Throwing

is performed upon a tool called the potter’s lathe. (See fig. , infrà. ) This consists of an upright iron shaft, about the height of a common table, on the top of which is fixed, by its centre, a horizontal disc or circular piece of wood, of an area sufficiently great for the largest stoneware ve el to stand upon. The lower end of the shaft is pointed, and runs in a conical step, and its collar, a little below the top-board, being truly turned, is embraced in a socket attached to the wooden frame of the lathe. The shaft has a pulley fixed upon it, with grooves for 3 speeds, over which an endle band pa es from a fly-wheel, by whose revolution any desired rapidity of rotation may be given to the shaft and its top-board. This wheel, when small, may be placed alongside, as in the turner’s lathe, and then it is driven by a treadle and crank; or when of larger dimensions, it is turned by the arms of a labourer. Sometimes, indeed, the wooden plate is replaced by a large thick disc of Paris plaster, which is whirled round by the hand of the potter, without the intervention of a pulley and fly-wheel, and affords sufficient centrifugal power for fashioning small ve els. The ma of dough to be thrown, is weighed out or gauged by an experienced hand. The thrower dashes down the lump on the centre of the revolving board, and dipping his hands frequently in an adjoining tub of water, he works up the clay into a tall irregular cylinder, and then down into a cake, alternately, till he has secured the final extrication of air-bubbles, and then gives the proper form to the ve el under a le speed of rotation, regulating its dimensions by wooden pegs and gauges. He now cuts it off at the base with a piece of fine bra wire, fastened to a handle at either end. The ve el thus rudely fashioned is placed in a situation where it may dry gradually to a proper point. At a certain stage of the drying, called the green state , it po e es a greater tenacity than at any other, till it is baked. It is then taken to another lathe, called the turning lathe, where it is attached by a little moisture to the vertical face of a wooden chuck, and turned nicely into its proper shape with a very sharp tool, which also smooths it. After this it is slightly burnished with a smooth steel surface. DESCRIPTION OF THE POTTER’S LATHE. IMG:4147767755307473660_illo1012.png:Potter A , fig. 891. , is the profile of the English potter’s lathe, for blocking out round ware; C is the table or tray; a is the head of the lathe, with its horizontal disc; a , b , is the upright shaft of the head; d , pulleys with several grooves of different diameters, fixed upon the shaft, for receiving the driving-cord or band; k is a bench upon which the workman sits astride; e , the treadle foot-board; l , is a ledge-board, for catching the shavings of clay which fly off from the lathe; h is an instrument, with a slide-nut i , for measuring the objects in the blocking out; c is the fly-wheel with its winch-handle r , turned by an a istant; the sole-frame is secured in its place by the heavy stone p ; f is the oblong guide-pulley, having also several grooves for converting the vertical movement of the fly-wheel into the horizontal movement of the head of the lathe. D is one of the intermediate forms given by the potter to the ball of clay, as it revolves upon the head of the lathe. In large potteries, the whole of the lathes, both for throwing and turning, are put in motion by a steam-engine. The vertical spindle of the lathe has a bevel wheel on it, which works in another bevel toothed wheel fixed to a horizontal shaft. This shaft is provided with a long conical wooden drum, from which a strap ascends to a similar conical drum on the main lying shaft. The apex of the one cone corresponds to the base of the other, which allows the strap to retain the same degree of tension (see the conical drum apparatus of the Stearine-pre ), while it is made to traverse horizontally, in order to vary the speed of the lathe at pleasure. When the belt is at the base of the driving-cone, it works near the vortex of the driven one, so as to give a maximum velocity to the lathe, and vice versâ . During the throwing of any article, a separate mechanism is conducted by a boy, which makes the strap move parallel to itself along these conical drums, and nicely regulates the speed of the lathe. When the strap runs at the middle of the cones, the velocity of each shaft is equal. By this elegant contrivance of parallel cones reversed, the velocity rises gradually to its maximum, and returns to its minimum or slower motion when the workman is about finishing the article thrown. The strap is then transferred to a pair of loose pulleys, and the lathe stops. The ve el is now cut off at the base with a small wire; is dried, turned on a power lathe, and polished as above described. The same degree of dryne which admits of the clay being turned on the lathe, also suits for fixing on the handles and other appendages to the ve els. The parts to be attached being previously prepared, are joined to the circular work by means of a thin paste which the workmen call slip , and the seams are then smoothed off with a wet sponge. They are now taken to a stove-room heated to 80° or 90° F., and fitted up with a great many shelves. When they are fully dried, they are smoothed over with a small bundle of hemp, if the articles be fine, and are then ready for the kiln, which is to convert the tender clay into the hard biscuit . A great variety of pottery wares, however, cannot be fashioned on the lathe, as they are not of a circular form. These are made by two different methods, the one called pre -work , and the other casting . The pre -work is done in moulds made of Paris plaster, the one half of the pattern being formed in the one side of the mould, and the other half on the other side: these moulding-pieces fit accurately together. All ve els of an oval form, and such as have flat sides, are made in this way. Handles of tea-pots, and fluted solid rods of various shapes, are formed by pre ure also; viz., by squeezing the dough contained in a pump-barrel through different shaped orifices at its bottom, by working a screw applied to the piston-rod. The worm-shaped dough, as it i ues, is cut to proper lengths, and bent into the desired form. Tubes may be also made on the same pre ure principle, only a tubular opening must be provided in the bottom plate of the clay-forcing pump. The other method of fashioning earthenware articles is called casting , and is, perhaps, the most elegant for such as have an irregular shape. This operation consists in pouring the clay, in the state of pap or slip, into plaster moulds, which are kept in a desiccated state. These moulds, as well as the pre ure ones, are made in halves, which nicely correspond together. The slip is poured in till the cavity is quite full, and is left in the mould for a certain time, more or le , according to the intended thickne of the ve el. The absorbent power of the plaster soon abstracts the water, and makes the coat of clay in contact with it quite doughy and stiff, so that the part still liquid being poured out, a hollow shape remains, which when removed from the mould constitutes the half of the ve el, bearing externally the exact impre of the mould. The thickne of the clay varies with the time that the paste has stood upon the plaster. These cast articles are dried to the green state, like the preceding, and then joined accurately with slip . Imitations of flowers and foliage are elegantly executed in this way. This operation, which is called furnishing , requires very delicate and dexterous manipulation. The saggers for the unglazed coloured stoneware should be covered inside with a glaze composed of 12 parts of common salt and 30 of potash, or 6 parts of potash and 14 of salt; which may be mixed with a little of the common enamel for the glazed pottery saggers. The bottom of each sagger has some bits of flints sprinkled upon it, which become so adherent after the first firing as to form a multitude of little prominences for setting the ware upon, when this does not consist of plates. It is the duty of the workmen belonging to the glaze kiln to make the saggers during the intervals of their work; or if there be a relay of hands, the man who is not firing makes the saggers. The English kilns differ from those of France and Germany, in their construction, in the nature of their fuel, and in the high temperature required to produce a surface sufficiently hard for a perfectly fine glaze. When the ware is sufficiently dry, and in sufficient quantity to fill a kiln, the next proce is placing the various articles in the baked fire-clay ve els, which may be either of a cylindrical or oval shape; called gazettes , Fr.; kapseln , Germ. These are from 6 to 8 inches deep, and from 12 to 18 inches in diameter. When packed full of the dry ware, they are piled over each other in the kiln. The bottom of the upper sagger forms the lid of its fellow below; and the junction of the two is luted with a ring of soft clay applied between them. These dishes protect the ware from being suddenly and unequally heated, and from being soiled by the smoke and vapours of the fuel. Each pile of saggers is called a bung . POTTERY KILN OF STAFF ORDS HIRE. IMG:4147767755307473660_illo1013.png:Staff ords hire kiln Figs. 892 , 893 , 894 , 895 , 896. , represent the kiln for baking the biscuit, and also for running the glaze, in the English potteries. a , a , figs. 892 , 893 , and 894 , are the furnaces which heat the kiln; of which b , in fig. 892. , are the upper mouths, and b′ the lower; the former being closed more or le by the fire-tile z , shown in fig. 896. f is one fireplace; for the manner of distributing the fuel in it, see fig. 896. g , y , figs. 892. and 896. , are the horizontal and vertical flues and chimneys for conducting the flame and smoke. l is the laboratory, or body of the kiln; having its floor k sloping slightly downwards from the centre to the circumference. x , y , is the slit of the horizontal register, leading to the chimney flue y of the furnace, being the first regulator; x , u , is the vertical register conduit, leading to the furnace or mouth f , being the second regulator; v is the register slit above the furnace, and its vertical flue leading into the body of the kiln; v′ , c , slit for regulating flue at the shoulder of the kiln; i is an arch which supports the walls of the kiln, when the furnace is under repair; c , c , are small flues in the vault s of the laboratory. h , fig. 893. , is the central flue, called lunette , of the laboratory. T , T , is the conical tower or howell , strengthened with a series of iron hoops, O ′ is the great chimney or lunette of the tower; p is the door of the laboratory, bound inside with an iron frame. A , is the complete kiln and howell , with all its appurtenances. B , fig. 893. , is the plan at the level d , d , of the floor, to show the arrangement and distribution of all the horizontal flues, both circular and radiating. C , fig. 894. , is a plan at the level e , e , of the upper mouths b , of the furnaces, to show the disposition of the fireplaces of the vertical flues, and of the horizontal registers, or peep-holes. D , fig. 894. , is a bird’s-eye view of the top of the vault or dome s , to show the disposition of the vent-holes c , c . IMG:4147767755307473660_illo1014.png:Details of kiln E , fig. 895. , is a detailed plan at the level c , c , of one furnace and its dependencies. F , fig. 896. , is a transverse section, in detail, of one furnace and its dependencies. The same letters in all the figures indicate the same objects.