﻿514 
  IsEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  In 
  some 
  clajs 
  containing 
  sulfate 
  of 
  iron 
  tlie 
  latter 
  may 
  be 
  de- 
  

   composed 
  by 
  chemical 
  reactions 
  taking 
  place 
  in 
  the 
  clay 
  and 
  sul- 
  

   furic 
  acid 
  being 
  set 
  free. 
  Tbis 
  acid 
  is 
  apt 
  to 
  attack 
  tbe 
  alumina 
  of 
  

   tbe 
  clay 
  substance, 
  and 
  if 
  potash, 
  soda 
  or 
  ammonia 
  be 
  present 
  they 
  

   give 
  rise 
  to 
  potash, 
  soda 
  or 
  ammonia 
  alum, 
  which 
  can 
  frequently 
  be 
  

   detected 
  by 
  tasting 
  the 
  clay. 
  

  

  Insoluble 
  alkaline 
  compounds. 
  All 
  the 
  sources 
  of 
  these 
  in 
  clay 
  

   are 
  minerals, 
  silicates 
  of 
  complex 
  composition. 
  Feldspar 
  and 
  mica 
  

   are 
  the 
  most 
  abundant 
  sources, 
  but 
  some 
  may 
  be 
  derived 
  from 
  

   garnet, 
  hornblende 
  and 
  pyroxene, 
  fragments 
  of 
  which 
  may 
  be 
  

   present 
  in 
  nearly 
  all 
  impure, 
  and 
  specially 
  ferruginous 
  clays. 
  

  

  The 
  feldspars 
  are 
  complex 
  silicates 
  of 
  alumina 
  and 
  potash, 
  or 
  

   alumina, 
  lime 
  and 
  soda. 
  Orthoclase, 
  the 
  most 
  common 
  of 
  the 
  feld- 
  

   spars, 
  contains 
  about 
  17 
  fo 
  of 
  potash, 
  while 
  the 
  lime-soda 
  feldspars 
  

   have 
  from 
  4^ 
  to 
  12 
  fc 
  of 
  soda, 
  according 
  to 
  the 
  species. 
  Feldspars 
  are 
  

   the 
  most 
  important 
  source 
  of 
  alkalis 
  in. 
  clay, 
  and, 
  as 
  the 
  species 
  

   vary 
  somewhat 
  in 
  their 
  fusibility, 
  they 
  may 
  exercise 
  a 
  varying 
  in- 
  

   fluence 
  on 
  the 
  fusing 
  point 
  of 
  the 
  clay. 
  Thus 
  the 
  lime-soda 
  feld- 
  

   spars 
  axe 
  more 
  fusible 
  than 
  the 
  potash 
  ones."^ 
  

  

  The 
  micas 
  are 
  complex 
  silicates 
  of 
  alumina, 
  with 
  iron, 
  magnesia 
  

   and 
  potash. 
  Muscovite, 
  the 
  commonest 
  species 
  of 
  the 
  group, 
  con- 
  

   tains 
  nearly 
  12^ 
  of 
  potash 
  and 
  may 
  contain 
  a 
  little 
  soda. 
  While 
  

   feldspars 
  fuse 
  completely 
  at 
  about 
  2300° 
  F., 
  mica 
  alone 
  is 
  very 
  

   refractory, 
  being 
  unaffected 
  by 
  a 
  temperature 
  of 
  2550° 
  F. 
  While 
  

   it 
  probably 
  serv^es 
  as 
  a 
  flux, 
  it 
  is 
  not 
  kno^vn 
  positively 
  at 
  just 
  what 
  

   temperature 
  it 
  begins 
  to 
  act 
  as 
  such. 
  

  

  Alkalis, 
  specially 
  in 
  the 
  form 
  of 
  silicates, 
  are 
  frequently 
  a 
  de- 
  

   sirable 
  constituent 
  of 
  clay, 
  on 
  account 
  of 
  their 
  fluxing 
  properties, 
  

   as 
  in 
  burning 
  they 
  serve 
  to 
  bind 
  the 
  particles 
  together 
  in 
  a 
  dense, 
  

   hard 
  body 
  and 
  permit 
  the 
  ware 
  being 
  burned 
  at 
  a 
  lower 
  tempera- 
  

   ture. 
  

  

  In 
  the 
  manufacture 
  of 
  porcelain, 
  white 
  earthenware, 
  encaustic 
  

   tiles 
  and 
  other 
  wares 
  made 
  from 
  kaolins, 
  and 
  having 
  a 
  body 
  which 
  

  

  ' 
  Seger. 
  Oes. 
  Schrift. 
  p. 
  41 
  J. 
  

  

  