Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/359

F L IT R 345 cellular siliciuus stone called buhr-stone, the best qualities of which are obtained from La-Ferte-sous-Jouarre, department of Seine-et-Maine, France. Millstones are generally built up of segments, bound together around the circumference by an iron hoop, and backed with plaster of Paris. The bed stone is dressed to a perfectly flat plane surface, and a series of grooves or shallow depressions are cut in it, generally in the manner shown in fig. 5, which represents FIG. 5. Upper Millstone, lower surface. the grinding surface of an upper or runner stone. The grooves on both are made to correspond exactly, so that when the one is rotated over the other the sharp edges of the grooves, meeting each other, operate like a rough pair of scissors, and thus the effect of the stones on grain sub mitted to their action is at once that of cutting, squeezing, and crushing. The dressing and grooving of millstones is generally done by hand-picking, but sometimes black amorphous diamonds (carbonado) are used, and emery wheel dressers have likewise been suggested. The upper stone or runner is set in motion by a spindle on which it is mounted, which passes up .through the centre of the bed stone, and there are screws and other appliances for adjust ing and balancing the stone. Further, provision is made within the stone case for passing through air to prevent too high a heat being developed in the grinding operation; and sweepers for conveying the flour to the meal spout are also provided. The ground meal delivered by the spout is carried forward in a conveyer or creeper box, by means of an Archimedean screw, to the elevators, by which it is lifted to an upper floor to the bolting or flour-dressing machine. The form in which this apparatus was formerly employed consisted of a cylinder mounted on an inclined plane, and covered externally with wire cloth of different degrees of fineness, the finest being at the upper part of the cylinder whore the meal is admitted. Within the cylinder, which was stationary, a circular brush revolved, by which the meal was pressed against the wire cloth, and at the same time carried gradually towards the lower extremity, sifting out as it proceeded the mill products into different grades of fineness, and finally delivering the coarse bran at the extremity of the cylinder. For the operation of bolting or dressing, hexagonal or octagonal cylinders, about 3 fest in diameter and from 20 to 25 feet long, are now commonly employed. These are mounted horizontally on a spindle for revolving, and externally they are covered with silk of different degrees of fineness, whence they are called "silks" or silk dressers. Radiating arms or other devices for carry ing the meal gradually forward as the apparatus revolves are fixed within the cylinders, and there is also an arrangement of beaters which gives the segments of cloth a sharp tap, and thereby facilitates the sifting action of the apparatus. Like all other mill machines, the modifica Flour.... Bran tions of the silk dresser are numerous. Mill products are differently assorted and classified in various localities and different mills some distinguishing many qualities of flour and bran, and others making only three or four divisions. The following (from Professor Church s Food) may be taken as a fair average representation of the product of 100 lb of good white wheat : 1. Finest flour 42 tt>. 2. Seconds flour 18 3. Biscuit flour 9 4. Tails or tailings 3 Middlings 5. Middlings or fine sharps 8 6. Coarse sharps 3 7. Fine pollard 3 8. Coarse pollard 6 9. Long bran 3 Loss by evaporation, &c 5 In the Crown Mills, Glasgow, belonging to John Ure & Son, the classification and general average mill products, dealing with 50 qrs., are as follows : Flour 60 to 62 bags of 280 Overheads or coarse flour .... 2J ,, 3 280 Fine thirds 5~ 6 168 Thirds 10 168 Bran 20 ,,22 112 Lightwheat 1 ,, 2 240 Loss 14 ,, 18 lt> per quarter. An additional proportion of fine flour is obtained by dressing und remilling tailings and middlings, and the purification and regrinding of these products have now become of much consequence in connexion with the changed systems of milling rapidly coming into use. A great variety of middlings purifying machines have been introduced and eagerly pushed within the last few years, showing that this branch of economic milling is now receiving great attention. The Hungarian System or High Milling. The object of the low or flat milling process, as practised in Great Britain, is to produce at one grinding operation as large a propor tion of good finished flour as possible. In high milling, on the other hand, the stones are kept so far apart that grain is merely bruised in the first operation, and by a series of such grindings or bruisings, alternated with elaborate sifting, the bran and all the outer envelopes with the cerealin are detached, and a nucleus of very pure semolina only left. In this way a large proportion of very inferior branny flour is obtained in these early millings, and the proportion of exceedingly fine strong flour for which Austro-Hungarian millers are famous is comparatively small. It is only to the hard brittle wheats that the Hungarian system of mill ing is applicable, and the method is only practicable under circumstances where there is a demand for the two extreme qualities of mill product which result from the system. Within the last few years the Hungarian millers have very largely adopted the roller mills, either to supplant entirely or to supplement the stone grinding. Roller Mills. In this form of mill a pair of horizontal rollers rotate face to face, and the grain or other material is submitted to their action by passing between them. The nature of that action varies according to the modification of roller surfaces, the closeness of the rollers to each other, and the equal or differential rate at which they revolve. Rollers of metal, either steel or chilled iron, having a toothed surface, revolving at different rates of speed and at definite distances apart, have a cutting action on the grain submitted to them. Such rollers are employed in the Buchholz system for reducing hulled wheat to the condition of semolina, and a similar arrangement is employed in the machine of Ganz & Co. of Buda-Pesth. Fig. 6 shows a section of the face of a pair of such rollers, where B revolv ing slowly serves as a holder for the grain, which is cut by the sharp edges of A, revolving at a speed three times IX. 44