What causes inclusions in the melt, and how to reduce them

Inclusions are the small non-metallic particles that end up in a casting and show as defects after machining. Some come from the alloy and the melt practice, but the crucible itself is a common source when its wall is porous or reactive with the metal.

A dense, low porosity body with a non-wetting surface sheds far less into the melt. That is why wall density and surface finish matter as much as the headline chemistry when you are chasing a lower scrap rate.