Page:The American Cyclopædia (1879) Volume XIII.djvu/814

 790 POTTERY AND PORCELAIN FIG. 23. Seggar. but each seggar forms a cover to its fellow beneath. Seggars are made of infusible fire clay mixed in a pug mill with old broken seg- gars, ground to a granular powder. The old seggars, being com- posed of already burned and shrunk- en material, allow of less contraction in baking, and preserve . their shape better during a hard firing. Experienced potters do not mix sand with them, because it is thought that when made as above described they are more homogeneous and less liable to crumble when shrinking. A seggar is made as follows. The mass, having been placed in a compact pile, is cut in slices by a sharp spade and thrown with a smart blow upon a stout plank table until a sheet sufficient for the sides of two or three seggars is formed. It is com^ pacted with a broad-faced mallet swung smart- ly with both hands, and a compressing roller or smoothing tool is then run over it, reducing it to a uniform thickness (about three fourths of an inch to an inch). A long straight-edge ruler is then laid upon the sheet, and it is cut into strips of the proper width "for the height of the seggar, and of a length equal to its cir- cumference. An assistant (usually a boy) in the FIG. 24. Section of Earthenware Kiln. mean time has formed a circular or oval sheet for the bottom of the seggar. The seggar ma- ker takes a wooden form and wraps the strip which he has cut around it, and connects the ends together. He then fits on the bottom and removes the form. It is now ready for the kiln, and is fired at a white heat for 36 or 48 hours. In these seggars the ware is placed, and they are then piled one upon the other in the kiln. In placing the ware in the seggar the bottom is lightly covered with infusible fire sand to prevent adhesion. Sometimes granu- lated quartz is used. The seggars are piled in columns as shown in fig. 24, which are so ar- ranged as to cause the most equable diffusion of heat. The kiln is then fired, and the ware raised to a white heat, which is continued for about 36 hours. This forms what is termed hard or earthenware biscuit. The fires are now allowed to go out and the kiln to cool, and as soon as the workmen can enter the seggars are removed and the biscuit taken out. It is very hard, ringing when struck, and so porous that it will absorb water rapidly. It is now dressed, all rough prominences being removed, when it is ready to receive the glazing by which it is rendered impervious to liquids and given a polished surface. The glaze of earthenware is always much more fusible than the body, and is baked with a dull cherry-red heat. It is usually composed of white clay, ground quartz, feldspar, and white lead, with small quantities of sal soda and boracic acid. Fine stoneware, sometimes called ironstone china and white granite (w. g.) ware, is made of rather more infusible materials than common earthenware, and is baked with a higher heat. The glazing is much the same, though less fusible and of more carefully selected materials. A "frit" is generally used as one of the ingredients of fine stoneware glaze, or the whole glaze may be made into a frit, which may afterward be ground to an impalpable pulp. The following is a frit commonly used : ground feldspar 25 per cent., ground quartz or flint 25, sal soda 25, plastic clay 15, boracic acid 10. The ma- terials are mixed together and fused in a frit furnace, which consists of a long chamber in the bottom of which the material is laid after having been well mixed. The fire enters at one end and passes out at the other, much after the manner of an ordinary reverbatory furnace. A white heat melts the mass to a liquid, which is then drawn off by the removal of a plug, and received into a brick or stone vat. When cool it is broken in pieces and ground to a fine paste with water in a circular tub-shaped vat, similar to those 'used in England in grinding Cornish stone for the preparation of porcelain clay. Such a mill is shown in fig. 25. A shaft, by which the mill is turned, passes up through .a hollow cylinder, and has attached to it strong arms, to which are fastened ver- tical slats, by which buhrstones are carrk ~ around over the bottom of the vat, which also composed of pieces of buhrstone. The frit after being ground is mingled with whit lead, and usually a small amount of cobalt blue, which gives fine stoneware its pearl whiteness. Before using, the glaze is mingle