It can be quite hard to extract the cores from the transformer without breaking them, since they are usually glued in - I have had reasonable success by cutting away the windings using a hacksaw.

I have been successful by heating cores in an oven (at 120C if I remember well) for some time. The glue gets weak and the windings can be removed (using gloves).

An alternative may be soaking the core in acetone (nail polish remover) or paint thinner.

73, Rik  ON7YD - OR7T