This can get a little tricky so here are a few pointers:
First, as has been stated by others, be careful. Wear gloves/mask/ goggles/smock and work in an area where the fungus can be contained/evacuated.
Second, never try to remove fungus from the surface of the tape while the fungus is still active. It will simply smear the fungus and cause more contamination. You must first drive the fungus into dormancy by exposing it to low humidity conditions. You can do this using a low humidity chamber, a vacuum or by incubation ("baking").
If there is a large amount of fungal growth on the tape edges, you may want to remove some of it before "baking" to avoid hardening the residue on the tape edges. To do this, remove the flanges for access to the tape edges and check the flanges (fungus can pit the inside of the flanges). The best chemical to remove the heavy fungus is simply a 50/50 mix of distilled water and 91+ % pure isopropyl alcohol. This is convenient to use as it also can remove some of the white powder/crystals (which are not fungus- more on that later). Dampen (do not soak) whatever you are using to wipe the edges and make sure everything evaporates before replacing the flange (do one flange at a time to avoid tape slippage off the core). WIPE IN ONE DIRECTION ONLY in a circular direction, following the tape wind. Do NOT wipe back-and-forth as this will loosen the binder and damage the tape. This chemical solution will not, necessarily, kill the fungus remaining on the tape but can be used to remove large fungal growths.
A better chemical for actually killing the fungus is methyl chloroform. Besides actually killing the fungus, this chemical has the benefit that the polymers in magnetic tape are not soluble in the chemical. The down sides are that the chemical is restricted, it is not good for removing the white powder/crystals, it can dissolve some plastic reels/cassettes and finally (IMPORTANT DANGER) it can damage hydrolyzed tape as it will dissolve the oligomers out of the tape. Do not use this chemical unless you really know what you are doing!
Unfortunately, most 2" audio tape is hydrolyzed. Tape with fungus is almost certainly hydrolyzed. If there is a large amount of fungus on the tape edges, there will also be fungus in the wraps. It is not a good idea to try and unwind a hydrolyzed tape to clean the fungus out of the tape pack. Fungus remediation should be done in certain steps: dehydrate to drive the fungus into dormancy, chemically remove large deposits from edges, incubate ("bake") to treat the hydrolysis and, at the end, clean the tape surface inside the wraps.
A final note: white powder or crystals can appear on the tape surface. This can occur alone or in conjunction with fungus. These are mostly chemicals left over from the manufacturing process that have oxidized and migrated to the tape surface (7 different residues have been identified in the lab). The one exception may be the "white powder" identified as cyclic trimers, but these are generated at the binder-base interface rather than within the binder itself such as are the oligomers related to "sticky shed". Unfortunately, if these residues are identified on the tape edge, they will also be on the tape surface throughout the tape pack. In addition, "baking" makes additional volumes of these residues migrate to the tape surface. This means that, AFTER "baking", the entire tape surface should be cleaned before attempting playback. Failure to do so has, occasionally, caused tape operators to throw up their hands and exclaim "It's snowing in the room" :) .
If you want any additional advice, give me a call or e-mail.
Best of luck,
SPECS BROS., LLC
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Audio and video restoration and re-mastering since 1983
From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of Blue Star
Sent: Tuesday, May 8, 2018 2:50 AM
To: [log in to unmask]
Subject: Re: [ARSCLIST] reel-to-reel with mildew
Hello list members,
I have just come into ownership of ten 2" reels (24 track) that have mildew on the edges. There are some ideas through google search to kill the mold then clean it but I would like to hear from you on the list. This is to clean the tapes before baking.