5.4.1 Introduction
5.4.1.1 Analogue magnetic tape recording technology has permeated every area of the recording industry since its mass distribution and popularisation in the post WWII era. Technological advancements made tape the primary recording format for professional recording studios, and manufacturing developments made the reel recorder affordable for the domestic market. The introduction of the Philips Compact Cassette in 1963 put a recording device within the grasp of many people and it became possible and practical for people to record whatever seemed important to them.Virtually every sound archive and library holds analogue magnetic tape recordings, and PRESTO (Wright and Williams 2001) estimates there are over 100 million hours of analogue tape recordings in collections throughout the world, a figure in no way contradicted by the IASA survey of endangered carriers (Boston 2003). Since the 1970s sound archivists recommended quarter inch analogue reel tape as the preferred archival carrier, and in spite of inherent noise and impending chemical decay, some still stand by them today as a stable carrier. Nonetheless, the imminent demise of the analogue tape industry and the consequent and almost total cessation of the production the replay equipment demand that immediate steps be taken to transfer this vast store of recorded cultural history to a more viable system of management.
5.4.1.2 Magnetic tape was first made commercially available in Germany in 1935, but it was the commercialisation of the American market after 1947 that drove its popularity and eventual standardisation. The first tapes were manufactured on a cellulose acetate backing and this continued until the introduction of polyester (polyethylene terephthalate PET, commercially known as Mylar). Tape manufacturers produced both acetate and PET tapes with an acetate binder, which was gradually, and most commonly, replaced from the late 1960s by a polyester urethane binder. BASF manufactured tapes on PVC from the mid 1940s until 1972, though it gradually introduced its own range of polyester from the late 1950s onward. Though PVC was primarily the province of the German manufacturer BASF, 3M also produced a PVC tape from around 1960; Scotch 311. Rarer are paper backed magnetic tapes, which date from the late 1940s to the early 1950s. Cassette tapes have always been manufactured on polyester. In 1939 the magnetic pigment used was γFe2O3, often called the oxide, and although subsequent improvements in particulate size, shape and doping increased performance and reduced noise, this formulation has remained virtually the same for almost all analogue reels and type I cassettes. Type II cassettes are CrO2 or cobalt doped Fe3O4, III (rarely encountered) are dual layered with both γFe2O3 and CrO2 and IV are metal (pure iron).
5.4.1.3 The materials that bind the magnetic particles to the tape substrate, called binders, are often identified as that part of the tape most susceptible to chemical breakdown. This is especially so with polyester urethane binder tapes which most commonly use a PET substrate from the 1970s, though AGFA and BASF and their subsequent owners, Emtec, used a PVC based binder on many of their studio and broadcast tapes, notably 468.
