Piezoelectric Diaphragm Discoloration

The electrode of the piezoelectric vibrating plate is made of silver, and the chemical reaction of silver and sulfur in the air results in discoloration (from silver to brown). This can adversely affect the ability to solder to it, so examine it before use.
In addition, if you touch the piezoelectric diagram with bare hand, you will leave a fingerprint and will cause a discoloration, you might want to consider wearing a finger cot, anti-static finger cot, glove, or anti-static glove when handling it.