In the early days of the blast-furnace the slag was handled as clinker or ashes, and this method of procedure survived until very recent times in the crane-operated shovel, fork, etc., which I have described in a previous chapter as adjuncts to charcoal furnaces in the seventies and which were used to clean out and "work down" the furnace very much as we find it necessray to do with the average domestic "heater" to-day. The difficulties of this method of operation, a growing knowledge of the use of flux, and the use of the hot blast with its correspondingly higher hearth temperatures, brought the possibility of discharging the slag in the liquid condition and this practice became general, though not universal, in the early part of the last half of the nineteenth century.

The slag was at first tapped out of the furnace with the iron, and after being skimmed off as previously described, was run off to a short distance into puddles in which it was allowed to cool. Slag being very brittle when cold is easily broken up by hand and was disposed of with carts or any convenient means, being simply loaded from the ground into the conveyance with shovels or forks. Carts dumped it into holes or low places, and enough of these were commonly found near the plant to take care of the slag output of those days for many years. Survivals of this method are in use even down to the present time, though they have now become rather rare.

When mineral coal was substituted for charcoal as fuel, the quantity of slag produced per ton of iron increased to some extent, but probably not in the ratio which the volume of charcoal slag bears to the volume of coke slag to-day, because in the early days of mineral fuel much leaner slags were used, it being impossible to carry as much lime as we can use with the high blast heats obtainable to-day. Nevertheless the increase in slag volume was very considerable and helped to bring about a change in the method of handling slag. The introduction of mineral fuel led also to larger outputs per day and this in conjunction with the considerably larger production of slag per ton made the slag per day to be handled with mineral fuel a much more important factor than it was with charcoal, so that the importance of slag disposal grew even more rapidly than the question of handling the pig iron output, and a vast number of methods have been devised and practiced in trying to handle it with maximum economy.

A comprehensive search would be required to obtain a reasonably complete list of the methods which have been used on an industrial scale in the past, but as only a few of them have survived this is not needed, a description of a few of the most commonly used will suffice.

The first important development was the choice of a definite dumping place, chosen with the idea of affording room for slag disposal within a limited area for a long term of years and the provision of railroad transportation for delivering the slag to it.

Probably the earliest improvement was to run the cinder into long parallel runners roughly made up in coke breeze so that the size of the individual bodies of cinder is limited, because while this material is very brittle as compared with iron, the difficulty of breaking it up grows very greatly with the size of the body into which it is cast since, if it is cast into puddles of several feet across and a foot or two thick, it cools very slowly, and this slow cooling anneals and greatly toughens it.

When cast in long runners of approximately semi-circular shape, a foot across and six inches thick, it was handled with hooks, one in the hands of each of a pair of men, who broke the runner of cinder up into pieces three or four feet long, and swung these on to a pile by catching them with a hook near each end. The sections of runner were piled roughly together and allowed to cool still further, after which they were loaded on to cars and taken away. This cleared the beds for the next fall of cinder.

In other cases quite large holes, approximately round, were dug into the ground, the cinder allowed to run into these, and a bar or hook of iron stuck into the center while it was still hot. After the cinder had hardened it was picked up by this hook, with the aid of a crane, and loaded for disposal. At the dump the cake was broken up and the hook or bar of iron recovered to be used again.

In other cases the cinder was cast into cakes on cars made entirely of iron, which had a perfectly flat top, bo hinged to the body of the car that it could be tipped laterally in either direction. On top of these were set cast-iron frames, the section of a truncated pyramid with the sides sloping inward at the top, without top or bottom. A joint between these and the deck of the car was made with breeze or fine cinder, and they were run full of cinder direct from the cinder trough at the furnace. After cooling, the iron frames or molds were lifted off with a crane, the train of cars taken to the cinder dump and the decks tipped, allowing the cakes to slide off over the dump. In this case of course, as with practically all dumps, the tracks must be moved either laterally or built ahead as the dumping space is used up. About thirty years ago the practice of handling the slag in the liquid condition began to receive quite general recognition, and has developed continuously and rapidly from that time. The first liquid cinder cars were simply huge rectangular tanks of steel plate, lined with firebrick, mounted on trucks, with a spout on each side closed by a hinged gate. The gate was closed, a little cold cinder thrown against it, and the tank was then filled with liquid cinder and hauled away. At the dump the gate was swung aside and the cinder chilled over the opening broken through, the contents of the car then ran out. But this method was open to serious objection because even when very quickly handled, slag forms quite a heavy skull under such conditions, and this skull fastening itself to the brickwork grows with each successive filling and emptying of the car until the capacity is so diminished that it must be cleaned out by hand, a laborious and time-consuming operation. If the car were allowed to stand until the cinder froze on top it made matters still worse as this additional skull also had eventually to be handled by hand.