In the early days of the blast-furnace it was probably the custom to allow the iron to run out in puddles in front of the furnace and there cool, but soon this crude practice developed into making a runner leading away from the furnace and putting short lateral runners off from one side of it so as to mold the iron into convenient shape for future handling. The runner was rather crudely formed as were the lateral runners, which were few in number.

The general appearance of the whole arrangement was roughly similar to that of a sow with a litter of suckling pigs. From this comes the name of "pig iron," while the runner from which the individual "pigs" are cast is known to this day as the "sow".

In still later times as outputs grew the pigs were not cast directly from the main central runner, but this branched into lateral runners at right angles to its course, and from these, at right angles again and therefore parallel to the main runner, ran the individual pigs. The combination of the branch runner or sow and the pigs which go with it are known as a "bed".

This system of casting the iron persisted as practically the only method of handling it until very recent years. Even at the time of the "Duquesne Revolution" when nearly everything else about the furnace was improved almost past recognition, the handling of the iron was changed in detail rather than in principle.

Casting In Sand Beds

The runner and beds were until about twenty years ago practically always made up in sand, since sand does not wash in a liquid with a high surface tension like a molten metal, and at the same time is porous enough to permit the free escape of the gases generated by the action of the intense heat of the iron on the water and organic matter the sand contains. This is a matter of the utmost importance because if free escape be not permitted, when the pressure of the gases generated has risen to that corresponding to the depth of metal upon them, they bubble up through the liquid metal and this action is very vigorous because the head decreases as the gas rises, while the volume of the gas simultaneously increases as it bubbles up through the metal.

The consequence of these ebullitions is that the sand is disturbed when the gas is released, and the hot iron is given access to wetter and fresher sand underneath, with consequent generation of more gas and, its release in turn, with a still greater tendency toward violence. This action continues progressively, if it once gets started, until a "boil" results.

These are spectacles of impressive beauty, like magnificent fireworks, for those not concerned in their results, but are frightfully expensive, not only in the loss of iron, but in the difficulty of removing that which remains in the center after the boil is over.

The depth to which one of these boils will dig in a short time is almost beyond belief, and if one occurs close to the front of the furnace so that there is no room to deflect the iron above it, it constitutes one of the worst interruptions that can befall the operation of the furnace.

I have seen a mass almost wholly of iron, but with a little cinder on top, about the shape of a huge strawberry, six feet in height and more in diameter, taken out after a boil at the end of the iron trough of only a few minutes duration. The difficulty of digging down to the bottom of such a boil and draining the liquid iron from the bottom of it is enormous, and a delay of hours in the operation of the furnace is practically certain to occur as a result of such a catastrophe in this location. For this reason the quality of the sand used in the pig beds, and particularly in the runner, is of vast importance. It must be coarse and open-grained, practically free from clay to prevent boils, and must also be free from lime, oxide of iron or any similar material which can unite with the silica at the high temperature of the iron and form a slag, since if such a slag be formed it causes great waste of sand, dirty iron and a tendency to make boils by sealing the pores of the sand beneath.

Later Developments

The first great change chronologically from this established method of handling the iron was the introduction of the mixer, of which, however, it will be more convenient to treat later.