OLAYS OF NEW YORK 639 



The following gives the maximiim, minimuni and average per- 

 centage of the different constituents in the brick clay analyses 

 given in the tables at the end of the report. 



Range Average 



Silica 34.35 —90.877 49.27^ 



Alumina 22.14 —44.00 22.774^ 



Ferric oxid .126 — 33.12 5.311^ 



lime 024 — 23.20 2.0W 



Magnesia 02—11.03 2.66^ 



Alkalis 17 —15.32 2.768^ 



Water 05 —13.60 5.749^ 



Moisture 17 — 9.64 2.502^ 



Clay for pressed brick. In addition to the characters mentioned 

 under common brick clays, it is highly necessary that the materials 

 used should burn to a uniform color. They should be as free as 

 possible from soluble salts, specially if the product is not vitrified. 



Many different grades of clay are utilized, but chief among them 

 may be mentioned white burning clays, buff burning clays (either 

 calcareous or semi-refractory), red burning clays, commonly high in 

 iron oxid. 



Semi-refractory, or refractory clays, form an important source 

 of material for making front brick, not only on account of the buff 

 color to which they burn, but also because this color permits the 

 admixture of manganese for the production of mottled and speckled 

 effects and various shades producible by the addition of the same 

 material in powdered form. 



The shrinkage of the clay in burning should be regular and 

 even, in order that the finished bricks may all be very nearly of the 

 same size. 



Burning of brick clay 



Brick clays when burned exhibit a variety of shades and colors 

 whose existence is influenced by several causes, such as the amount 

 of ferric oxid in a clay, the percentage of other constituents as- 



