﻿CLAYS 
  OF 
  KEW 
  YORK 
  545 
  

  

  Another 
  method 
  requires 
  that 
  the 
  clay 
  be 
  formed 
  into 
  a 
  bar 
  of 
  

   known 
  cross-section. 
  When 
  dried, 
  the 
  bar 
  is 
  held 
  in 
  a 
  horizontal 
  

   position 
  by 
  supports 
  under 
  the 
  two 
  ends, 
  and 
  the 
  weight 
  noted 
  

   which 
  is 
  needed 
  to 
  cross-break 
  it 
  when 
  the 
  pressure 
  is 
  applied 
  to 
  

   the 
  central 
  portion 
  of 
  the 
  bar. 
  

  

  An 
  objection 
  to 
  this 
  method 
  is 
  that 
  very 
  plastic 
  clays 
  are 
  apt 
  

   to 
  develop 
  structural 
  peculiarities, 
  which 
  cause 
  their 
  tensile 
  strength 
  

   to 
  appear 
  much 
  lower 
  than 
  it 
  really 
  is. 
  In 
  such 
  cases, 
  it 
  has 
  been 
  

   suggested 
  that 
  the 
  clay 
  be 
  pulverized 
  and 
  mixed 
  with 
  an 
  equal 
  

   quantity 
  of 
  fine 
  sand. 
  If 
  when 
  made 
  into 
  briquets 
  this 
  mixture 
  

   shows 
  a 
  higher 
  tensile 
  strength 
  than 
  the 
  clay 
  alone, 
  it 
  is 
  an 
  indi- 
  

   cation 
  that 
  the 
  low 
  tenacity 
  of 
  the 
  original 
  clay 
  was 
  due 
  to 
  flaws. 
  

  

  The 
  tensile 
  strength 
  of 
  air-dried 
  clays 
  is 
  highly 
  variable. 
  The 
  

   following 
  figures 
  may 
  be 
  taken 
  as 
  representing 
  the 
  average 
  for 
  

   different 
  clays, 
  in 
  pounds 
  per 
  square 
  inch. 
  

  

  Pounds 
  

  

  Kaolins 
  5- 
  15 
  

  

  Brick 
  clays 
  60- 
  75 
  or 
  even 
  100 
  

  

  Pottery 
  clays 
  . 
  . 
  . 
  150-175 
  

  

  Some 
  very 
  plastic 
  clays 
  200-300 
  

  

  Shrinkage 
  

  

  All 
  clays 
  shrink 
  in 
  drying, 
  and 
  again 
  in 
  burning. 
  The 
  first 
  is 
  

   known 
  as 
  air, 
  the 
  second 
  as 
  fire 
  shrinkage. 
  Some 
  clays 
  shrink 
  

   more 
  in 
  drying, 
  others 
  in 
  burning, 
  consequently 
  the 
  amount 
  is 
  

   variable. 
  

  

  Air 
  shrinkage 
  depends 
  partly 
  on 
  the 
  amount 
  of 
  water 
  absorbed, 
  

   and 
  partly 
  on 
  the 
  grain 
  or 
  texture 
  of 
  the 
  clay. 
  

  

  Air 
  shrinkage. 
  As 
  soon 
  as 
  evaporation 
  of 
  water 
  takes 
  place 
  

   from 
  a 
  clay, 
  it 
  begins 
  to 
  shrink, 
  and 
  within 
  certain 
  limits, 
  the 
  

   greater 
  the 
  amount 
  of 
  water 
  absorbed, 
  the 
  greater 
  the 
  air 
  shrink- 
  

   age. 
  Plastic 
  clays 
  show 
  this 
  property 
  in 
  a 
  marked 
  degree. 
  The 
  

   shrinkage 
  continues 
  till 
  all 
  the 
  clay 
  particles 
  are 
  in 
  contact 
  with 
  

  

  