Why piezo's ceramic has got copper color?

[elrules]

SOm of my piezos, after weeks, have got a kind of red like copper color on the ceremic. They seem like old, rusty, and the give the impression of not working but in deed they work. Other piezos I had in the same place did not get this color. Why some piezos adquire this color? Maybe air contact? I have now covered the ceramic of the new rim piezos I am installing with tape, in case air is making them rusty.

Do you know why this phenomenom happen?

[Bencthemusicman]

Hi, i've made a post about this recently.

on mine its the ceramic that is turing a brownish grey.
I got mine from Maplins (UK).

I dont think that its caused any problems so i wouldn't worry about it.