‘Baking Tapes’ or Analogue Audio Restoration

Last Friday was a bit of a milestone for me, since, in the 6 or so months that I have been involved in the audio preservation side of things at PARADISEC, I hadn’t yet actually cleaned a damaged audio tape. Unfortunately for me, the process isn’t quite as straight-forward as it is for a CD – warm soapy water, a non-abrasive cloth, wipe across the grain – rather, the entire process can take weeks, depending on how badly affected the tapes are.

A while back, we received a collection of cassette tapes that are less than ten years old, but had been affected by mould growing on the edge of the magnetic tape itself. This is unfortunately a common scenario. Tape that is affected by mould will not give a clear, continuous signal when played back. A single speck of dust or mould can separate the tape from the head far enough to cause a complete drop-out. To get one good, clear play of the tape for digitisation purposes, this mould has to be neutralised and removed.

First things first, exposure to moulds can have seriously deleterious effects on one’s health over a long period, so a well-ventilated room at the very least, as well as gloves and a filter mask, are quite necessary. The cassettes are opened up, exposing the tape reels and the mould on them. Every exposed surface has to be wiped down with a 2% cetrimide solution, including, most importantly, the edge of the packed tape, which means carefully removing it from the housing, cleaning one side and turning it over to clean the other side, careful not to let the tape unwind itself (which would be an absolute disaster). Finally, the tape including all those small components, guide-rollers, the pressure pad and the magnetic shield, which are all pictured below, are placed back in the housing. This process takes about half an hour for each cassette.

Anatomy of a cassette.

This is where the fun begins. The tape looks clean, but the mould will continue to grow if given the right amount of moisture. PARADISEC has a vacuum oven specifically designed to ‘kill’ mould (you can’t really kill it, but you can make it non-viable for long enough to play it again). The way it works is to reduce the air-pressure inside the oven chamber slightly, and allow air to ‘bleed’ back into the chamber through a tube of silicon beads that absorb moisture. The net result is that the air inside gradually gets drier and drier. Also, the heat inside is raised to around 35ºC (95ºF, 308ºK) to aid the drying process. The tapes, including their cases, sit in this chamber for a while (as in the photo), depending on the severity of the mould. The tapes that I have just been cleaning will have been ‘baked’ for a week by the time I get around to finishing them off.

Tapes being baked.

After the baking process, the tapes need to be cleaned again, exactly the same as before. But even then, every time you play the tape, more mould will accumulate on the pressure pad and around the tape-deck head. Apparently there are machines that will play the tape through, cleaning every inch of it, but I can’t find one (email me [aidan at-sign usyd.edu.au] if you know where I should look). I just have an old Philips portable personal tape player (I’d have called it ‘Walkman’ except doing so might infringe on someone’s (Sony’s) trademark) with the head ripped out that just serves to wind the tape back and forth, expelling a little bit more mould each time. Then I simply clean it (yet) again, and soon enough the tape is ready for digitisation!

Friday was my first attempt at putting a tape through the entire process from cleaning to archive. It was a bit of a new experience for me, but I was quite happy with the result, despite the fact that the signal I retrieved from the tape was a little bit dodgy in some areas. In fact, I have earmarked it for potential re-ingestion if I’m able to clean it up a little more. Regardless, I’m pleased that I was able to extract anything at all.

The moral of the story is, if you have cassette tapes that contain valuable data, then try to keep them in good condition. And make backups, and keep those backups in good condition too! Alternatively, you can send them to us and we’ll do all the work (for a small fee of course). The suggested climate conditions that will minimise any mould growth are somewhere below 18ºC (64.4ºF, 291ºK) but not below 4ºC (so the fridge isn’t the best idea) and under relative humidity of 35%, though 60% RH is the real danger-zone (this is unfortunate if your tapes are kept in the tropics).

Thanks go to Mick Newnham of The National Film and Sound Archive for his generous advice in these matters.

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