Most of these must await discussion in subsequent chapters, but one was vastly important because it made a radical alteration in the character of the product, and in the process. The original product was tough and pasty, and could not be melted but could be "wrought" directly under the hammer, while the product of the improved furnace could not be worked under the hammer, either hot or cold, passing immediately from the liquid state in which it could be "cast" or poured to the brittle solid state. Hence the names of the two principal varieties of iron, "cast" and "wrought".

The production of the iron in the liquid condition contributed further to the continuity of the process since the iron could be allowed to gather in a pool at the bottom of the furnace instead of requiring to be worked up through the fire at intervals in lumps or "loups", with fire tools; and the subsequent introduction of a fresh charge of ore.

The shaft-furnace along with these advantages brought a requirement of far-reaching importance. The column of stock must be penetrable to a considerable gas volume at a reasonable pressure. This requirement is just as important to-day as it was in the early day of which I have spoken, though it must be confessed that our ideas of a "reasonable pressure" have undergone great changes.

This penetrability of the stock column may be seriously if not fatally impaired by either of two conditions.

First, the presence of tarry or sticky matter which would fill up the interstices of the charge and make it practically gas-tight.

Second, great fineness of the charge, either of the ore or of the fuel. Moreover, the fuel is subject to a certain solvent action by the gas as will be described later, and this is more active in proportion to the fineness of the fuel so that serious loss occurs if the latter be too fine.

The two principal fuels available, wood and bituminous coal, in their native condition both contain tarry material, generally in important quantities. The vast majority of their varieties are, therefore, not adapted to use as furnace fuels without preliminary treatment to drive off the tar, so that this condition is industrially of paramount importance.

We can now combine all these necessary conditions to form a fundamental definition as follows:

The blast-furnace is a vertical shaft-furnace blown with air under pressure for reducing iron from Us oxides and delivering it in a liquid condition. Its fuel must be not only solid but in lumps of considerable size and must be sufficiently free from tarry components to permit the passage of the gas resulting from the combustion in the hearth.

Additional Requirements

We have given a definition of the fundamental requirements of the blast-furnace. To these may be added two more, which are necessities in the case of more than 99 per cent of the iron produced.

First, the slag must be sufficiently fusible to run freely from the hearth at a temperature easily attainable.

Second, the blast must be warmed before being delivered to the furnace.

The single exception to these two conditions occurs in the case of furnaces making cold-blast charcoal iron of which probably not over twenty or twenty-five thousand tons per year are produced out of a total of some thirty-five million tons in the United States. In this case the temperature attainable is very low and in consequence the slag is not heated hot enough to run freely and has to be removed from the furnace with the assistance of mechanical means such as slag-hooks, etc., or, if it comes from the furnace without assistance, it chills before going more than a few feet and is accordingly troublesome to remove.

Leaving out of account this very small percentage, all blast-furnaces are blown with blast heated from 500 deg. to 1400 deg. F. and their slag is heated to a degree sufficient to enable it to run from the furnace without assistance and far enough to reach some convenient means of ultimate disposal. The heated blast was introduced by the Englishman, Neilson, about 100 years ago, and it has made possible the modern development of the blast-furnace because the velocity of combustion of the fuel is so greatly augmented by preheating the blast, and the continuity of the operation has so increased by raising the molten iron and slag well above their fusion points, that outputs have been increased almost a hundredfold since.

Characteristics Required Of The Fuel

In addition to these fundamental requirements of a blast-furnace fuel, that it shall not be tarry and shall be in lumps, there are others of nearly equal importance.

The three characteristics next in importance to those mentioned are first, purity, particularly freedom from sulphur and phosphorus.

Second, porosity or ratio of surface to mass. Third, physical strength or resistance to crushing.