Slag is the waste material that forms on top of melted metal during the process of refining ore. When workers heat iron ore in a blast furnace to extract pure iron, impurities like rock and dirt float to the surface as a glassy, crusty substance called slag. Think of it like the foam that rises when you boil pasta: the useful part stays below while the unwanted material collects on top.

Slag gets skimmed off and removed, leaving behind cleaner, purer metal. Ironworkers and steelworkers have been dealing with slag for thousands of years. The ancient Romans left behind huge piles of slag near their metalworking sites, and archaeologists study these slag heaps to learn about early manufacturing techniques.

Today, people have found clever uses for slag instead of just throwing it away. Crushed slag makes excellent material for road construction and railroad ballast. Some types of slag get ground up and mixed into cement. What was once considered worthless waste has become a valuable building material, proving that even leftovers from one process can fuel another.