﻿Ill] 
  Special 
  Report 
  No. 
  4 
  : 
  Clays 
  269 
  

  

  causes 
  the 
  particles 
  to 
  soften 
  still 
  more 
  under 
  the 
  action 
  of 
  the 
  

   heat, 
  so 
  that 
  they 
  pack 
  together 
  in 
  a 
  smaller 
  space, 
  leaving 
  no 
  

   interstices. 
  At 
  this 
  point 
  the 
  maximum 
  shrinkage 
  of 
  the 
  clay 
  

   has 
  been 
  reached 
  and 
  the 
  mass 
  will 
  also 
  be 
  impervious 
  or 
  very 
  

   nearly 
  so. 
  It 
  is 
  what 
  is 
  known 
  as 
  the 
  condition 
  of 
  vitrification. 
  

   This 
  is 
  the 
  state 
  to 
  which 
  stoneware, 
  paving 
  brick 
  and 
  sewer 
  pipe 
  

   should 
  be 
  burned. 
  But 
  all 
  claj^s 
  do 
  not 
  yield 
  the 
  best 
  results 
  if 
  

   burned 
  to 
  this 
  point. 
  

  

  If 
  the 
  clay 
  is 
  heated 
  still 
  higher 
  it 
  begins 
  to 
  soften 
  still 
  more 
  

   and 
  at 
  a 
  certain 
  point 
  becomes 
  viscous 
  or 
  flows. 
  This 
  is 
  there- 
  

   fore 
  spoken 
  of 
  as 
  the 
  point 
  of 
  viscosity. 
  In 
  clays 
  which 
  are 
  to 
  

   be 
  burned 
  to 
  the 
  condition 
  of 
  vitrification 
  the 
  points 
  of 
  vitrifica- 
  

   tion 
  and 
  viscosity 
  should 
  be 
  at 
  least 
  150 
  degrees 
  Fahr. 
  apart 
  and 
  

   preferably 
  200 
  degrees. 
  In 
  limy 
  clays 
  the 
  points 
  of 
  incipient 
  

   fusion 
  and 
  viscosity 
  are 
  very 
  close 
  together 
  and 
  consequently- 
  it 
  

   is 
  not 
  possible 
  to 
  burn 
  a 
  kiln 
  full 
  of 
  ware 
  to 
  vitrification 
  without 
  

   danger 
  of 
  running 
  it 
  beyond 
  to 
  the 
  point 
  of 
  viscosity. 
  In 
  some 
  

   fireclays 
  the 
  differcence 
  between 
  the 
  points 
  of 
  incipient 
  fusion 
  

   and 
  viscosity 
  may 
  be 
  as 
  much 
  as 
  600 
  degrees 
  Fahr. 
  

  

  In 
  very 
  impure 
  clays 
  incipient 
  fusion 
  may 
  begin 
  at 
  as 
  low 
  a 
  

   temperature 
  as 
  1,700 
  degrees 
  Fahr., 
  while 
  in 
  fireclays 
  this 
  same 
  

   point 
  may 
  not 
  be 
  attained 
  below 
  2,700 
  degrees, 
  and 
  indeed 
  it 
  

   really 
  should 
  not 
  in 
  order 
  to 
  permit 
  calling 
  the 
  clay 
  refractory. 
  

  

  Another 
  effect 
  of 
  heating 
  is 
  the 
  change 
  of 
  color 
  that 
  is 
  brought 
  

   about, 
  especially 
  by 
  iron, 
  for 
  this 
  is 
  the 
  great 
  coloring 
  agent 
  of 
  

   clay 
  in 
  both 
  the 
  burned 
  and 
  unburned 
  condition. 
  With 
  a 
  given 
  

   percentage 
  of 
  iron, 
  the 
  clay 
  when 
  lightly 
  burned 
  will 
  be 
  light 
  

   red, 
  but 
  as 
  the 
  temperature 
  of 
  the 
  firing 
  increases 
  the 
  color 
  

   deepens, 
  passing 
  into 
  deep 
  red 
  and 
  finally, 
  when 
  the 
  clay 
  fuses, 
  

   into 
  bluish 
  black. 
  Again, 
  the 
  greater 
  the 
  quantity 
  of 
  iron 
  oxide 
  

   in 
  a 
  cla}^ 
  the 
  deeper 
  will 
  be 
  the 
  color 
  produced 
  at 
  any 
  given 
  

   temperature. 
  This 
  production 
  of 
  the 
  red 
  color 
  assumes 
  that 
  the 
  

   condition 
  of 
  the 
  kiln 
  fires 
  is 
  oxydizing, 
  that 
  is 
  that 
  there 
  is 
  a 
  

   supply 
  of 
  air. 
  If 
  there 
  is 
  an 
  insufficient 
  supply 
  of 
  the 
  latter 
  the 
  

   the 
  fire 
  will 
  act 
  reducing 
  and 
  the 
  color 
  of 
  the 
  burned 
  clay 
  will 
  be 
  

   bluish 
  instead 
  of 
  red. 
  

  

  There 
  are 
  certain 
  ingredients 
  which 
  tend 
  to 
  destroy 
  the 
  red- 
  

   dening 
  power 
  of 
  iron, 
  and 
  these 
  are 
  lime 
  and 
  alumina. 
  It 
  has 
  

  

  