I've seen corrosion on brass strings which may be the so-called 'bronze 
disease'. This takes the form of a powdery green deposit which if 
unchecked will eat right through the string. According to Stewart 
Pollens' recent book* it develops in copper-alloy objects where the 
ambient RH is 63% or more and chlorine ions are also present. The 
chlorine ions can come from actual salt in the air, but more likely they 
result from perspiration transferred from someone's hands, perhaps when 
winding the loops on a set of stored strings.

The moral seems to be that strings should be stored at RH below about 
50%, and that in making loops one should avoid touching the wire with 
bare hands.

*Stewart Pollens, *The Manual of Musical Instrument Conservation*, 
Cambridge University Press 2015
-- 
Peter Bavington
Clavichord Maker
291 Sprowston Mews
LONDON
E7 9AE
www.peter-bavington.co.uk

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