These large particles, semolina, are next passed between one or more pairs of smooth rolls known as reduction rolls (Fig. 15). These are set rather nearer together than the break rolls, and the difference in speed between each roll and its partner is quite small. The object of reduction is to reduce the size of the large particles of semolina and to produce thereby finely divided flour. The stock from the first pair or pairs of reduction rolls contains much finely ground flour mixed with coarser particles of kernel with or without adherent husk. It is at once submitted to the separation and purification processes as above described. This yields a large quantity of finished flour which is very white and free from husk. It represents commercially the highest grade of flour separated in the mill and is described technically as patents. A small amount of finished offal is also separated at this stage. The coarse particles of kernel with adherent husk from which the flour and offal have been separated are now passed through a second pair of break rolls more finely fluted than before, known as the second break. These are set closer together than the first break rolls. Their object is to rub off more kernel from the husk. The stock from them is again separated, the flour and finished offal being removed as before. The coarser particles are again reduced by smooth reduction rolls, and a second large quantity of flour separated. This is commercially high grade flour and is usually mixed with the patents already separated. The coarse particles left after this separation are usually subjected to a third and a fourth break, each of which is succeeded by one or two reductions. Separation of the stock and purification of the flour take place after each rolling, so that as soon as any flour or husk is finely ground it may be at once separated without further grinding. The last pair of fluted rolls, the fourth break, are set so closely together that they practically touch both sides of the pieces of husk which pass through them. They are intended to scrape the last particles of kernel from the husk. This is very severe treatment, and usually results in the production of much finely powdered husk which goes through the sifting silk and cannot be separated from the flour. The flour from the fourth break is therefore usually discoloured by the presence of much finely divided husk. For this reason it ranks as of low commercial grade. The later reductions too yield flours containing more or less husk, which darkens their colour. They are usually mixed together and sold as seconds.