Page:The American Cyclopædia (1879) Volume XIII.djvu/812

 788 POTTERY AND PORCELAIN of the lathe, and then presses it firmly with his hands, which he wets in a vessel of water conveniently near, forming it first into a con- ical shape,- represented at 5, fig. 16, by which means the remaining portions of air are work- ed out of it, and it is also rendered more plas- tic. The workman then forces his thumbs into the centre of the mass, holding his fingers on the outside, and gives it the form shown at c. Then, by placing one hand upon the in- side and the other upon the outside, the forms shown at d and e are given ; and afterward, by means of the simplest toc.s, made of wood or leather, which are kept wet, the thickness of the article is still further reduced, its general dimensions enlarged, and its shape perfect- ed. With a fine wire which he holds at each end the workman separates the piece of ware from the block, and it is then removed to a shelf, usually in an adjoining room, to dry, after which it is placed in the kiln with other FIG. 17 Kiln for Common Stoneware. like articles. The kilns for common stone- ware vary in form. In England they often have much the form of an ordinary reverbera- tory furnace, as shown in fig. 17, where r is the grate, a the ash pit, e the baking cham- ber, and d the chimney. A vertical form, such as that for earthenware shown further on, 1 1 1 I FIG. 18. Cells of Stoneware Kiln. is generally used in the United States. The articles are placed in cells formed of baked slabs, as shown in fig. 18. The hot air from the furnace, passing through these open cells, raises the clay to the proper temperature. The firing may continue from 24 to 4$ hours, de- pending upon the size of the pieces, the fusi- bility of the clay, and the extent of vitrifica- tion which may be desired. Toward the close of the operation a quantity of common salt is thrown into the kiln, which is volatilized by the heat, and coming in contact with the ware is decomposed ; the sodium combines with the silica of the clay, forming a soda glass, which again fusing with the clay beneath forms a tough, vitreous, earthy substance of great du- rability. On the continent of Europe common stoneware often receives no glaze other than that conferred by high firing. The manufac- ture of earthenware requires more care in the selection and preparation of materials and more skill in the workmen. The best materials are kaolin, pipe clay, quartz or flints, and feldspar, the kaolin and quartz to impart hardness, and the pipe clay and feldspar to yield a flux suffi- cient to bind the mass firmly together. The proportions of course vary with the quality of the clays and the fineness of the ware which is to be made. When the fineness and hardness exceed a certain degree, the ware ceases to be called common earthenware, but is technical- ly called fine stoneware. The proportions of materials for a good article of Staffordshire cream-colored (c. c.) ware may be stated as follows : pipe clay 40, kaolin 25, quartz 20, and feldspar about 15 parts in 100. If the pipe clay is quite infusible, less must be taken of it, with a greater proportion of feldspar. The quartz and feldspar are procured in this country of the manufacturer, already ground to a fine dust. Many of the best potters mix the mate- rials together in the dry state, and then thor- oughly incorporate them in a rotary " blunger " moved by steam power. This blunger is a horizontal cylindrical or octagonal box 10 or 15 ft. long by 3 or 4 ft. in diameter, within which is a horizontal shaft armed with strong paddles placed like the teeth of a threshing machine, only larger and longer and revolving with much less velocity. Water enough is added to make a thin mixture called " slip," which, after being worked in the blunger for a sufficient time to effect thorough incorpora- tion, is drawn off through a trough and also a fine silk gauze strainer, and received in a vat, from which it is forced by a powerful hy- draulic pump into a system of strainers or consolidating- apparatus patented by Needham and Kite. The machine consists of a number of flat shallow vessels composed each of two parts, as shown in fig. 19, and containing a stout fine hempen bag, suspended on hooks, and having a nozzle into which the slip is forced from the pipes d and c, c, c, fig. 19, which shows the apparatus in working posi- tion. Grooves against which the bags rest are shown at 5, and the holes in both a and are for the purpose of allowing the water escape. It is received in a channel in the floe beneath the machine. The mass is thus