Very superficial black spots on a good and smooth casting
Indicates that the metal reacted with oxygen forming Chromium (Cr) and Manganese (Mn) oxides during melting, casting and solidification
Causes of this defect are evident in the metal and other portions of the process; to cure, R&R recommends taking the following actions:
High metal and mold temperatures.
Melt under Argon (Ar) blanket and adjust temperatures.
Use a ceramic filter or carbon stucco.
Slow cooling down curve.
Separate molds after casting. Can molds after casting.