SAND    AND    LIMESTONE    FOR    GLASS    MAKING. 
453 
DEFINITION    AND    COMPONENTS    OF    GLASS. 
Glass  is  a  fused  mixture  of  the  silicates  of  alkalies,  alkaline  earths,  and  of  more  common 
metals.  Usually  the  alkaline  bases  are  sodium  and  potassium ;  the  alkaline  earth  is  calcium, 
arid  the  common  metallic  element  is  lead.  All  of  these  are  used  in  the  form  of  salts.  In 
melting  together  the  various  ingredients  employed  in  the  hatch  or  mixture  it  appears  that 
silica  under  the  influence  of  heat  in  the  presence  of  a  flux  forms  silicates  with  sodium  or 
potassium,  and  calcium,  lead,  etc.,  and  the  alkaline  silicate  then  dissolves  the  remaining 
silicates.  It  is  this  solution  that  solidifies  into  glass  on  cooling.  Glass  may  be  divided 
into  four  general  classes— plate,  window,  green  bottle,  and  flint.  Owing  to  manufacturing 
conditions,  few  makers  follow  the  same  formula,  but  the  following  table  shows  the  general 
proportion  by  weight  which  sand  bears  to  the  other  ingredients  commonly  used  in  certain 
types  of  American-made  glass. a 
General  proportions  by  weight  of  various  component*  of  glass. 
Component. 
['late 
glass. 
100 
Window 
glass. 
Green 
bottle. 
Lead 
flint. 
Lime 
Hint. 
Sand  (Si02) 
100 
42 
100 
38 
100 
100 
Salt  cake  (Na2S04) 
Soda  ash  (Na2C03).., 
36 
24 
1 
30 
Limestone  (CaC03) 
40 
6 
2 
34 
5 
Carbon  (C) 
Arsenic  ( As203) 
.15 
"  .02 
Slaked  lime  (Ca(OII)2) 
12 
Potash  (K2C03) 
34 
48 
6 
.00 
.02 
Red  lead  (2PbO-+-Pb02) 
Niter  (NaN03) . . .   . 
1 
0. 06 
.23 
REQUIREMENTS    OF    GLASS    SAND. 
Sand  is  therefore  the  major  constituent  of  glass,  constituting  from  52  to  65  per  cent  of 
the  mass  of  the  original  mixture,  or  from  60  to  75  per  cent  of  the  finished  product  after 
melting  has  driven  off  carbon  dioxide,  sulphur  dioxide,  and  other  volatile  materials.  To 
the  sand  is  due  the  absence  of  color  (according  to  its  purity),  the  transparency,  brilliancy, 
and  hardness  of  glass.  In  other  words,  the  quality  of  the  glass  depends  largely  on  the 
quality  of  the  sand.  For  the  finest  flint  ware,  such  as  optical  and  cut  glass,  "  water  white- 
ness," absolute  transparency,  great  brilliance,  and  uniform  density  are  required,  and  only 
the  purest  sand  can  be  employed,  since  slight  impurities,  especially  small  quantities  of  iron, 
tend  to  destroy  these  effects.  For  plate  and  window  glass,  which  are  commonly  pale 
green,  absolute  purity  is  not  so  essential,  but  generally  the  sand  should  not  carry  more 
than  two-tenths  per  cent  of  ferric  oxide.  Green  and  amber  glass  for  bottles,  jars,  and  rough 
structural  work  can  be  made  from  sand  relatively  high  in  impurities.  An  excess  of  the 
chief  impurity,  iron,  is  usually  avoided  in  the  quarries  by  a  careful  selection  of  the  whitest 
sand,  although  the  whitest  sand  is  not  invariably  the  purest.  Repeated  washing  tends 
to  remove  the  iron.  Magnetic  separators  also  have  been  resorted  to,  especially  when  the 
iron  is  present  in  the  form  of  magnetite.  Clay  materials  are  objectionable  because  they 
cloud  the  glass.  Washing  helps  to  remove  them,  since  they  occur  usually  in  a  very  finely 
divided  state.     Magnesia,  which  is  more  apt  to  be  introduced  into  glass  materials  through 
a  Data,  except  for  lime  flint,  are  taken  from  Linton,  Robert,  Glass:  Mineral  Industry  for  1899,  vol  8, 
1900,  pp.  244-245. 
