The wooden frame of a piano gives the instrument its unique sound.
A transparent material instead could adversely alter that sound.
To demonstrate the works in my instrument I remove the upper panel
to expose the air motor, tracker pneumatic and hammers in action.

The "key" pneumatics are enclosed in the pneumatic chest in such
a position as be difficult to see in operation.  I suppose one could
install a transparent upper panel so the works could be seen without
being exposed to dust, etc.

Peter Vander Veld