The average ouput of blast-furnaces has increased in little more than a generation from about thirty tons per day to about five hundred tons per day. This great increase has occurred simultaneously with a decrease in the supply of labor relative to the demand, and a great increase in its cost.

These two circumstances have brought about great changes in the methods of handling the raw materials for pig-iron manufacture. These materials are so generally ore, coke and limestone, that unless definitely stated to the contrary, they will be meant when the term raw materials is used. In furnace parlance these are also known under the name of "stock," and the building in which the supply is stored to cover irregularities in daily receipts is known as the "stock house," a term which has survived the existence of any actual house.

The prime condition for successful operation of the furnace is that it must be run continuously. This is a fact that it will be necessary to emphasize many times in subsequent chapters. This term means literally twenty-four hours a day, seven days a week, fifty-two weeks a year, and for several years at a stretch. To enable furnaces to be operated in this way a supply of raw materials must always be on hand, in spite of interruptions to transportation and other conditions which make regular daily receipts impossible. The first and obvious one of these conditions is the occurrence of Sunday, when transportation is always partly, very often wholly, suspended. This makes it indispensable to have a supply of raw materials large enough as a minimum for two or three days' run, and rising as high as a six months' supply in many cases.

The small charcoal furnaces of the old days were generally built close alongside of a very steep hill, and had a bridge running from the top of the furnace over to a leveled yard or space on the hill, served by a wagon road with an approach long enough to permit the hill to be surmounted. All raw materials were delivered by team into the stock house built on this level space, and unloaded, at first by shoveling from the wagons, later by wagons with loose bottoms, which could be pulled out, one board at a time. These materials were shoveled by hand into filling barrows, which were then wheeled onto a platform scale set for the correct weight of the given materials. The weight was adjusted by shoveling into or out of the barrows to make the balance correct, and the barrows were then wheeled across the bridge to the top of the furnace and there dumped. Of other equipment besides shovels, barrows and scales there was none. The number of men employed was not very great because the quantity of material handled in a day was small. Fifty years ago it probably did not average much over one hundred tons in twenty-four hours, and as a small number of men could handle this amount of material, equipment would not have been needed had it been available, because the saving would have been too small.

With the great increase in the size of the units, conditions became more and more difficult. The gangs of men required to fill a furnace grew to be large. The work was hard and it was very difficult to maintain full gangs. It must be borne in mind that no delays could be tolerated, a short suspension of filling would let the furnace get "down," which would probably be reflected in poor iron or in some other form of bad furnace work the next day.

More than 80 per cent. of the iron produced in the United States is made from ores mined in the Lake Superior region and transported wholly or partly by water to the points of consumption. Navigation on the Great Lakes is open nominally seven months, but actually only about six in each year. Therefore furnaces which derive their ore supply from this region are under the necessity of having on hand a six months' stock of ore at the close of navigation. If the furnaces are some distance removed from the receiving port of the ore, the latter may be stored on the docks or at storage yards maintained by the railroads. But the best practice increasingly tends toward shipping the ore directly to the furnace plant, and storing it in an ore yard devoted to that exclusive purpose, and not infrequently equipped with powerful and expensive machinery for putting the ore into the stock pile and reclaiming it as needed throughout the year.

A plant of four furnaces, each of a daily capacity of five hundred tons of metal, is not a large one as furnace plants go to-day, but such a plant consumes four thousand tons of ore per day, or 1,460,000 tons per year, and all of this must be stored during six months or less, and half of it, say 750,000 tons, must be reclaimed from storage during the six months which includes the winter weather, and principally in the severe climate which characterizes the central district of the United States. As it is characteristic of the Lake Superior ores to carry from a minimum of five or six, to a maximum of twenty-two or twenty-three per cent. of moisture, with an average of ten or twelve per cent.; and as this moisture is not combined with the ore, but only held by it in the shape of moisture, these great stock piles freeze to a depth of several feet, and the ore being soft and plastic, like clay or sandy loam in most instances, the frozen mass is exceedingly hard. Machinery well able to handle the material in its unfrozen condition would be unable to stand the service on frozen ore for the briefest period. This adds another very severe condition which must be met in handling the raw materials for iron manufacture in a preponderating portion of the industry.