There is scarcely any feature of the construction and operation of the blast furnace of more importance than this and probably none concerning which less is known by the great majority of those to whom it is so important, the furnace managers.

The problem of firebrick has never been solved in terms of chemistry, or if so the solution is not known to furnacemen. This is probably because several other factors, such as the fineness of the grinding of the clay, and the method by which is it molded, the temperature and time of burning of the bricks, and other factors not reflected in their analysis, have a vital influence on their character.

For this reason firebrick are to a great extent bought and sold on the basis of workmanship, freedom of the bricks from warpage, and general reputation. It is true that iron and alkalies in more than moderate amounts are barred because both unite with the silica at low temperatures and cause the fluxing of the brick under conditions far less severe than can be met with impunity by bricks free from these impurities. But further than this chemistry does not play a heavy role in the purchase or sale of firebrick.

It was customary a generation or more ago to use very large bricks, a size 6 in. square on the small end, by 9 in. or 14 in. long was the standard in certain of the older iron-producing regions. But this was followed by a period in which a thickness of about 2 1/2 in. became standard, virtually the same as that of red-brick. These were made in lengths of 9 in. and 13 1/2 in. so that two of one would equal three of the other. The straight, commonly known as the "square" brick, is 13 1/2 in. long, 6 in. wide, or in the case of the tapered or circle brick, known as "keys," 6 in. wide at the larger end. Similarly the straight brick, 9 in. long, is 4 1/2 in. wide, and the "keys" 4 1/2 in. wide at the large end.

These are relatively small bricks and require much labor for laying and involve a great number of joints, but it was claimed by some fur-nacemen for many years that thicker bricks could not be burned satisfactorily, though I have seen years ago thousands of brick 6 in. thick which gave complete satisfaction in the furnaces in which they were used, which were small coke furnaces.

In very recent years firebrick manufacturers began to make furnace bricks 3 in. thick, and these are very much freer from warpage in the burning process than the thinner bricks as well as requiring less joints and less labor to lay them. They are, therefore, very much to be preferred from practically all points of view except that of cost.

In this respect the brickmakers have established certain customs which have not been permitted to any other trade. The standard price of bricks is based on the 9-in. by 4 1/2-in. by 2 1/4-in. brick. Smaller bricks than the 9-in. are charged for at the same rate per brick as if they were the 9-in. size. This is explained by the brick maker on the ground that the clay in the brick amounts to very little but that the workmanship on it is just as much on the small brick as it is on the large one. The 13 1/2-in. bricks, on the other hand, take a price of nearly 20 per cent. higher than the 9-in. based on the same volume.

When he is asked why this is so the brick maker replies that they are more difficult to handle and to burn.

Similarly when it comes to the 3-in. lining instead of the standard thickness, the brick maker insists that he must have an increase of about 20 per cent. in the price per unit of volume of these, although warpage in this case is very much less, the labor decidedly less, and the burning but little more.

It is, however, no more than fair to the brick manufacturer to say that quality does not always receive the consideration from the furnaceman that it should, and the manufacturer of a superior grade of brick is often forced to compete against a poor grade of brick on a piece basis, which is neither fair nor sensible, but is to a large extent the outcome of the difficulty in determining what real quality is, already described.

Firebrick are made in three grades known respectively as "hearth and bosh," "inwall," and "top," intended for use in those regions of the furnace for which they are named. Formerly the "hearth and bosh" quality was usually continued a number of feet above the top of the bosh, but in the best recent practice this quality stops at the top of the bosh.

These differences in quality are adapted to meet the difference in conditions in the different portions of the furnace. In the hearth and bosh there is little or no abrasion but intense temperature, and intense scouring action of slag and iron. To meet these conditions the bricks are made with a relatively large proportion of flint clay and a correspondingly smaller proportion of plastic clay. These bricks are not exposed to much abrasion and are therefore not particularly designed to resist it, but through their composition and burning, are made to stand extremely high temperatures successfully.

In the middle zone of the furnace in which the lining is known as the "inwall" the conditions of temperature and chemical action are considerably less severe, while those of abrasion due to the presence of some of the solid portions of the charge and the sand-blast action of its fine portions at the velocity imparted by the blast are important, and the cone in this portion of the furnace being convergent upward the sand-blasting action is a factor to be reckoned with, whereas in the bosh with its divergent upward flare this is not the case. Inwall bricks, therefore, are made with more plastic and less flint clay than hearth and bosh brick to meet these conditions. At the same time they still have high heat-resisting qualities.