PowerCore Grain Oriented Steel is usually delivered with a thin inorganic coating C5G applied on both surfaces, on top of the so called glass film layer formed during annealing. The coating thickness of 2-5 microns provides good electrical resistance simultaneously ensuring a high stacking factor. The stress relief annealing of wound cores or punched laminations, usually performed at temperature around 840°C. It is unaffected by, and in turn does not affect, the various types of oils used in transformers.