A blast furnace.
Iron has a huge amount of impurities in it. Until 1828, most iron was made by puddling: A trough or pit was lined with heat-resistant brick and then heated. Sometimes, the heat came from below, which meant the furnace had no cover. Sometimes, the heat came from above, which meant the furnace was covered. The heat in this "reverberatory furnace" bounced off the roof and went down into the iron in the trough. Either way, the iron melted. As it did, men would stir it with iron paddles on long iron shafts. This exposed the iron to oxygen, which helped reduce the carbon content. Limestone, and sometimes other elements, would be added to the iron. This would adhere to the unwanted impurities in the iron, causing them to float to the surface -- where they'd be skimmed or poured off.
Puddling was extremely hot, dangerous work. The heat could burn. The fumes destroyed lungs. Accidents were common, and men were consumed by molten metal. Many puddlers didn't live very long...
In 1828, two Scottish men invited the blast furnace. It had been understood for three centuries that a vertical furnace worked better than a horizontal one. Charcoal would be loaded into the furnace first, then limestone, then raw iron. When the charcoal was lit, it burned the limestone, causing a chemical reaction that worked better than just tossing in untreated limestone. This "slake" would gradually mix with the iron as it melted and dripped down from above. At the bottom of the furnace, tubes wrapped in water ("tuyeres") could be opened, and air would bubble up through the molten iron. This meant no stirring.
But these early blast furnaces were fuel-hogs. It took eight tons of charcoal to make a single batch of iron.
What the Scots discovered is the "hot blast". The air would be preheated first in nearby stoves, then allowed to bubble into the iron. Moreover, the hot air coming out the top of the blast furnace could also be recaptured, helping to heat the stoves and further reducing fuel needs. The iron melted much faster. All of this reduced fuel requirements by nearly 40 percent.
The blast furnace changed the world. Suddenly, pure iron could be manufactured swiftly and easily.
Blast furnaces were monsters. Even early ones were 100 feet high, and 20 feet in width. When the tuyeres were opened, the furnace really got going -- and the roar of the gases coming out of the iron could be heard for three miles. The light of a blast furnace at night would light the surrounding area as if it were day. The smoke was immense.