Scum and Scoria

Scum noun - People looked down upon as ignorant and of the lowest class.

Scoria and scum are semantically related in slag topic. In some cases you can use "Scoria" instead a noun "Scum".

Scoria noun - The scum formed by oxidation at the surface of molten metals.

Scum and scoria are semantically related in remains topic. You can use "Scum" instead a noun "Scoria", if it concerns topics such as slag.