Oh, the youths and their fascination with old technology! We knew vinyl was back, but were you aware that the cassette tape has reached critical mass … again? Against all odds, they have, and the leading cassette-maker National Audio Company is running into trouble filling all its orders due to a global shortage of high-grade gamma ferric oxide, a key ingredient in magnetic tape. It’s not exactly because of cassette demand — actually, the only gamma ferric oxide factory in the world is being renovated — but the increased demand doesn’t help, with cassette sales up nearly 25 percent last year. (Are that many people really still buying the Guardians of the Galaxy soundtrack?) But if you’re waiting on a cassette of something like Frankie Cosmos’ Close It Quietly or Black Belt Eagle Scout’s At the Party With My Brown Friends, two recent albums out on tape, don’t fret — the NAC is expecting 11 tons of gamma ferric oxide this month and hiring more staff to make up for the shortage, promising that things will be back to “normal” by the end of the year. Just not in the sense of, you know, not mining the earth to be able to produce recorded music.