The modern tendency to reduce milling-costs by increasing the mill-capacity has demanded a greater duty from rolls than ever before, and in the larger mills rolls are now employed from 36 up to 54 in. in diameter, and from 15 to 28 in. width of face. Such rolls are used mainly for coarse crushing ; that is, they take the product from the jaw or the gyratory crusher, from 1.5 to 2.5 in. in size, and reduce it to about 0.5 in. These coarse or No. 1 rolls are then followed by other rolls set closer together for finer crushing, and possibly by others which re-crush certain middlings products obtained in the process of ore-treatment, or even tailings, dependent upon the nature of the ore and its association. Rolls of this general character require massive construction and excellent workmanship. Rolled-steel tires can now be obtained up to 54 in. in diameter. Special hard steels, such as chrome- and manganese-steels, are also used for certain ores, either in the form of tires or of plates bolted to a central mandrel. In this way the life of the crushing-surfaces has been much prolonged.