﻿268 
  Geological 
  Survey 
  of 
  Louisiana 
  [Sect. 
  

  

  resistance 
  very 
  strongly 
  the 
  tendency 
  to 
  be 
  pulled 
  apart, 
  then 
  

   there 
  is 
  less 
  danger 
  of 
  cracking.) 
  This 
  resistance 
  of 
  the 
  parti- 
  

   cles 
  to 
  tearing 
  is 
  spoken 
  of 
  as 
  the 
  tensile 
  strength 
  and 
  is 
  expressed 
  

   in 
  pounds 
  per 
  square 
  inch. 
  It 
  is 
  measured 
  by 
  forming 
  the 
  clay 
  

   into 
  the 
  form 
  of 
  briquettes 
  of 
  the 
  same 
  shape 
  and 
  size 
  as 
  those 
  

   used 
  in 
  the 
  testing 
  of 
  cement. 
  When 
  dry 
  these 
  are 
  placed 
  in 
  a 
  

   cement 
  testing 
  machine 
  and 
  the 
  number 
  of 
  pounds 
  per 
  square 
  

   inch, 
  which 
  is 
  required 
  to 
  pull 
  them 
  apart, 
  is 
  measured. 
  This 
  is 
  

   a 
  variable 
  quantity, 
  and 
  may 
  run 
  from 
  lo 
  or 
  15 
  lbs., 
  to 
  400. 
  The 
  

   tensile 
  strength 
  shown 
  by 
  the 
  various 
  types 
  of 
  clay 
  in 
  their 
  

   air 
  dried 
  condition, 
  is 
  

  

  Kaolins 
  10- 
  25 
  lbs. 
  per 
  sq. 
  in. 
  

  

  Brick 
  clays 
  60- 
  75 
  ' 
  ' 
  

  

  Pottery 
  clays 
  100-150 
  " 
  

  

  Paving 
  brick 
  clays 
  75-150 
  " 
  

  

  Fire 
  shrinkage. 
  — 
  If 
  after 
  a 
  clay 
  has 
  been 
  air 
  dried 
  it 
  is 
  put 
  in 
  

   a 
  furnace 
  or 
  kiln 
  and 
  subjected 
  to 
  a 
  slowly 
  rising 
  temperature, 
  

   it 
  begins 
  to 
  shrink 
  more, 
  beginning 
  at 
  a 
  low 
  temperature 
  and 
  

   continuing 
  upwards. 
  This 
  second 
  shrinkage 
  is 
  known 
  as 
  

   the 
  fire 
  shrinkage 
  and 
  may 
  be 
  just 
  as 
  variable 
  as 
  the 
  air 
  

   shrinkage. 
  It 
  is 
  due 
  partly 
  to 
  the 
  loss 
  of 
  chemically 
  combined 
  

   water 
  which 
  the 
  clay 
  contains, 
  and 
  also 
  to 
  the 
  presence 
  of 
  other 
  

   volatile 
  materials 
  such 
  as 
  organic 
  matter 
  in 
  the 
  clay. 
  It 
  is 
  just 
  

   as 
  important 
  that 
  the 
  fire 
  shrinkage 
  should 
  proceed 
  slowly 
  in 
  

   order 
  to 
  prevent 
  cracking 
  and 
  warping 
  of 
  the 
  ware. 
  Fine 
  

   grained 
  clays 
  usually 
  shrink 
  more 
  in 
  burning 
  than 
  coarse, 
  sandy 
  

   ones, 
  and 
  it 
  is 
  on 
  this 
  account 
  that 
  sand 
  is 
  sometimes 
  added 
  to 
  

   the 
  clay 
  in 
  the 
  process 
  of 
  mixing 
  and 
  molding. 
  

  

  Effect 
  of 
  heat 
  on 
  clay. 
  — 
  In 
  addition 
  to 
  the 
  fire 
  shrinkage 
  which 
  

   has 
  just 
  been 
  mentioned, 
  there 
  are 
  certain 
  other 
  changes 
  which 
  

   take 
  place 
  during 
  the 
  burning 
  of 
  a 
  clay 
  and 
  these 
  depend 
  on 
  the 
  

   temperature 
  and 
  also 
  on 
  the 
  clay, 
  whether 
  pure 
  or 
  impure. 
  If 
  

   the 
  clay 
  is 
  heated 
  to 
  a 
  certain 
  point, 
  which 
  will 
  be 
  the 
  higher, 
  

   the 
  purer 
  the 
  clay, 
  a 
  softening 
  of 
  the 
  particles 
  takes 
  place, 
  or 
  

   fusion 
  begins. 
  If 
  the 
  clay 
  is 
  not 
  heated 
  further 
  than 
  this 
  point, 
  

   it 
  will 
  on 
  cooling 
  be 
  of 
  a 
  hard, 
  rock-like 
  nature, 
  but 
  will 
  still 
  be 
  

   porous. 
  This 
  is 
  known 
  as 
  the 
  condition 
  of 
  incipient 
  fusion. 
  A 
  

   further 
  heating 
  to 
  a 
  temperature 
  of 
  from 
  75 
  to 
  200 
  degrees 
  Fahr. 
  

  

  