This Qi fanboy tried different materials:
- A regular black sock.
- A comic book.
- A plastic chopping board (4-5 mm thick).
- A Bluray Disc
- A stainless steel pan
- Two centimeters of air
Unsurprisingly, charging through stainless steel doesn''t work (you can see the white LED on the charger blinking fast, which means that power transfer is not possible).
Note also the large freedom of positioning. Alignment between charger and phone is not at all critical.