BRICK 



521 



horse-mill. The kneading of the clay is, in fact, the most laborious but indispensable 

 part of the whole business ; and that on which, in a great measure, the quality of the 

 brick depends. All the stones, particularly the ferruginous, calcareous, and pyritous 

 kinds, should be removed, and the clay worked into a homogeneous paste with as little 

 water as possible. 



Mr. F. W. Simrns, C.E., communicated to the Institution of Civil Engineers, in 

 April and May, 1843, an account of the process of brick-making for the Dover 

 Eailway. The plan adopted is called slop-moulding, because the mould is dipped into 

 water before receiving the clay, instead of being sanded as in making sand-stock 

 bricks. The workman throws the proper lump of clay with some force into the 

 mould, presses it down with his hands to fill the cavities, and then strikes off the 

 surplus clay with a stick. An attendant boy, who has previously placed another 

 mould in a water trough by the side of the moulding table, takes the mould just filled, 

 and carries it to the floor, where he carefully drops the brick from the mould, on its 

 flat side, and leaves it to dry ; by the time he has returned to the moulding table, and 

 deposited the empty mould in the water trough, the brickmaker will have filled the 

 other mould for the boy to convey to the floor, where they are allowed to dry, and 

 are then stacked in readiness for being burned in clamps or kilns. The average pro- 

 duct is shown in the following Table : 



It appears that while the produce in sand-stock bricks is to that of slop-bricks, in 

 the same time, as 30 to 16, the amount of labour is as 7 to 4 ; while the quantity of 

 land, and the cost of labour per thousand, are nearly the same in both processes. The 

 quantity of coal consumed in the kiln was at the rate of 10 cwt. 8 Ibs. per 1000 

 bricks. The cost of the bricks was 21. Is. 6d. per thousand. The slop-made bricks 

 are fully 1 pound heavier than the sand-stock. Mr. Bennett states that at his 

 brick-field at Cowley, the average number of sand-stock t-icks moulded per day 

 was 32,000 ; but that frequently so many as 37,000, or even 50,000, were formed. 

 The total amount in the shrinkage of his bricks was Tths of an inch upon 10 inches 

 in length ; but this differed with the different clays. Mr. Simms objects to the use of 

 machinery in brick-making, because it causes economy only in the moulding, which 

 constitutes no more than about one-eighth of the total expense. 



The principal machines which have been worked for this purpose are three 1st, 

 the pug-mill ; 2nd, the wash-mill ; 3rd, the rolling-mill. 



The pug-mill is a cylinder, sometimes conical, generally worked in a vertical posi- 

 tion, with the large end up. Down the centre of this is a strong revolving vertical 

 shaft, on which are hung horizontal knives, inclined at such an angle as to form por- 

 tions of a screw, that is, the knives follow each other at an angle forming a series of 

 coils round this shaft. The bottom knives are larger, and vary in form, to throw off 

 the clay, in some mills vertically, in others horizontally. Some have on the bottom 

 of the shaft one coil of a screw, which throws the clay off more powerfully where it 

 is wished to 'give pressure. 



The action of this mill is to cut the clay with the knives during their revolution, 

 and so work and mix it, that on its escape it may be one homogeneous mass, without 

 any lumps of hard untempered clay; the clay being thoroughly amalgamated, and in 

 the toughest state in which it can be got by tempering. This mill is an excellent 

 contrivance for the purpose of working the clay, in combination with rollers ; but 

 if only one mill is worked, it is not generally adopted, for, although it tempers, 

 mixes, and toughens, it does not extract stones, crush up hard substances, or 

 free the clay from all matters injurious to the quality of the ware when ready for 

 market. This mill can be worked by either steam, water, or horse-power; but 

 it takes much power in proportion to the quantity of work which it performs. 

 If a brick is made with clay that has passed the pug-mill, and contains stones, 

 or marl not acted on by weather, or lime-shells, (a material very common in clays), 

 or any other extraneous matter injurious to the brick, it is apparent from the 

 action of this mill that it is not removed or reduced. The result is this, the bricks 

 being when moulded in a very soft state of tempered material, or mud, considerably 

 contract in drying, but the stones or hard substances not contracting, cause the clay 



