A violent explosion is caused by molten iron running on to even the smallest wet spot on anything made of iron, but it cannot bore into the iron as it does into the sand in case of a boil, so that the explosion, while violent and dangerous to the men, is limited to a single shot and the iron cannot work down to any depth as it can in the sand. A bad boil in sand at the tap-hole would prevent its being shut and would put the furnace out of commission for an almost indefinite time, constituting a catastrophe. For this reason a few feet of iron runner or trough has been used next to the tap-hole for many years. This is thoroughly grouted and carefully dried because the grout prevents the iron from burning to the trough which it would do if the latter were bare, while careful drying eliminates the danger of explosion.

Up to within a few years the runners from this point on were entirely of sand and the necessary arrangements for skimming the cinder from the iron were also built up of sand. The cinder has a specific gravity only about one-fourth to one-third that of the iron, and while the two come out of the tapping hole together during the latter part of the cast, the cinder immediately rises to the surface and can be skimmed off without difficulty. To accomplish this the portion of the runner next to the iron trough is made very large relative to the portion below this section, and a dam of sand some six to twelve or more inches high is put into the bottom of it four to eight feet from the end of the iron trough; just above or in front of this dam is set a skimmer, formerly a thick plate of iron cast in open sand in the floor of the cast house, and guided by two pairs of pins driven into the sand of the runner on each side of it. When the cinder begins to show on the iron this skimmer is driven down with a hammer until its edge projects below the surface of the metal sufficiently to prevent the head of cinder floating on the iron from pushing under it, the cinder then rises and flows over a lateral dam, a low place in the side of the sand trough, and is led from there to its point of ultimate disposition. With large and rapid streams of iron which were the consequence of the growing outputs of the nineties, this operation became more diffi-cult and less certain. Either cinder was allowed to get down on to the iron, which created a fearful nuisance, as the cinder broke off when cold and fouled the pig beds, or the iron itself would be forced out over the cinder dam and wasted. Moreover there was left in the basin formed by the iron and cinder dams at the end of the cast a considerable volume of iron with a heavy layer of cinder on top of it. To direct this iron to its proper place and keep the cinder from following it was by no means an easy matter, and much iron was likely to be lost in the cinder without the most careful practice.

In order to overcome these difficulties Mr. Michael Killeen at the Edgar Thompson Works devised the Killeen skimmer. This consists of an additional section of iron trough linked to the section next the furnace. It has cast with it the cinder and iron dams, a slot in which to set the skimmer (preferably made of firebrick in an iron frame to prevent its cutting away during the cast, as the old iron skimmers did), and a supplementary lateral outlet extending clear to the bottom of the trough and closed by an iron gate with a staple in the top. Before cast this gate is set to place and the joint between it and the body of the skimmer trough well protected with sand and grout. With this arrangement the skimmer can project far enough below the surface of the iron to prevent the possibility of cinder getting down the iron runner, while at the same time the cinder dam can be raised so high as to prevent the iron from rising over it and running to waste. The considerable volume of iron remaining in the trough at the end of the cast is released by withdrawing the supplementary lateral gate and allowing the iron to run into a special runner which takes it either into a ladle or to a special bed set below the level of the other beds as may be preferred. The cinder is prevented from following it by a hand gate dropped when the iron is all discharged and before much of the cinder can follow. This not only prevents the iron from being wasted and the cinder from getting down the iron runner, but also drains the iron trough effectively to the bottom so that easy access is thereby afforded "to the tapping hole in order to stop it.

Other forms of skimmers have since been brought out, but are only variations of the general principle of the Killeen design.

Following the introduction of the iron skimming trough, iron runners were substituted for the sand runner all over the cast house. These are provided with grooves into which gates are set to cut off the flow of the metal, and are grouted after each cast with a mixture of sand and clay kept hot by a steam coil, the gate grooves being filled up flush with this mixture. With good practice only a very light skull of iron is left in these iron runners when the cast is over, and the runner requires only a new coating of sand and grout to make it ready for use again.

On account of its smoothness and uniformly sloping bottom the iron runner drains itself better than the sand runner, thus preventing the formation of any but light scrap; moreover, with the iron runner it is possible to raise a gate which has once been set and allow iron to flow again down a section of trough from which it had been cut off, which it is never safe to do with sand runners because the withdrawal of the gate from the sand is extremely likely to cause a boil. These advantages have resulted in the universal introduction of iron runners where the iron is run direct to the ladles and not cast in sand beds.