548  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
kilns  10  to  14  feet  high,  placed  6  to  10  feet  apart,  and  are  supplied 
with  gas  by  lateral  pipes  which  tap  a  common  main  laid  through 
their  midst.  A  large  Bunsen  burner  attachment  is  placed  at  the 
kiln  end  of  the  lateral,  so  that  the  amount  of  air  to  be  mixed  with 
the  gas  is  well  regulated.  It  usually  requires  from  six  to  nine  days' 
firing  to  burn  such  a  kiln  of  brick  and  from  three  to  five  days  for  the 
mass  to  cool  after  the  gas  is  shut  off.  Owing  to  the  iron  oxides  in 
the  shale  all  the  building  brick  in  the  area  under  consideration  burn 
to  a  beautiful  uniform  brick-red  color. 
Vitrified  brick. — Thus  far  the  demand  for  building  brick  has  been 
so  great  that  some  of  the  companies  have  confined  their  efforts  entirely 
to  the  production  of  this  class  and  have  not  made  vitrified  or  paving 
brick.     The  latter  are  made,  however,  by  all  the  larger  companies. 
The  only  essential  difference  between  making  a  building  brick  and  a; 
paving  brick  is  in  the  firing.     The  latter  requires  a  much  higher 
temperature  than  the  building  brick — a  temperature  that  will  bring 
its  entire  mass  to  a  state  of  incipient  fusion.     To  accomplish  this* 
a  special  style  of  kiln  is  necessary.     The  one  usually  employed  is^ 
some  form  of  the  circular  down-draft  kiln,  with  a  low,  dome-shaped! 
roof  and  open-draft  spaces  connecting  from  the  floor  with  the  chim- 
ney.    Gas  mixed  with  air  is  admitted  on  the  side,  similar  to  the 
method  already  described  for  rectangular  kilns.     Upon  entering  the 
kiln  it  is  deflected  upward  to  the  concave  roof  and  then  reflected:! 
down  through  the  mass  of  brick  to  the  opening  in  the  floor  and  out  | 
through  proper  conduits  and  chimneys.     It  usually  requires  about 
two  days1  longer  firing  to  burn  a  kiln  of  vitrified  brick  than  one  oi 
common  brick. 
Dry-pressed  brick. — The  essential  differences  in  the  methods  oi 
manufacturing  common  building  brick  and  dry-pressed  brick  are  the 
amount  of  water  used  and  the  enormous  amount  of  pressure  to  which 
the  latter  are  subjected.  For  molding  common  building  brick  and 
vitrified  brick  a  sufficient  amount  of  water  is  added  to  produce  a  stiff 
mud,  but  with  the  dry-pressed  brick  the  amount  of  water  added  is  sc 
small  it  can  scarcely  be  noticed  in  the  clay. 
This  use  of  a  small  amount  of  moisture  keeps  shrinkage  and  warp 
ing  in  the  dry-pressed  brick  at  a  minimum  and  is  one  of  the  reasons 
for  the  superiority  of  this  brick. 
The  operations  of  the  Federal  Betterment  Company  at  Cherryvah 
show  that  where  there  is  no  culling  or  discarding  dry-pressed  bricl 
can  be  placed  on  the  market  almost  as  cheaply  as  others. 
Of  the  plants  enumerated  only  seven  produce  dry-pressed  brick 
namely,  the  two  each  at  Cherry  vale  and  Coffey  ville,  and  those  a'i 
Independence,  Caney,  and  Sycamore. 
Roofing  tile. — A  plant  for  the  manufacture  of  roofing  tile,  ownec 
by  the  Western  Roofing  Tile  Company,  is  operated  at  Coffey  ville 
