or arched soundboard shrinks it merely reduces the curvature a little but a flat soundboard can only pull itself apart if humidity drops dramatically - a constant worry in climates like Australia.

Modern solvent based lacquers have a different expansion co-efficient & are less flexible than most woods & tend to crack even if the wood doesnt. Shellac & latest water based finishes tend to set with a little more flexibility.

 Design & construction are important in difficult climates.

cheers,

Kiwi

[Arash]

Hi Dean,

Thanks for your reply. It hasn't seen a dramatic climate change. The soundboard is curved. I have tried your solution and it doesn't look visible from the behind. It's hardly even visible from the outside. A local luthier said that it's a "mirror crack", not sure what it means. Anyway, he suggested that I should wait about 5 months to see how it turns out. Do you think it's crack?

[Dean Sibley]

Hi Arash,

 to be honest it's difficult to tell from pix. If a luthier has seen it then the mirror crack most likely means the glue join where the 2 pcs of spruce are joined  has separated - in this case a very small amount. Try tapping - gently, with fingertip -  the soundboard in various places. You will get a dull sound near outside edges with a better "tone" in the middle. Violin & archtop makers use tapping to tune a soundboard while it is being carved. If sound is dull at crack that would confirm your suspicion. Best leave it for a luthier to repair. Also best repaired when humidity in environment is driest - this will help open the crack for glue. I woudn't lose too much sleep over it. Wait untill you finish travelling.

good luck,

Kiwi

[Arash]

Thanks, I just tried tapping and didn't hear a dull sound! I guess that's a good sign!