Which of the following is the reason for black casting?
Black casting
Can be due to two reasons:

Overheating the investment above 700oC causes it to decompose liberating sulfur or sulfur compounds. They readily combine with the metals in gold alloy forming a sulfide film. This gives a dark casting which cannot be cleaned by pickling.
A black casting can also be due to incomplete elimination of the wax pattern, as a result of heating the mold at too low temperature. A carbonized wax remains, which sticks to the surface of the casting. It can be removed by heating over a flame.
Overheating of metal
Underheating of metal
Incomplete removal of wax
None of the above