Brain preservation difficulty for tracing

How difficult is it to preserve a brain sample at a state that can be traced? I do mean, so that you are able to reconstruct the conectome very well.

Like… can you take some random brain that has pickled in formaldehyde in a museum for 100 years and trace it?

If the brain sample is frozen, the ice crystal damage the neurones to a irrecoverable degree? Or it does work? Or it has to be in liquid nitrogen? Or as long as it’s solid ice it’s fine even in 1000 years? Could you trace the brain of a Mammoth? Or a dehydrated brain? A mummy? Einsteins brain? If it’s a bit rotten? Or must use special chemicals?

You see what i mean?
If a brain looks macroscopically good, it wouldn’t necessarily mean that microscopically the structure is preserved.

It doesn’t matter if it’s hard or easy. Just if it’s possible in principle.

…feel free to give me your best educated guess if you don’t know.