What factor may cause ejection of molten metal from the crucible?

The ejection of molten metal from the crucible during thermite welding can occur when moisture is present. When moisture is introduced into the reaction, it can quickly vaporize due to the high temperatures involved. The rapid expansion of steam can create pressure that forces molten metal out of the crucible. This explosive reaction can lead to dangerous situations, especially given the extreme temperatures of thermite welding, which can reach up to 4500°F (2500°C).

In contrast, while wind can influence the cooling of the molten metal and may affect the overall process, it does not directly cause the ejection of molten material from the crucible. Similarly, contamination refers to unwanted materials that can alter the quality and properties of the weld but do not inherently cause the ejection of molten metal. Low temperature is also not a factor in causing ejection, as lower temperatures would typically lead to solidification rather than ejection of molten metal.