In recent years, since the very general introduction of concrete, it has been realized that slag allowed to cool slowly made one of the best aggregates that could be found, much better than limestone, because being of igneous origin it does not break up under the action of heat, as limestone has been known to do in case of fire. Cinder is exceedingly rough and therefore furnishes an excellent bond for the cement, and in general cinder concrete is very highly esteemed by those who have used it.

Fig. 232. Browning grab bucket open over pit.

Cinder is often recovered for concreting and road filling purposes by blasting the dumps made by ladle cars and loading them up with a steam shovel, but the cinder has generally run out in thin sheets which do not weld together, bo that the mass when it is reloaded breaks up into quite small pieces. Moreover, this process requires the double operation of removing the cinder in pots and then picking it up with the steam shovel.

Mr. D. T. Croxton, of the Cleveland Furnace Company, some years ago, initiated the plan of casting the cinder directly from furnace in the old-fashioned way, but handling it by mechanical means after it was cold, crushing, screening and selling it for concrete material.

After a course of development for several years in which he installed a considerable quantity of special machinery, he finally came to the simple practice of bedding heavy chains into the cinder bed before casting the cinder on to it, these chains being covered up so as to prevent the slag from burning them. The slag bed is served by an overhead crane which, after the slag is cold, takes hold of the ends of the chains at the sides of the bed and pulls them up through it. These chains are spaced a few feet apart and this breaks the slag bed into a material which can readily be picked up by a modern clam-shell bucket and loaded into cars for delivery to the crushing and screening plant.

This system without the mechanical equipment for handling the slag has been in use at other places where the slag has a commercial value as filling or building material.

One considerable advantage of handling the slag in this way is that a magnetic separator can be introduced into the path of the crushed slag and used to recover from it a considerable quantity of iron in pieces from the size of a pinhead up to several pounds weight. This is true even with good furnace practice and where no important quantity of iron is visible to the eye, since the iron is so much heavier than the cinder that it goes to the bottom and is covered over by the latter.

It has been the experience of almost everyone who made any attempt to recover iron from the slag to find more than was expected when the attempt was begun. I had once the pleasant experience of putting in a magnetic separator in the hope of recovering four or five tons a month from a charcoal furnace, and actually recovered fifteen to twenty-five.

Many other methods of handling cinder have doubtless been used and perhaps a considerable number of these are still in operation, but the ones here described cover all the important methods in use on a large industrial scale.