Burning

Kilns. — Bricks  are  burned  in  a  variety  of  kilns,  ranging  from  tempo-
rary structures,  which  are  torn  down  after  each  lot  of  brick  is  burned
(PI.  XIV,  Fig.  2),  to  patented  or  other  permanent  forms  of  complicated
design.  They  are  built  on  one  of  two  principles,  either  up-draft  or  down-
draft.  In  the  former  the  heat  from  the  fire-boxes  at  the  bottom  passes
directly  into  the  body  of  the  kiln  and  up  through  the  wares,  escaping
from  suitable  chimneys  or  openings  at  the  top.  In  the  latter  the  heat
from  the  fire-boxes  is  conducted  first  to  the  top  of  the  kiln  chamber,  by
means  of  suitable  flues  on  the  interior  wall,  and  then  down  through  the
wares;  being  carried  off  through  flues  in  the  bottom  of  the  kiln  to  the
stack  (PL  XV,  Fig.  2).  The  down-draft  system  is  growing  in  favor,
as  the  burning  can  be  regulated  better.  Furthermore,  since  the  bricks
at  the  top  receive  the  greatest  heat,  and  these  at  the  bottom  the  least,
there  is  less  danger  of  the  bricks  in  the  lower  courses  being  crushed  out
of  shape  if  heated  too  high.
The  amount  of  heat  required  for  burning  brick  will  vary  with  the
clay  and  the  color,  density  or  degree  of  hardness  desired,  the  same
clay  giving  different  results  when  burned  at  different  temperatures.
Common  bricks  are  rarely  burned  any  higher  than  cone  05,  and  usually
not  above  cone  010,  while  pressed  brick  are  frequently  fired  to  cone
7  or  8,  because  the  clays  generally  used  have  to  be  burned  to  that  point
to  render  them  hard.
