THE NONMETALLIC MINERALS. 345 



fusion sufficient to make a body that will not absorb water. And its 

 surface can be made smooth and clean by a suitable plain or orna- 

 mented glaze. Ware of this kind is porcelain or china. 



The analyses on page 349, compiled from works believed to be authori- 

 tative, show the varying character, so far as chemical composition is 

 concerned, of the clays. In most of the analyses, it will be observed, 

 the silica existing in the form of quartz is given in a separate column 

 from the combined, while in column 4 is given the actual calculated 

 percentage of kaolin which the analyses indicates each sample contains. 



Refractory materials. Modern improvements in metallurgy, and in 

 furnaces for all purposes, are dependent to a great degree on having 

 materials for construction which will stand intense heat without 

 fusing, cracking, or yielding in any way. The two materials to 

 which resort is had in almost all cases, are pure clay, and quartz in 

 the form of sand or rock. They are both infusible at the highest 

 furnace heats. The clay, however, is liable to have in it small quan- 

 tities of impurities which are fusible, and it shrinks very much when 

 heated to a high temperature. Quartz rocks are very liable to crack 

 to pieces if heated too rapidly, and both the rocks and sand are rap- 

 idly melted when in contact with alkalies, earths or metallic oxides, at 

 a high temperature. They do not shrink in heating. Sandstone, or 

 quartz rock, is not as much used as a refractory material as it was 

 formerly. Bricks to resist intense heat are made of cla} T , of sand, 

 and of a mixture of clay and sand. The different kinds are specially 

 adapted to different uses. 



Fire bricks made of clay, or clay and sand, are the ones which have 

 been generally made in the United States. To make these, the clay 

 which stands an intense heat the best, is selected as the plastic mate- 

 rial of the brick. This is tempered so that it may not shrink too 

 much or unevenly in burning, by adding to the raw clay a portion of 

 clay which has been burned till it has ceased to shrink and then 

 ground, or a portion of coarse sand, or a quantity of so-called feld- 

 spar. These materials are added in the proportions which the experi- 

 ence of the manufacturer has found best. The formula for the 

 mixture is the special property of each manufacturer, and is not 

 made public. The materials, being mixed together and properly wet, 

 are molded in the same way as common bricks arei, and after they 

 have dried a little, they are put into a metallic mould and subjected to 

 powerful pressure. They are then taken out, dried, and burned in a 

 kiln at an intense heat. 



It does not appear which is the best for tempering, burned and 

 ground clay, or coarse sand, or feldspar. Reputable manufacturers 

 are found who use each of these materials, and make brick that stand 

 fire well. It is of the utmost importance to select the materials care- 



