sullage

Sullage is the thin, crusty layer of oxidized metal and slag that forms on the surface of molten metal as it sits in a ladle. It is not the same as the denser slag that settles at the bottom of the ladle, but rather a skin that develops as the metal cools slightly and reacts with air and the ladle lining. The formation happens naturally during the time between pouring and casting, and its presence is nearly universal in foundry and steel shop work.

The composition of sullage depends on what metal is being held. In steel pouring, it consists mainly of iron oxide mixed with silicates and alumina from the ladle refractory. In aluminum work, the layer is aluminum oxide (corundum), which forms rapidly and can be quite thick. The color and appearance change as the metal cools: fresh sullage may be dark and glossy, while older material becomes dull and brittle. The layer typically measures a few millimeters thick, though this varies with holding time and metal temperature.

Before casting, sullage must be removed or at least managed carefully. If it passes into the mold with the metal, it becomes a hard inclusion in the finished casting and can cause defects that fail quality inspection. Foundry workers use skimming tools, often simple flat paddles or ladles, to skim the surface and push the sullage to the side of the ladle, away from the pouring spout. Some operations use ceramic or steel bars to break and separate it. The timing matters: skim too early and new sullage forms; wait too long and the material cools and becomes harder to remove.

The term sullage appears most commonly in British and European foundry practice, though American texts sometimes use scoria or simply slag to describe the same thing. The word's origin is uncertain but may relate to soil or sediment terms in older trade language. It is distinct from dross, which usually refers to light oxides that float on non-ferrous molten metals, and from skull, which is partially solidified metal that breaks loose from the ladle walls.

Excessive sullage formation signals problems worth investigating. Too much suggests the metal is losing temperature too quickly, the ladle lining is degrading, or the ladle is being held too long before casting. Some foundries deliberately add flux powders to the surface of the metal to promote a more manageable slag layer and reduce inclusions. The skill of ladle management, including sulfage control, is part of what separates experienced pour crews from inexperienced ones.