
Watch out, it is an indication of damage caused by overheating of aluminum wiring and could be a fire hazard. Aluminum wiring was installed in some homes in the 60's and 70's and though it is not officially "recalled" it is a known hazard due to overheating. Educate yourself on aluminum wiring here
https://inspectapedia.com/aluminum/Aluminum_Wiring_Hazards.php
lots of great information.