[pianotech] Expanding leads

[tnrwim at aol.com]

So it would appear that it's the kind of wood used 
in keys, rather than anything else, that is the culprit with this problem. Also, 
if the air is moving more around the piano it's probably a good thing. So 
perhaps having a closed fallboard will contribute to a concentration of the 
acids in the micro-environment around the keys?



Again, if it is the wood, why are some leads corroding, and some are not, in the same piano? Or is there that much difference in the fibers of a set of keys that makes some leads corrode, and not others? Or, since keys are made from three or four boards end glued together, with one or more boards coming from different trees, is there that much difference in the acid content from one pine tree to another? 

So it seems that in order for the corroding problem not to surface again, perhaps the only real solution is a new set of keys, made from basswood, weighted with a set of electroplated leads. Yeah, right. Try to sell that job to a customer with a 50 year old Kimball spinet. 

Wim


 



-----Original Message-----
From: Allen Wright <akwright at btopenworld.com>
To: pianotech <pianotech at ptg.org>
Sent: Tue, Mar 12, 2013 1:41 pm
Subject: Re: [pianotech] Expanding leads



On Mar 12, 2013, at 10:47 PM, pianotech-request at ptg.org wrote:

David,

Like Wim, I always assumed that this was caused somehow by humidity, and was 
simply "oxidation". So that perhaps pianos kept in damper environments might 
suffer the worst. But it seemed to show up in the most expensive fine older 
instruments as well as low quality ones.

It's not the quality of the piano (nor humidity) that has any effect on this 
phenomenon; rather, it's acetic acid (and formic acid to a far lesser extent) in 
the wooden keys themselves that is the cause. 

I only just learned about this last week when I noticed (by sheer coincidence in 
the archives) a mention by Laury Libin about this subject, either on this list 
or perhaps CAUT some months ago. He references an article from a US Navy 
website, on the challenge of dealing with deteriorating lead parts in ship 
models. A couple of the most relevant quotes:

The chemical process is this: "acetic and some other acids, in the presence of 
carbon dioxide, catalyze with lead to produce lead acetate and lead hydroxide. 
Lead acetate and lead hydroxide together react with carbon dioxide and form lead 
carbonate. Lead carbonate then releases acetic acid and the process becomes 
self-sustaining. ...the formed lead carbonate is not just a substance clinging 
to the surface of a casting, it is the surface of the casting transformed to 
powder (... ) a portion of the lead is gone and lead carbonate is left in its 
place. The lead carbonate releases acetic acid which can continue the process 
until the lead part is progressively consumed from the outside, inward."

"A simple way to prevent woods from off-gassing acetic acid would seem to be to 
seal the wood using an acid-impervious coating. But most kinds of wood sealers, 
paints, and clear finishes are not impervious to the passage of acetic acid from 
woods, and indeed, the coatings might further contribute to the 
micro-environmental problem. To date, researchers have found no product which 
can be applied as a liquid and which fully seals wood to suppress the emission 
of acids. Two part epoxy and some urethane paints appear to offer a limited 
degree of barrier. Shellac, while not an acid producer, does not offer any 
protection."

Three categories of Very Harmful, Moderately Harmful, and Less Harmful woods are 
given. Basswood is in the very harmful column, whereas Sitka spruce and Pine are 
in the less harmful column. So it would appear that it's the kind of wood used 
in keys, rather than anything else, that is the culprit with this problem. Also, 
if the air is moving more around the piano it's probably a good thing. So 
perhaps having a closed fallboard will contribute to a concentration of the 
acids in the micro-environment around the keys?

To extrapolate from all this a bit further, it would seem that lead is perhaps a 
fundamentally problematic and ultimately unsatisfactory material to be used in 
weighing keys, at least in the long term. I'm not sure what else could be used, 
although the article does say that electroplating the lead may be an effective 
preventive measure. That might be prohibitively expensive.

I'm glad to finally know that this is not an "oxidation" problem, and to have 
some of the mystery explained. For anyone interested in reading the whole 
article, here it is (and once again, all thanks to Laurence Libin for posting 
this originally).

 http://www.navsea.navy.mil/nswc/carderock/pub/cnsm/lead/lead_01.aspx

Best regards,

Allen Wright, RPT
London, UK

p.s. I hope I've posted this correctly in terms of format. I get the daily 
digest of the list.
> 
> Message: 4
> Date: Tue, 12 Mar 2013 13:27:30 -0700
> From: "David Love" <davidlovepianos at comcast.net>
> To: <pianotech at googlegroups.com>
> Cc: pianotech at ptg.org
> Subject: [pianotech] Expanding leads
> Message-ID: <00bf01ce1f60$054f07f0$0fed17d0$@net>
> Content-Type: text/plain;	charset="UTF-8"
> 
> While I have run into expanding lead problems periodically generally they seem 
to have been reserved for "lesser" pianos.  Today I found a 1950's Boesendorfer 
200 with leads expanding both in the back action and the keys to the extent that 
they were binding on their neighbors and on the verge of splitting things.  Of 
course I will have to remove and replace, a large task considering the new 
weigh-off at the key end that will be required.
> 
> My question is what is it that causes that reaction and why after such a time 
does it happen.  This is some 60 years later. 
> 
> 
> David Love
> www.davidlovepianos.com
> 
> 
> 


 
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