[chuckm]

Any conductive metal can be used for shielding. An EM field will induce a counteracting current in the shielding, which also generates a field. The result is that, outside of the shielding, the sum of the two fields in zero (or close to it). It does not need to be a ferrous material. In fact, many shoplifters use bags lined with thin aluminum foil, a crude Farraday cage, to prevent the RFID from working.