How Does Metal Affect RFID Reads?

If you were to take an ordinary passive RFID transponder and place it on a metal object, the metal would detune the antenna. This is similar to what happens when you take an AM or FM radio and touch the antenna with a piece of metal. The antenna is no longer tuned to the frequency of the incoming radio waves, so you don't get good radio reception. Similarly, the RFID tag antenna will no longer be tuned to receive radio waves from the reader, and will be rendered unreadable or have an extremely short read range.