This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
[{'content': 'Which of the following is the reason for black casting? \nSelect and explain. \nA. Overheating of metal, B. Underheating of metal, C. Incomplete removal of wax, D. None of the above', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Black casting\nCan be due to two reasons:\n\nOverheating 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.\nA black casting can also be\xa0due 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.', 'role': 'assistant'}]