Which of the following is the reason for black casting?
Overheating of metal
Underheating of metal
Incomplete removal of wax
None of the above
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.