The slate, when burned, has a light yellowish colour, which would change the colour of the product. Figs. 30 to 32 represent a front elevation of the kiln and two sections at right angles to each other. The kiln is 22 feet high and 16 feet square on the outside. The interior is cylindrical, 5 feet in diameter, with a fire-brick lining _a_ of the best quality. The interior lining slopes from the fire-place _b_ to the door _c_, by which the charges are withdrawn; this facilitates the removal of the calcined ore. The casing _d_ is of sandstone, 5½ feet thick, and tied together with the best white-oak timber _e_. When charged, a kiln holds 16 tons of ore, and the kiln is kept constantly full. The heat passes from the fire-places _b_--of which there are two, placed diametrically opposite each other--through a checker-work _f_ of brick into the centre of the charge. The charge enters at _g_ and is withdrawn by a door _c_ in the front wall, 2 feet long and 18 inches high. The ashpit is at _i_. The fire is kept at a cherry-red heat, and about one cord of wood is burned every twenty-four hours. The kiln works continuously, calcined ore being withdrawn and fresh charges made without interruption. The ore is subjected for forty-eight hours to the heat, which expels the moisture, sulphur and carbon-dioxide. About 1½ tons of calcined ore are withdrawn every three hours during the day. The outside of the lumps of calcined ore has a light brown colour, while the interior shows upon fracture a darker brown. Great care is necessary to regulate the heat [Illustration: Figs. 30, 31, 32.--RUTHERFORD AND BARCLAY KILN.] so that the ore is not over burned. When this happens, the product has a black scoriaceous appearance, and is unfit for the manufacture of metallic paint, as it is extremely hard to grind. The calcined ore is carried from the kiln in wagons to the mill, where it is broken to the size of grains of corn in a rotating crusher. The broken ore is carried by elevators to the stock-bins at the top of the building, and thence by shutes to the hoppers of the mills, which grind it to the necessary degree of fineness. Elevators again carry it to the packing-machine by a spout, and it is packed into barrels holding 500, 300, or 100 lb. each. _Characters._--Ochres owe their colour to hydrated oxide of iron, besides which body they contain clayey matter (silicate of alumina), earthy matters, barytes, carbonate and sulphate of calcium, &c., dependent upon the locality from whence they are obtained; thus Derbyshire ochres contain mostly calcareous earthy matters, barytes, gypsum, &c., while Oxford ochres and French ochres contain clayey matter; Welsh ochres are variable, and usually contain a good deal of silicious matter.