The modern ones I pull the backing plate off & use a cold chisle down thru the groove, & then lever the 'C's off with a flat screwdriver, then punch out the squares inside the coil.
The coils covered in paint so I bash it & most of the paint falls off & they sell as domestic copper.
The older ones, tar filled,
I bash the outer case off or heat it in a fire so the tar softens. Then heat the tar off, then pull the iron core - a long legged 'U' shape out of the coil, the coils covered in tar but cleans up easy like.
Theres other sorts, I have found 3 sorts, but they seem to be just variations of the two.
About 100gms per ballast, the last tar filled ones had a wooden block in one end to fill up space.
And a very thin tar soaked ricepaper inside the coil, when I unravelled the wire the ricepaper fell off, made a huge mess of the place & there wasn't much ricepaper, in volume, after all.
If you think its worth doing, just pull one apart & learn from it & do the rest 'production style'.
Its a dirty job, but I like to get every little bit possible.
I have found the wire in the newer ones is a solid single strand, I don't see that very often.