420 Introduction to the Study of Science 



later into slate, by a very slow process of nature. (From what 

 mineral and by what process was clay formed?) By making 

 use of the conditions nature employed, that is, moisture, pres- 

 sure, and heat, men convert clay into brick which may be 

 harder than slate. To-day brick may be made from the raw 

 materials in a few days, whereas in nature's course similar 

 products may have required thousands of years. 



Vast deposits of clay are found in almost every land. Some 

 of these deposits were utilized for brick and pottery in very 

 early history. The ancient inhabitants of the valleys of the 

 Tigris, Euphrates, and the Nile made brick for use in buildings. 

 To-day bricks of very many kinds are produced in great quan- 

 tities, the average value of the annual output being more than 

 $140,000,000. 



The first step in the manufacture of brick and other clay 

 products is mixing the clay. This was originally done by hand, 

 later by a crude machine operated by man or animal power, 

 and to-day by machines operated by steam, gas, or electricity. 

 The mixed clay was molded into the desired forms at first by 

 hand, later by wooden molds, and is now molded by machines 

 operating at a high rate of speed. 



The second step of the process is baking the brick. This 

 was formerly accomplished by slow drying in the sun ; but 

 to-day it is done rapidly by the aid of a kiln in which the clay 

 products are heated to a temperature just below the fusion 

 point of clay. This high temperature causes the particles to 

 cohere closely. As the clay shrinks or contracts with the process, 

 the bricks are generally porous. This character may be ob- 

 served by placing a dry brick in a shallow dish of water ; and 

 the quantity of water absorbed may be measured accurately. 



Exercise : Brick and tile making. What different kinds of brick 

 and tile do you find used in your community? If a brick and tile 

 factory is located nearby, it will be profitable for you to visit it and 

 observe the successive steps in the manufacture of clay products. 

 Examine the clay, and ascertain whether it is the ordinary red clay, 



