One of the most important details of the whole system of charging a furnace, which as a whole is probably the most important single part of blast-furnace operation, is the weighing of the materials. It is customary to look upon the blast-furnace as a rough, not to say crude, apparatus. This view is justified in part by the enormous forces which it must handle and keep under control. But as a matter of fact in spite of its size it is one of the most sensitive pieces of apparatus in industrial use and runs on smaller margin of safety in fuel than any other of our great industrial operations. Changes of the "burden" (a technical name for the weight of ore for a given fuel charge) as small as one per cent are not infrequently used in adjusting the operation of the furnace to make the kind of iron desired with a minimum consumption of fuel. Changes of two or three per cent are as large as are often made to adjust for the ordinary variations of operation. Such adjustments necessitate accurate weighing, and the lack of this has often been accountable for bad furnace operation, and even more often, for cases in which claims of remarkable furnace work were based upon inadequate or incorrect knowledge of the weights of the different materials actually used. Not infrequently the consequence has been a bitter disappointment when the book inventory of the material on hand was compared with the actual stock of such material.

Cases are on record too numerous to mention in which the books have shown the presence in the stock house of thousands of tons of material, and the financial returns have been estimated on this basis only to find on investigation that the material, so plainly shown by the books, had been put through the furnace and was conspicuous by its absence from the stock house. This has sometimes happened with the knowledge, or even through the deliberate action of a dishonest superintendent, but has also often happened to an honest one who had too great confidence in his scale weights. This condition is now so well understood that the financial management generally insists on a monthly or weekly inventory of the actual material on hand, and its comparison and reconciliation with the figures taken from the furnace books.

The scales used for charging purposes have to meet certain special conditions, and therefore were early designed by the scale makers for this purpose, and given the name of "charging scales".

Fig. 24. Furnace charging scale.

The type of these in common use with the barrow system of filling is shown by Fig. 24 (Fairbanks & Co.). These are platform scales with the base sunk in the floor of the stock house so as to bring the platform level with the floor. In recent years these have nearly always been of the suspended type, the platform being carried by four suspension rods from the scale-levers over head. This takes the latter up out of the way and out of the dirt in which they were necessarily more or less buried with the older type of scales, in which the levers were in a pit below the platform.

A special condition that these scales have to meet is that as a general thing at least three and generally more kinds of material have to be weighed - fuel, ore, and flux (limestone or dolomite), and a definite amount of each is required for each charge.

In order to meet these conditions the scales are built with from three to six separate beams, any one of which can be lifted up out of operation or lowered onto a frame depending from the main beam, so that the two operate as one. The main beam is usually used to balance the weight of the empty barrows, and one of the secondary beams is then thrown in for each material to be filled, its pea being locked fast for the charge weight wanted, this particular beam then rises and falls according to the weight of the given charge in the barrow. When this has been adjusted by adding or subtracting material this beam is thrown out and that for the next material thrown in.

The introduction of mechanical filling brought about the necessity for a different arrangement of the scales, and the trucks which carry the buckets to the bins in the bucket system of filling are provided with scales under the platform on which the bucket rests; or in the case of the larry car used with the skip, the hopper is carried directly by the scales which constitute a part of the truck. The same system of beams has in some cases been used as in the stationary charging scales, but in more recent years it has been found extremely desirable to use a recording type of scale.

The first of these ever installed, so far as known to me, was built by the Streeter-Amet Company of Chicago, for the order of Mr. W. L. Kluttz, then superintendent of the Thomas plant of the Republic Iron and Steel Company.

This scale carries a large circular dial, around which a pointer travels as the weight on the scale increases. Certain fixed markers are attached to the face of the scale, in positions corresponding to the burden of the different materials wanted, and the scale-car operator can watch the gradual rising of this pointer toward the desired weight, so securing much more accurate weights than is possible when the only indication given by the scale occurs during the brief instant while it is in balance, since then it is generally overloaded before the supply of material running in can be cut off.