[pianotech] Expanding leads

[Allen Wright]

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
> 
> 
>