﻿Ill] 
  Special 
  Report 
  No. 
  4 
  : 
  Clays 
  267 
  

  

  which 
  they 
  exhibit. 
  Very 
  plastic 
  clays 
  are 
  called 
  " 
  f 
  at 
  " 
  while 
  

   those 
  which 
  are 
  low 
  in 
  plasticity 
  are 
  called 
  " 
  lean." 
  One 
  of 
  the 
  

   important 
  effects 
  of 
  plasticity 
  is 
  that 
  it 
  permits 
  the 
  clay 
  to 
  be 
  

   molded 
  more 
  readily, 
  and 
  obviates 
  the 
  danger 
  of 
  its 
  cracking 
  

   while 
  it 
  is 
  being 
  formed. 
  

  

  Plasticity 
  is 
  developed 
  by 
  mixing 
  the 
  clay 
  with 
  water, 
  and 
  the 
  

   plasticity 
  of 
  any 
  clay 
  increases 
  with 
  the 
  amount 
  of 
  water 
  added 
  

   up 
  to 
  a 
  certain 
  point, 
  while 
  the 
  addition 
  of 
  more 
  water 
  causes 
  

   the 
  clay 
  to 
  soften 
  and 
  turn 
  into 
  a 
  condition 
  of 
  mud. 
  In 
  sedi- 
  

   mentary 
  clays, 
  especially 
  the 
  very 
  plastic 
  ones 
  an 
  appreciable 
  

   amount 
  of 
  water 
  can 
  be 
  added 
  to 
  the 
  clay 
  after 
  the 
  point 
  of 
  maxi- 
  

   mum 
  plasticity 
  has 
  been 
  reached 
  without 
  destroying 
  the 
  cohe- 
  

   siveness 
  of 
  the 
  clay, 
  but 
  with 
  residual 
  clays 
  the 
  addition 
  of 
  a 
  very 
  

   slight 
  excess 
  of 
  water 
  in 
  mixing 
  is 
  often 
  very 
  noticeable 
  as 
  the 
  

   material 
  softens 
  to 
  the 
  mud 
  like 
  condition 
  very 
  rapidly. 
  

  

  Very 
  plastic 
  clays 
  may 
  require 
  as 
  much 
  as 
  35 
  or 
  40 
  per 
  cent 
  of 
  

   water 
  by 
  weight 
  to 
  develop 
  their 
  greatest 
  plasticity, 
  while 
  the 
  

   lean 
  ones 
  may 
  not 
  need 
  over 
  15 
  or 
  even 
  20 
  per 
  cent 
  to 
  accomplish 
  

   the 
  same 
  result. 
  

  

  Shrinkage. 
  — 
  If 
  a 
  mass 
  of 
  clay 
  is 
  set 
  aside 
  after 
  being 
  molded 
  

   and 
  allowed 
  to 
  dry, 
  because 
  of 
  the 
  evaporation 
  of 
  the 
  water, 
  the 
  

   particles 
  begin 
  to 
  draw 
  together 
  and 
  the 
  mass 
  shrinks. 
  This 
  is 
  

   known 
  as 
  the 
  air 
  shrinkage. 
  Claj^s, 
  which 
  are 
  very 
  plastic 
  and 
  

   contain 
  a 
  large 
  quantity 
  of 
  fine 
  particles 
  in 
  their 
  mass 
  tend 
  to 
  

   shrink 
  the 
  most, 
  while 
  the 
  very 
  lean 
  clays 
  as 
  a 
  rule 
  shrink 
  the 
  

   least. 
  The 
  more 
  water 
  that 
  is 
  added 
  to 
  the 
  clay 
  in 
  molding, 
  the 
  

   greater 
  will 
  be 
  the 
  air 
  shrinkage. 
  It 
  is 
  in 
  this 
  stage 
  that 
  the 
  

   effects 
  of 
  dry-pressing 
  make 
  themselves 
  beneficially 
  felt, 
  that 
  is 
  

   when 
  the 
  clay 
  is 
  molded 
  in 
  the 
  form 
  of 
  a 
  dry, 
  or 
  nearly 
  dry 
  

   powder 
  there 
  is 
  little 
  or 
  no 
  water 
  to 
  escape 
  and 
  consequenth^ 
  the 
  

   air 
  shrinkage 
  is 
  very 
  low. 
  The 
  air 
  shrinkage 
  of 
  lean, 
  sandy 
  

   clays 
  ma}^ 
  be 
  as 
  low 
  as 
  two 
  per 
  cent, 
  while 
  for 
  ver}^ 
  fat 
  plastic 
  

   clays 
  it 
  may 
  be 
  as 
  high 
  as 
  12 
  per 
  cent. 
  There 
  are 
  certain 
  dangers 
  

   which 
  attend 
  the 
  high 
  shrinkage 
  in 
  the 
  air 
  drying, 
  i. 
  e. 
  that 
  the 
  

   clay 
  does 
  not 
  tend 
  to 
  dry 
  equally 
  fast 
  throughout 
  the 
  mass 
  

   unless 
  the 
  operation 
  is 
  carried 
  on 
  very 
  slowly, 
  and 
  consequently 
  

   there 
  is 
  danger 
  of 
  cracking 
  in 
  the 
  ware, 
  or 
  twisting 
  of 
  the 
  form. 
  

   (If 
  the 
  particles 
  of 
  the 
  clay 
  interlock 
  in 
  an 
  intimate 
  manner, 
  and 
  

  

  