﻿546 
  NEW 
  YOEK 
  STATE 
  MUSEUM 
  

  

  one 
  another, 
  but 
  this 
  does 
  not 
  mean 
  that 
  all 
  tlie 
  mechanicallj 
  com- 
  

   bined 
  water 
  has 
  passed 
  off 
  j 
  for 
  there 
  may 
  remain 
  spaces 
  which 
  hold 
  

   some. 
  Consequently 
  a 
  clay 
  will 
  continue 
  to 
  lose 
  weight 
  after 
  the 
  

   air 
  shrinkage 
  has 
  ceased. 
  This 
  fact 
  is 
  shown 
  by 
  the 
  following 
  

   tests 
  made 
  on 
  samples 
  of 
  clay 
  of 
  'New 
  York 
  state. 
  

  

  A 
  sample 
  of 
  soft, 
  moderately 
  plastic 
  shale 
  was 
  mixed 
  with 
  water 
  

   and 
  molded 
  into 
  briquets 
  and 
  then 
  allowed 
  to 
  dry. 
  

  

  Weight 
  

   When 
  molded 
  Air 
  in 
  

  

  shrinkage 
  grams 
  

  

  When 
  molded 
  , 
  41 
  .220 
  

  

  End 
  of 
  12 
  hrs 
  IJ^ 
  38 
  . 
  030 
  

  

  End 
  of 
  1 
  day 
  2^ 
  37.616 
  

  

  End 
  of 
  3 
  " 
  2^ 
  37 
  . 
  500 
  

  

  End 
  of 
  4 
  '' 
  2^ 
  37.496 
  

  

  Coarse-grained 
  clays 
  commonly 
  shrink 
  less 
  than 
  the 
  fine-grained; 
  

   they 
  may 
  at 
  the 
  same 
  time 
  absorb 
  a3 
  much 
  water. 
  Having 
  larger 
  

   pores, 
  they 
  will 
  permit 
  the 
  water 
  to 
  escape 
  m,ore 
  rapidly, 
  and 
  

   hence 
  can 
  often 
  be 
  dried 
  more 
  quickly 
  than 
  fine-grained 
  clays, 
  

   from 
  which 
  the 
  water, 
  on 
  account 
  of 
  the 
  smallness 
  of 
  the 
  pores, 
  

   can 
  not 
  escape 
  so 
  quickly. 
  Again, 
  if 
  fine-grained 
  clays 
  are 
  dried 
  

   rapidly, 
  the 
  surface 
  shrinks 
  more 
  quickly 
  than 
  the 
  interior, 
  and 
  

   cracking 
  may 
  ensue, 
  more 
  specially 
  if 
  the 
  clay 
  has 
  a 
  low 
  tensile 
  

   strength, 
  or 
  if 
  it 
  is 
  highly 
  plastic. 
  

  

  Air 
  shrinkage 
  begins 
  as 
  soon 
  as 
  the 
  clay 
  is 
  molded 
  and 
  set 
  out 
  to 
  

   dry, 
  at 
  first 
  taking 
  place 
  very 
  rapidly, 
  but 
  with 
  decreasing 
  speed. 
  

   It 
  is 
  in 
  nearly 
  all 
  cases 
  completed 
  before 
  the 
  brick 
  or 
  wares 
  are 
  

   placed 
  in 
  the 
  kiln. 
  The 
  final 
  traces 
  of 
  moisture 
  are 
  not 
  driven 
  

   off, 
  however, 
  till 
  the 
  first 
  stages 
  of 
  burning. 
  

  

  Fire 
  shrinkage. 
  This 
  generally 
  commences 
  when 
  the 
  combined 
  

   water 
  begins 
  to 
  pass 
  off, 
  or 
  at 
  about 
  1200° 
  F. 
  It 
  varies 
  in 
  differ- 
  

   ent 
  clays 
  and 
  may 
  reach 
  any 
  point 
  between 
  2^ 
  and 
  15^. 
  

  

  The 
  shrinkage 
  in 
  burning 
  may 
  be 
  just 
  as 
  variable 
  as 
  that 
  in 
  dry- 
  

   ing; 
  it 
  does 
  not 
  depend 
  on 
  the 
  same 
  causes 
  but 
  is 
  influenced 
  by 
  the 
  

   teanperature 
  to 
  which 
  the 
  clay 
  is 
  exposed, 
  percentage 
  of 
  combined 
  

   water 
  and 
  organic 
  matter. 
  

  

  