﻿522 
  Is'EW 
  YOKK 
  STATE 
  MUSEUM 
  

  

  or 
  to 
  obtain 
  a 
  cream 
  colored 
  ware. 
  For 
  the 
  latter 
  purpose 
  semi-fire 
  

   clays 
  yield 
  tlie 
  best 
  results, 
  but 
  are 
  not 
  always 
  obtainable; 
  hence 
  

   calcareous 
  clays 
  must 
  be 
  used. 
  It 
  is 
  therefore 
  desirable 
  to 
  know 
  the 
  

   amount 
  of 
  lime 
  carbonate 
  which 
  is 
  allowable. 
  A 
  good, 
  but 
  not 
  at 
  

   the 
  same 
  time 
  vitrified, 
  brick 
  can 
  be 
  made 
  from 
  a 
  clay 
  containing 
  

   20^-25^ 
  of 
  lime 
  carbonate, 
  provided 
  it 
  is 
  evenly 
  and 
  finely 
  dis- 
  

   tributed 
  through 
  the 
  x3lay. 
  

  

  The 
  objection 
  to 
  highly 
  calcareous 
  clays 
  is 
  that 
  the 
  points 
  of 
  in- 
  

   cipient 
  fusion 
  and 
  vitrification 
  lie 
  so 
  close 
  together 
  that 
  it 
  is 
  not 
  

   safe 
  to 
  burn 
  them 
  hard, 
  because 
  of 
  the 
  risk 
  of 
  fusing 
  them. 
  It 
  has 
  

   been 
  found 
  possible 
  to 
  separate 
  these 
  points 
  however 
  by 
  the 
  addition 
  

   of 
  quartz 
  and 
  feldspar 
  to 
  the 
  clay, 
  or 
  by 
  adding 
  sand 
  containing 
  a 
  

   large 
  proportion 
  of 
  them."^ 
  

  

  Aside 
  from 
  lowering 
  the 
  fusibility 
  of 
  a 
  clay, 
  and 
  affecting 
  its 
  

   color 
  when 
  burned, 
  lime 
  also 
  exerts 
  a 
  powerful 
  effect 
  on 
  the 
  

   shrinkage. 
  

  

  Soger 
  ^ 
  found 
  that 
  calcareous 
  or 
  marly 
  clays 
  required 
  usually 
  only 
  

   20^-24^ 
  of 
  water 
  to 
  convert 
  them 
  from 
  a 
  dry 
  condition 
  into 
  a 
  work- 
  

   able 
  paste, 
  whereas 
  other 
  clays 
  needed 
  28^35^ 
  of 
  water 
  to 
  ac- 
  

   complish 
  the 
  same 
  change. 
  

  

  In 
  burning, 
  such 
  clays 
  lose 
  not 
  only 
  their 
  combined 
  water 
  but 
  

   also 
  carbon 
  dioxid, 
  and 
  consequently 
  they 
  are 
  more 
  porous 
  than 
  

   other 
  clays 
  up 
  to 
  the 
  point 
  of 
  sintering, 
  and 
  this 
  porosity, 
  attended 
  

   by 
  diminution 
  of 
  shrinkage, 
  increases 
  with 
  the 
  amount 
  of 
  lime 
  

   carbonate 
  contained 
  in 
  the 
  clay. 
  The 
  shrinkage 
  may 
  indeed 
  bo- 
  

   come 
  zero, 
  or 
  the 
  brick 
  even 
  swell. 
  

  

  The 
  small 
  difference 
  between 
  the 
  points 
  of 
  incipient 
  fusion 
  and 
  

   viscosity 
  have 
  already 
  been 
  mentioned. 
  

  

  Gypsum, 
  the 
  hydrated 
  sulfate 
  of 
  lime, 
  is 
  not 
  uncommon 
  in 
  some 
  

   clays, 
  specially 
  those 
  which 
  originally 
  contained 
  carbonate 
  of 
  lime 
  

   and 
  pyrite. 
  The 
  oxidation 
  and 
  deco^mposition 
  of 
  the 
  latter 
  produce 
  

   sulfuric 
  acid, 
  which 
  attacks 
  the 
  lime 
  carbonate, 
  producing 
  lime 
  

  

  1 
  See 
  " 
  Glazed 
  brick 
  ", 
  p. 
  652. 
  

  

  2 
  Seger. 
  Ges. 
  Schrift, 
  p. 
  265. 
  

  

  