SAFE STRENGTH OF MATERIALS 93 



quantity is to be used. If it sets up quickly it is, in all proba- 

 bility, a safe cement to use. 



MORTAR 



Working Quality. In the mixing of mortar it is very 

 important to get a mortar that will be conveniently plastic 

 under the trowel. A good mortar is usually spoken of as 

 tough. A sand and cement mortar is not tough, but very 

 short. In order to improve the working qualities of such a 

 mortar it is necessary to use some lime with the cement. 



Proportions. The proportion of 1 part of cement to 2 or 

 2}/2 parts of sand and % P ai> t or 1 part of lime putty (slaked 

 lime) , will result in a very good mortar. A good mortar saves 

 time and sets up rapidly enough to permit the building of a 

 considerable height of wall each day. 



CLAY BLOCKS 



Vitrified Brick. Brick clay is made up of two classes of 

 materials, one which melts at a temperature within the kiln, 

 and the other a nonfusible substance which does not melt but 

 holds the brick in shape. The flux or fusible material, in 

 melting, fills the pores between the other particles of clay and 

 melts out over the surface, giving it a glassy appearance, that 

 is, what is known as vitrified brick. If either the shale clay 

 or surface clay tile have these materials properly propor- 

 tioned, the result of thorough burning will be very dense or 

 vitrified brick. 



The modern kiln is so arranged that the fire passes up the 

 side of the wall and down among the brick or other material 

 to be burned, so that the highest temperature occurs in the 

 top of the kiln, and the most thorough burning occurs there. 

 So even though the clay is right, only that portion located 

 in the top portion of the kiln is right for silo construction. 



