Bert said:

"I always thought that heat destroys magnetism, so baking would be an absolutely last resort."

Excess heat demagnetises but the right amount of heat consolidates the oxide with minimal disruption of the magnetic signal. That's why it's a specialist job but it's only a matter of knowing the right temperature and having a bit of experience with different makes of tape etc. Yes, it's used as a once only solution to getting the the signal off the tape and onto another medium.