221. Porcelain and Pottery.--Genuine porcelain and china-ware are made of a fine clay, kaolin, which results from the disintegration of feldspathic rocks. Bricks are baked clay. The FeO in common clay is oxidized to Fe2O3, on heating, a process which gives their red color. Some clay, having no Fe, is white; this is used for fire-bricks and clay pipes. That containing Fe is too fusible for fire-clay, which must also have much SiO2. The electric arc, however, will melt even this, and the most refractory vessels are of calcium oxide or of graphite. Pottery is clay, molded, baked, and either glazed, like crockery, or unglazed, like flower-pots. Jugs and coarse earthenware are glazed by volatilizing NaCl in an oven which holds the porous material. This coats the ware with sodium silicate. To glaze china, it is dipped into a powder of feldspar and SiO2 suspended in water and vinegar, and then fused. If the ware and glaze expand uniformly with heat, the latter does not crack. Chapter XLIII. METALS AND THEIR ALLOYS. 222. Comparison of Metals and Non-Metals.--The majority of elements are metals, only about a dozen being non-metallic in their properties. The division line between the two classes is not very well defined; e.g. As has certain properties which ally it to metals; it has other properties which are non-metallic. H occupies a place between the two classes. The following are the more marked characteristics of each group: - METALS. 1. Metals are solid at ordinary temperatures, and usually of high specific gravity. Exceptions: Hg is liquid above -39.5 degees; Li is the lightest solid known; Na and K will float on water. 2. Metals reflect light in a way peculiar to themselves. They have what is called a metallic luster. 3. They are white or gray. Exceptions: Au, Ca, Sr are yellow; Cu is red. 4. In general they conduct heat and electricity well. NON-METALS. 1. Non-metals are either gaseous or solid at ordinary temperatures, and of low specific gravity. Exceptions: Br is a liquid; I has the heaviest known vapor. 2. Non-metallic solids have different lusters, as glassy, resinous- silky, etc. Exceptions: I, B, and C have metallic luster. 3. Non-metals have no characteristic color. 4. They are non-conductors of heat and electricity. Exceptions: C and some others are conductors. 5. They are usually malleable and ductile. 6. They form alloys, or "chemical mixtures," with one another, similar to other solutions. Exceptions: Some, as Ph and Zn, will not alloy with one another. 7. Metals are electro-positive elements, and unite with O and H to form bases. Exceptions: Some of the less electro-positive metals, with a large quantity of O, form acids, as Cr, As, etc. Numbers 2, 6, and 7 are the most characteristic and important properties. 5. They are deficient in malleability and ductility. 6. They often form liquid solutions, similar to alloys in metals. 7. Non-metals are electronegative, and with H, or with H and O, form acids.