Jim Carroll wrote:

When he took his collection out to present it to an archive, he found that the cupboard had a heating pipe running through it; when the tapes were unreeled the coating stripped off like orange peel.

This is a common problem and old tapes should never be unwound until they have been checked. If they are shedding oxide they need to be baked to consolidate the coating for a single pass transfer (hoping for the best). This is possible to do in an amateur way if professional facilities are not available - put them in a (fireproof) box with a 60 watt light bulb for 48 hours.

The other test is to sniff the tapes for 'vinegar syndrome' - if they smell acetic then they need specialist attention.

Otherwise tapes stored properly, tail out and wound and rewound once a year should last for a very long time.