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Most Common Cause of Soundpost Cracks in Violin Tops?


GeorgeH
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53 minutes ago, Strad O Various Jr. said:

Dropping the violin has to be right up there!

In addition:  banging around a violin in a cheap case where the bridge is the first thing to hit the inside of the case.  Or forgetting to remove a practice mute when closing the case.

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22 minutes ago, GeorgeH said:

I am wondering also about more subtle causes, and how to avoid them. 

Due to the way the violin is constructed and stressed, the soundpost (and area below it) see forces pushing outward on a convex shape, putting the outside surface in tension and thus prone to cracking.  Weak wood, or anything that causes higher stress (vertical impact on the bridge, too-tight soundpost or bad fit) will worsen the situation.  Thin graduations at and around the post I think would also contribute.

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I recently found a very nice violin c. 1925 that had (probably) been in a case for decades. Soundpost was up with a lovely saddle crack running over it.

:(

I was wondering if this crack was likely started by plate shrinkage.

While this crack is definitely worth patching, I began thinking about the violins I own that simply don't get played often (not never, but not often). All of these have had soundposts installed by an excellent luthier.

I keep the humidity controlled, but I am wondering how often do seemingly spontaneous soundpost cracks occur, and what I can do to avoid them. 

sp_crack.jpg

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12 minutes ago, GeorgeH said:

I was wondering if this crack was likely started by plate shrinkage

Does the crack go all the way to the lower edge of the plate?  Plate shrinkage would cause a crack to initiate at the edge, and with the aforementioned undesirable post stress, the crack could propagate up to the post.  Additionally, plate shrinkage would increase the pressure on the post.

If the post had been re-fit to the shrinking plate, I don't think that the plate shrinkage would cause a crack to start at the post, but most likely be a continuation of an edge crack.

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17 minutes ago, Don Noon said:

Does the crack go all the way to the lower edge of the plate? 

Yes. I expect that it started as a saddle crack.

17 minutes ago, Don Noon said:

If the post had been re-fit to the shrinking plate, I don't think that the plate shrinkage would cause a crack to start at the post, but most likely be a continuation of an edge crack.

I was thinking that, too. So it would be unlikely then that a sound post crack would develop initially at the sound post due to plate shrinkage? 

I wonder if sound post cracks that start at the sound post without apparent cause ("spontaneous" soundpost cracks) are ever observed.

Don, do you think that keeping tension on the strings of a violin in storage reduces the chances of a soundpost crack?

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48 minutes ago, GeorgeH said:

So it would be unlikely then that a sound post crack would develop initially at the sound post due to plate shrinkage? 

Don, do you think that keeping tension on the strings of a violin in storage reduces the chances of a soundpost crack?

Plate shrinkage would cause some added crossgrain tension in the top, which would increase the forces tending to cause a crack.  However, I think this tension would be minimal if the crossarch is allowed to flatten out by fitting s shorter post.  If the post isn't re-fit, then you could get more significant tension.

Tension from the strings puts downward force on the bridge foot, and resulting upward reaction at the post (and behind the post).  It would increase the cracking forces if you leave tension on the strings.

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The OP question is what, “in the experience of luthiers & restorers here” is the “most common cause of sound post cracks in the belly”. In my experience it is certainly the various forms of violent occurrence such as dropping it, sitting on it, falling down the stairs with it, or running it over with the car

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Impact will produce brief but extremely high forces, and it doesn't take much time to create a crack (unlike the Hollywood movie cracks that slowly grow in a menacing and fear-producing manner, similar to rope failing strand by strand).

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I see a lot of soundpost cracks caused by tight sound posts where there is no evidence of any other damage. I have also seen someone put a soundpost crack into a brand new instrument while moving a post that was too tight.

If the post only comes from into contact on an edge, that edge can act like a knife edge and put a lot of pressure into the top. Often a palpable ridge will appear, and it will eventually turn into a crack.

Impact is definitely a common cause. A good number of the cello sound post patches I’ve done have been on cellos that fell over while in their cases.  

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27 minutes ago, The Violin Beautiful said:

I see a lot of soundpost cracks caused by tight sound posts where there is no evidence of any other damage. I have also seen someone put a soundpost crack into a brand new instrument while moving a post that was too tight.

Ouch! 

Is there a way for a non-luthier to tell if a soundpost is too tight? Do soundposts installed in a humid summer get tighter in a drier winter?

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6 hours ago, GeorgeH said:

Thank you, Don and Jacob.

@jacobsaunders, have you ever seen a soundpost crack that started at the soundpost without evidence of any physical impact?

 

A fine violin I know with a highly figured "pioppo" (poplar) back, had the flames very "upright" in the region of the post, causing an spontaneous crack there, but that is a very rare case.

 

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On 7/10/2021 at 3:21 AM, GeorgeH said:

Ouch! 

Is there a way for a non-luthier to tell if a soundpost is too tight? Do soundposts installed in a humid summer get tighter in a drier winter?

Yes. I had long ago seen a cello where I could not move the sound post and that was in winter. It was of course very well fitted. 
 

I saw the customer in summer again and I could move the sound post, though it was still tight.

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On 7/14/2021 at 8:35 AM, Marty Kasprzyk said:

I used to glue a 20mm diameter round disk about of craft paper on the inside surface of the top plate at the sound post location to strengthen the wood and prevent cracking without adding much weight.

 

I should have added that I first thoroughly soak the kraft paper in hide glue.  When it is dried it is very hard and strong.  I also put these kraft paper patches above and below the f-hole eyes to prevent cracks there.

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The Krutz shop puts a 'prophylactic' round sound post patch in their shop violins, and as far as I know, Anton does, too, on his own violins.

I have had to shorten sound posts on follow-up maintenance as new violins shrank and settled in, but the only sound post cracks I know of for sure have been associated with rough handling.

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37 minutes ago, Michael Richwine said:

I have had to shorten sound posts on follow-up maintenance as new violins shrank and settled in, but the only sound post cracks I know of for sure have been associated with rough handling.

Good to know! 

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On 7/16/2021 at 8:08 PM, Dwight Brown said:

At least a few very famous makers put in a sound post patch in new violins. I believe they use a thin maple veneer.

Also some not-very-famous makers, like me.  I only use the bridge footpads when using very low density spruce, as the pads are visible and are annoying when varnishing.

On 7/17/2021 at 5:06 AM, Marty Kasprzyk said:

I should have added that I first thoroughly soak the kraft paper in hide glue.  When it is dried it is very hard and strong.  I also put these kraft paper patches above and below the f-hole eyes to prevent cracks there.

For wing crack prevention, I prefer to leave that zone very thick, and feed hide glue into the endgrain for reinforcement.

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