Outside of special cases, the surface of a molten metal is covered with a thin, solid layer of oxide.
If the surface of the liquid happens to fold in on itself, or if a separate droplet falls into it, there will be a solid-on-solid interface between the two films.
The presence of such crack-like entrainment in solid metal articles constitutes a metalworking defect.
The continuous casting process used in the initial forming of most wrought alloys is also susceptible to entrainment.
This phenomenon is able to partially bond the two facing oxide surfaces, thus reducing the severity of the defect.