Burning in transformers
Transformer magnetism
Magnetism not something analog. It is quantized. The magnetic material, is divided into micro sized, or rather nano sized, small units, and each of those can have only one magnetic direction. If these directions are random, there is no magnetic field. Yet if they all get a similar direction, there will be an effective field. These internal magnets are called the elementary magnets. If these are exposed to an external field, some will change their direction to that field, which direction will stay, if the field is removed. Some others will fall back to their original condition. The stronger the field, the more effect it will have, no surprise with that.
Something strange will happen if the field is only weak, but the magnet is hit by something. The shock will shake them up, and they tend to fall back then, NOT in their original direction, but in the direction of the (weak) external field. By this, an un magmetised material can become magnetic because of this, to a remarkable amount. Even the earth magnetic field, though weak, can achieve this. Most transformer cores can (and will) hold some permanent magnetism. It is not much, and the manufacturer treated them carefully, we hope so. However at the post office, they know not much about magnetism, and the package throwing will have it's result. Also the mechanical rattling in the car may have some result. The remaining magnetism causes distortion of the electric signal. That is why we ship Lundahl transformers always in foam, same as Lundahl does, when shipping to us.
What will burn in do?
Interestingly, just for the sake of the word, heat above 150°C will also remove permanent magnetism, but that is too hot for the transformers. However, magnetism will disappear slowly over time, but only when the transformer are used. Transformer cores have as little as possible memory effect. (Much unlike a permanent magnet, which is intentionally made to remember it's condition). So by using the transformer, the particles are shaked up, and eventually return to their original, neutral condition. This does however take time, lots and lots of time. This is why indeed people praise older transformers so much.