﻿544 
  I^EW 
  YORK 
  STATE 
  MUSEUM 
  

  

  position 
  but 
  different 
  plasticity; 
  viz, 
  washed 
  Zettlitz 
  kaolin 
  and 
  

   plastic 
  refractory 
  clay 
  from 
  Mlihlheim, 
  near 
  Coblenz. 
  Both 
  have 
  

   only 
  a 
  small 
  admixture 
  of 
  quartz 
  sand, 
  viz, 
  about 
  1-|^, 
  the 
  balance 
  

   being 
  nearly 
  pure 
  clay 
  substance, 
  while 
  in 
  the 
  percentage 
  of 
  ferric 
  

   oxid 
  they 
  differ 
  by 
  only 
  1^. 
  

  

  Briquets 
  (air-dried) 
  of 
  the 
  Zettlitz 
  kaolin 
  were 
  loose, 
  and 
  rubbed 
  

   easily, 
  while 
  their 
  porosity 
  was 
  42^. 
  Those 
  of 
  the 
  Miilheim 
  

   clay 
  were 
  hard, 
  and 
  showed 
  only 
  28^ 
  porosity. 
  If 
  both 
  are 
  heated, 
  

   the 
  latter 
  gets 
  thoroughly 
  dense 
  at 
  1100° 
  — 
  1150° 
  O., 
  while 
  the 
  

   kaolin 
  retains 
  its 
  porosity 
  up 
  tO' 
  a 
  high 
  temperature. 
  The 
  exhibi- 
  

   tion 
  of 
  density 
  by 
  kaolin 
  is 
  not 
  to 
  be 
  regarded 
  as 
  even 
  the 
  be- 
  

   ginning 
  of 
  fusion, 
  for 
  the 
  clay, 
  after 
  assuming 
  it, 
  retains 
  it 
  unaltered 
  

   up 
  to 
  a 
  high 
  temperature. 
  Many 
  plastic 
  and 
  hard 
  drying 
  clays 
  act 
  

   in 
  this 
  respect 
  like 
  the 
  Muhlheim 
  material 
  ; 
  they 
  sinter 
  however 
  at 
  

   a 
  much 
  lower 
  temperature. 
  

  

  Tor 
  the 
  manufacture 
  of 
  glass 
  pots, 
  this 
  is 
  of 
  high 
  importance; 
  

   for 
  it 
  is 
  not 
  the 
  most 
  refractory 
  clays 
  that 
  are 
  the 
  best, 
  but 
  those 
  

   which 
  burn 
  dense 
  at 
  a 
  low 
  temperature, 
  and 
  are 
  consequently 
  less 
  

   attacked 
  by 
  the 
  molten 
  glass. 
  

  

  The 
  same 
  is 
  true 
  of 
  brick 
  used 
  in 
  coke 
  ovens 
  holding 
  coal 
  with 
  

   soluble 
  salts. 
  

  

  Plasticity, 
  whatever 
  its 
  cause, 
  is 
  an 
  important 
  property 
  from 
  a 
  

   commercial 
  standpoint, 
  and 
  interesting 
  from 
  a 
  scientific 
  one. 
  

  

  The 
  amount 
  of 
  water 
  required 
  to 
  develop 
  the 
  maximum 
  plasticity 
  

   varies. 
  If 
  too 
  little 
  is 
  added, 
  the 
  clay 
  cracks 
  in 
  molding 
  and 
  is 
  

   stiff 
  and 
  hard 
  to 
  work. 
  If 
  too 
  much 
  water 
  is 
  used, 
  the 
  paste 
  be- 
  

   comes 
  soft 
  and 
  retains 
  its 
  shape 
  with 
  difficulty. 
  Lean 
  clays 
  usually 
  

   require 
  less 
  water 
  to 
  produce 
  a 
  workable 
  mass 
  than 
  fat 
  ones. 
  

  

  Tensile 
  strength 
  

  

  The 
  tensile 
  strength 
  or 
  binding 
  power 
  of 
  a 
  clay 
  often 
  stands 
  in 
  

   relation 
  to 
  its 
  plasticity, 
  but 
  not 
  always. 
  It 
  is, 
  however, 
  an 
  im- 
  

   portant 
  property, 
  and 
  exerts 
  an 
  important 
  effect 
  in 
  connection 
  with 
  

   the 
  cracking 
  in 
  drying. 
  One 
  way 
  of 
  testing 
  the 
  tensile 
  stiengtK 
  

   is 
  the 
  briquet 
  method 
  mentioned 
  under 
  " 
  Plasticity," 
  p. 
  539. 
  

  

  