﻿782 
  Is'EW 
  YORK 
  STATE 
  MUSEUM 
  

  

  many 
  PennsYlvania 
  mines 
  tkere 
  often 
  occurs 
  considerable 
  iron 
  

   oxid, 
  and 
  iron 
  carbonate, 
  wbicL 
  are 
  known 
  as 
  ore-balls.^ 
  The 
  iron 
  

   oxid 
  concretions 
  are 
  generally 
  near 
  the 
  surface, 
  while 
  the 
  iron 
  car- 
  

   bonate 
  is 
  confined 
  to 
  the 
  interior 
  of 
  the 
  bank. 
  In 
  the 
  mi-m'r>ig 
  of 
  the 
  

   clay, 
  these 
  concretions 
  have 
  to 
  be 
  picked 
  out. 
  This 
  collecting 
  of 
  the 
  

   iron 
  by 
  natural 
  processes 
  has 
  therefore 
  served 
  to 
  purify 
  the 
  clay 
  

   mass; 
  where 
  it 
  has 
  not 
  taken 
  place 
  the 
  material 
  loses 
  its 
  value 
  as 
  

   a 
  refractory 
  clay. 
  Flint 
  clays 
  are 
  hard, 
  they 
  lack 
  plasticity, 
  hav 
  

   a 
  conchoidal 
  or 
  shell-like 
  fracture, 
  and 
  sometimes 
  a 
  very 
  faint 
  

   luster. 
  At 
  times 
  they 
  occur 
  in 
  beds 
  interstratified 
  with 
  other 
  rocks, 
  

   or 
  again 
  they 
  may 
  occur 
  as 
  basin-shaped 
  deposits, 
  in 
  which 
  case 
  

   they 
  seem 
  to 
  have 
  been 
  formed 
  by 
  chemical 
  precipitation. 
  

  

  The 
  color 
  of 
  a 
  fire 
  clay 
  is 
  an 
  indication 
  of 
  its 
  refractoriness 
  only 
  

   to 
  a 
  limited 
  extent. 
  Many 
  fire 
  clays 
  are 
  bluish 
  in 
  color, 
  while 
  

   others 
  are 
  light 
  gray 
  or 
  yellowish 
  white. 
  A 
  given 
  amount 
  of 
  iron 
  

   will 
  color 
  a 
  sandy 
  clay 
  more 
  strongly 
  than 
  a 
  less 
  silicious 
  one; 
  the 
  

   same 
  is 
  true 
  of 
  organic 
  matter. 
  The 
  latter 
  might 
  even 
  mask 
  the 
  

   color 
  of 
  iron. 
  The 
  condition 
  of 
  oxidation 
  of 
  the 
  iron 
  would 
  also 
  

   cause 
  a 
  difference 
  in 
  color, 
  ferric 
  compounds 
  being 
  red 
  or 
  yellow, 
  

   while 
  ferrous 
  ones 
  may 
  be 
  blue 
  or 
  gTay. 
  

  

  Whatever 
  the 
  color 
  of 
  the 
  fire 
  clay 
  in 
  its 
  unburned 
  condition, 
  in 
  

   the 
  fired 
  state 
  it 
  is 
  always 
  buff, 
  unless 
  ^atrified, 
  when 
  it 
  may 
  be- 
  

   come 
  red. 
  

  

  The 
  refractoriness 
  of 
  a 
  fire 
  clay 
  is 
  dependent 
  almost 
  entirely 
  

   on 
  its 
  chemical 
  composition. 
  It 
  can 
  be 
  said 
  in 
  general 
  that 
  the 
  

   more 
  powerful 
  fluxing 
  impurities, 
  such 
  as 
  ferric 
  oxid, 
  lime, 
  alkalis, 
  

   and 
  magnesia 
  should 
  not 
  exceed 
  1^ 
  if 
  possible. 
  Many 
  fire 
  brick 
  

   manufacturers 
  do 
  not 
  seem 
  to 
  recognize 
  the 
  fact 
  that 
  silica 
  acts 
  

   as 
  a 
  flux 
  at 
  high 
  temperatures. 
  

  

  The 
  recent 
  experiments 
  of 
  H. 
  O. 
  Hofman 
  tend 
  to 
  indicate 
  that 
  

   in 
  the 
  case 
  of 
  fire 
  clays 
  at 
  least 
  the 
  size 
  of 
  grain 
  has 
  no 
  effect 
  on 
  

   the 
  fusibility 
  of 
  the 
  mass. 
  The 
  tests 
  which 
  he 
  carried 
  out 
  are 
  

   described 
  under 
  " 
  Fusibility 
  of 
  clays 
  " 
  p. 
  563. 
  

  

  ' 
  Annual 
  report 
  Pa. 
  atate 
  college. 
  1897. 
  p. 
  52. 
  

  

  