﻿CLAYS 
  OF 
  Is'EW 
  YOKK 
  641 
  

  

  terior 
  of 
  tlie 
  burned 
  brick 
  may 
  be 
  due 
  to 
  the 
  deposition 
  of 
  foreign 
  

   substances 
  brought 
  there 
  by 
  the 
  fire 
  gases, 
  which 
  may 
  exert 
  a 
  

   colorizing 
  action 
  either 
  by 
  their 
  presence 
  alone 
  or 
  by 
  their 
  forming 
  

   a 
  glaze 
  on 
  the 
  surface 
  of 
  the 
  brick 
  as 
  a 
  result 
  of 
  their 
  union 
  with 
  

   the 
  silica 
  in 
  it. 
  This 
  is 
  often 
  to 
  be 
  seen 
  on 
  the 
  surface 
  of 
  arch 
  

   brick 
  in 
  an 
  up-draft 
  kiln, 
  and 
  on 
  the 
  surface 
  of 
  the 
  brick 
  which 
  

   line 
  the 
  bag 
  walls 
  of 
  any 
  down-draft 
  kiln. 
  

  

  The 
  coloration 
  of 
  most 
  brick 
  is 
  due 
  to 
  iron; 
  unless 
  the 
  brick 
  is 
  

   heated 
  beyond 
  vitrification 
  it 
  is 
  probable 
  that 
  much 
  of 
  the 
  iron 
  re- 
  

   mains 
  in 
  the 
  ferric 
  condition. 
  

  

  With 
  ferric 
  oxid 
  in 
  a 
  clay 
  it 
  is 
  possible 
  to 
  obtain 
  all 
  shades 
  rang- 
  

   ing 
  from 
  pink 
  to 
  reddish 
  black, 
  and 
  with 
  an 
  excess 
  of 
  lime 
  all 
  

   shades 
  of 
  yellow, 
  while 
  manganese, 
  which 
  sometimes 
  accompanies 
  

   the 
  iron 
  in 
  small 
  amounts, 
  tends 
  to 
  give 
  a 
  brownish 
  coloration. 
  

  

  Ferrous 
  oxid 
  produces 
  colors 
  ranging 
  from 
  green 
  to 
  black. 
  

  

  It 
  should 
  also 
  be 
  remembered 
  that 
  with 
  any 
  given 
  amount 
  of 
  

   iron 
  in 
  a 
  clay, 
  the 
  higher 
  the 
  temperature 
  to 
  which 
  the 
  material 
  is 
  

   exposed 
  the 
  deeper 
  will 
  be 
  the 
  color 
  obtained. 
  Iron 
  in 
  the 
  ferric 
  

   condition 
  tends 
  to 
  color 
  the 
  mass 
  red 
  as 
  long 
  as 
  it 
  is 
  at 
  all 
  porous, 
  

   but 
  with 
  the 
  beginning 
  of 
  fusion 
  it 
  generally 
  passes 
  over 
  to 
  black. 
  

  

  When 
  the 
  clay 
  also 
  contains 
  carbonate 
  of 
  lime, 
  the 
  latter 
  serves 
  

   as 
  a 
  flux, 
  and 
  causes 
  fusion 
  to 
  set 
  in 
  at 
  a 
  lower 
  temperature 
  than 
  it 
  

   otherwise 
  would, 
  the 
  result 
  being 
  the 
  formation 
  of 
  a 
  complex 
  

   silicate, 
  containing 
  iron, 
  alumina 
  and 
  lime, 
  which 
  with 
  the 
  proper 
  

   proportion 
  of 
  iron 
  and 
  lime 
  shows 
  a 
  yellow 
  color. 
  Up 
  to 
  the 
  time 
  

   that 
  fusion 
  sets 
  in 
  the 
  ferric 
  oxid 
  still 
  imparts 
  its 
  red 
  color 
  to 
  the 
  

   clay, 
  but 
  as 
  the 
  heat 
  rises, 
  this 
  gradually 
  turns 
  to 
  flesh 
  red, 
  white, 
  

   yellow, 
  and 
  finally 
  yellowish 
  green, 
  and 
  at 
  viscosity 
  passes 
  to 
  green, 
  

   and 
  sometimes 
  black. 
  

  

  The 
  coloration 
  which 
  is 
  induced 
  superficially 
  is 
  a 
  matter 
  of 
  great 
  

   importance, 
  and, 
  as 
  before 
  stated, 
  it 
  may 
  be 
  due 
  either 
  to 
  a 
  coating 
  

   of 
  soluble 
  salts, 
  or 
  a 
  deposit 
  of 
  impurities 
  from 
  the 
  fire 
  gases. 
  The 
  

   former 
  are 
  described 
  under 
  ^' 
  Efflorescence 
  on 
  bricks,'' 
  p. 
  679. 
  

  

  The 
  discoloration 
  caused 
  by 
  the 
  action 
  of 
  fire 
  gases 
  on 
  the 
  clay 
  

  

  