﻿CLAYS 
  OF 
  iS'EW 
  YORK 
  

  

  551 
  

  

  farther, 
  the 
  combined 
  Tvater 
  begins 
  to 
  pass 
  off 
  at 
  a 
  dull 
  red 
  beat, 
  

   and 
  the 
  clay 
  shrinks 
  to 
  an 
  additional 
  extent, 
  becoming 
  not 
  only 
  

   harder 
  but 
  denser, 
  till 
  it 
  reaches 
  a 
  condition 
  approaching 
  imper- 
  

   viousness, 
  and 
  a 
  hardness 
  of 
  about 
  6. 
  (See 
  scale 
  of 
  hardness, 
  p. 
  

   855) 
  This 
  condition 
  of 
  hardness 
  commonly 
  indicates 
  the 
  beginning 
  

   of 
  fusion, 
  not 
  of 
  the 
  whole 
  clay 
  mass, 
  but 
  of 
  the 
  more 
  fusible 
  con- 
  

   stituents, 
  which 
  soften 
  slightly 
  and 
  bind 
  the 
  whole, 
  together. 
  It 
  

   is 
  called 
  the 
  stage 
  of 
  incipient 
  fusion. 
  In 
  clays 
  that 
  have 
  been 
  

   burned 
  to 
  this 
  condition, 
  the 
  clay 
  particles 
  are 
  commonly 
  still 
  

   recognizable. 
  

  

  ^Vith 
  an 
  increase 
  of 
  the 
  temperature 
  ranging 
  from 
  50° 
  to 
  

   200° 
  F., 
  or 
  sometimes 
  even 
  more, 
  an 
  additional 
  amount 
  of 
  shrinkage 
  

   occurs, 
  and 
  most 
  or 
  all 
  of 
  the 
  particles 
  have 
  become 
  sufficiently 
  

   soft 
  to 
  allow 
  their 
  adjustment 
  to 
  the 
  most 
  compact 
  condition, 
  leav- 
  

   ing 
  no 
  interspaces, 
  or, 
  in 
  other 
  words, 
  making 
  the 
  burned 
  clay 
  im- 
  

   pervious. 
  This 
  is 
  spoken 
  of 
  as 
  vitrification, 
  and 
  brick 
  or 
  other 
  clay 
  

   products 
  burned 
  to 
  this 
  stage 
  are 
  vitrified 
  or 
  completely 
  sintered. 
  

   The 
  particles 
  are 
  no 
  longer 
  recognizable, 
  and 
  'the 
  maximum 
  shrink- 
  

   age 
  has 
  been 
  reached. 
  With 
  a 
  farther 
  rise 
  in 
  temperature 
  the 
  clay 
  

   becomes 
  viscous 
  or 
  flows. 
  

  

  We 
  can 
  therefore 
  recognize 
  three 
  stages 
  in 
  the 
  burning 
  of 
  a 
  

   clay 
  : 
  

  

  Incipient 
  fusion 
  ^ 
  

  

  Vitrification 
  

  

  Viscosity 
  

  

  The 
  three 
  stages 
  are 
  not 
  by 
  any 
  means 
  sharply 
  marked, 
  they 
  do 
  

   not 
  show 
  the 
  same 
  difference 
  in 
  temperature, 
  nor 
  does 
  incipient 
  

   fusion 
  begin 
  at 
  the 
  same 
  temperature 
  in 
  all 
  clays. 
  

  

  In 
  general 
  we 
  can 
  say, 
  that 
  other 
  things 
  being 
  equal, 
  the 
  greater 
  

   the 
  percentage 
  of 
  total 
  fluxes, 
  the 
  lower 
  the 
  temperature 
  of 
  in- 
  

   cipient 
  fusion, 
  vitrification 
  and 
  viscosity. 
  

  

  The 
  difference 
  in 
  temperature 
  between 
  incipient 
  fusion 
  and 
  

   viscosity 
  varies 
  with 
  the 
  composition 
  of 
  the 
  clay. 
  In 
  calcareous 
  

  

  1 
  These 
  three 
  terms 
  have 
  been 
  suggested 
  by 
  H. 
  A. 
  Wheeler. 
  Vitrified 
  pav- 
  

   ing 
  brick, 
  p. 
  12. 
  1895. 
  

  

  