CLAYS OF NEW YOKK Y6Y 



SEWER PIPE 

 Clays used 



The qualities of clay required for this purpose are in general 

 the same as those demanded for any ware with a vitrified body. 

 They should therefore be sufficiently plastic to permit molding 

 without cracking; a high tensile strength, while desirable, is not 

 absolutely necessary. Many clays used in the manufacture of 

 sewer pipe have a tensile strength as high as 125 or even 150 

 pounds a square inch, while on the other hand shales are used 

 whose tensile strength when ground to 30 mesh is not over 

 90 pounds a square .inch. The clay should bum to- a hard, 

 •dense, impervious body; the amount of iron in such clays or shales 

 is usually sufficient to color it a red, or deep red. The drying 

 should be rapid, and the ware should not warp or crack in drying. 

 Owing to the thinness of the body, sewer pipe may be burned more 

 rapidly than paving brick. 



An excess of fluxing impurities may render a clay so fusible that 

 in burning it softens and loses shape. It is a very common prac- 

 tice to use a mixture of clays, the one being fusible to form a bond 

 in burning, the other more refractory to preserve the shape of the 

 ware. 



Sewer pipe are usually glazed by means of salt, thrown into the 

 fireplaces when the temperature of the kiln is at its highest, the 

 vapors, passing through the kiln and uniting with the silica and 

 the alumina of the clay, forming a glaze over the surface of the 

 ware. The following is the reaction which occurs: 



NaCl+HaO^HCl+Na OH. 



Na OH+nSiO=NaO+nSi02H20. 



Glazing requires one to two hours; some manufacturers add 

 manganese to the salt in order to produce a glaze of the proper 

 •color. An excess of silica in the clay seems to be detrimental to 

 the formation of a good glaze. 



