 Tapes were initially made of paper coated with magnetite powder.In 1947/48 Minnesota Mining & Manufacturing Company (3M) replaced the paper backing with cellulose acetate or polyester, and coated it first with black oxide, and later, to improve signal-to-noise ratio and improve overall superior quality, with red oxide (gamma ferric oxide).American audio engineer John T. Mullin and entertainer Bing Crosby were key players in the commercial development of magnetic tape.