Originally Posted By: Run
Looks like it was baked in the oven for a few hours.
Clearly hard to tell, but that's probably a factor.
The big power transformers are paper/oil insulated, using kraft paper (that brown glossy stuff from kindergarden), and a fairly pure oil with low additive levels.
As it ages, acids attack the lignin(???I think) that is like the glue that binds the cellulose fibres together...making them brittle.
But higher temperatures also do it, and pretty markedly.
Degraded transformer paper flakes off just like that, so acid yes, temperature (maybe an overheat), and maybe wrong oil in the first place.