英文科学读本 第二册·Lesson 23 Iron

Lesson 23 Iron

Fred, said Norah, "do tell me what becomes of that melted iron at the bottom of the blast furnace."

There is a hole in the side of the furnace, said Fred. "The men keep it plugged up close and strong with fire-clay. Every twelve hours they come with long iron rods, and break away this clay plug. Then of course out comes the red-hot liquid iron.

The floor all round the furnace is laid thick with sand, and channels about four inches wide are made in the sand. As the red-hot liquid metal flows out through the hole in the furnace wall it runs into these moulds of sand. When it gets cold it forms solid blocks of iron. The blocks are called pigs of iron. The iron itself is known as pig-iron. It is also called cast-iron, because it is cast or poured in the liquid state into the sand moulds, and left to cool.

And is this pig-iron fit now to make all sorts of things? asked Norah.

Well, no, said Fred. "Cast-iron is not fit for every purpose. These cast-iron pigs have to be melted again in great pots in the puddling furnace. This is an intensely hot furnace where a flame is made to play over the iron, although the iron itself is never allowed to touch the fuel which is burning. In the blast furnace the iron got mixed up with some of the charcoal of the coal and coke. In the puddling furnace this is all burnt away, and then the melted iron is poured out into molds. It cools in the molds and forms solid blocks of iron. But you see it is still only cast-iron.

The blocks of iron are again put into the puddling furnace. This time the iron is not allowed to melt. It is simply heated till it is soft and plastic. Each block is then taken out of the furnace, and beaten while it is red-hot with immense hammers worked by steam engines. The iron is heated and hammered in this way again and again. When it has been worked up enough, it is cut into bars while it is soft.

It is now ready for use, and is called wrought iron.

SUMMARY

The red-hot, liquid iron is run off from the bottom of the blast furnace, just as we might run off water, or any other liquid, from a tap. It flows into sand molds on the floor, and when it is cold it forms great blocks or pigs of cast-iron. The pigor cast-iron is again melted in great pots, in the puddling furnace, and left to cool, after which it is heated till it is redhot and pliable, and in that state it is beaten for a long time with immense hammers, and cut up into bars while it is soft. It is then called wrought iron.