452 
SILVERING  AND  GILDING  OF  GLASS. 
The  solution  of  potash  or  soda  must  be  free  from  chlorides  ; 
pure  carbonate  of  soda  or  potash  must  be  dissolved  in  water,  and 
rendered  caustic  by  hydrate  of  lime  previously  freed  from  all 
chloride  by  washing  with  distilled  water.  The  solution  is  not 
filtered,  but  left  to  stand  until  it  becomes  perfectly  clear. 
Immediately  before  the  application  of  this  fluid,  it  is  mixed 
with  one-tenth  to  one-eighth  of  its  volume  of  solution  of  sugar 
of  milk,  containing  1  part  in  19  parts  of  water. 
In  silvering  small  concave  or  convex  mirrors,  a  stick  or  brass 
hook  is  attached  to  the  back  of  the  glass  by  a  resinous  cement, 
so  as  to  enable  the  glass  to  be  suspended  horizontally.  It  is 
suspended  over  a  suitable  glass  or  porcelain  saucer,  with  the  sur- 
face to  be  silvered  about  half  an  inch  from  the  bottom  of  the 
vessel,  and  the  fluid  previously  mixed  with  the  solution  of  sugar 
of  milk  is  poured  in  until  the  whole  surface  of  the  glass  is  im- 
mersed. 
For  the  production  of  flat  mirrors  the  author  recommends 
vessels  of  gutta  percha,  cut  out  of  a  flat  piece  so  as  to  have  a 
margin  of  about  an  inch  all  round  the  glass.  This  is  turned  up 
after  the  gutta  percha  has  been  softened  in  hot  water,  and  the 
corners  are  made  water-tight  by  the  application  of  a  hot  spatula 
or  knife.  The  glass  is  supported,  at  a  distance  of  half  an  inch 
from  the  bottom  of  the  vessel,  by  means  of  small  cones  of  gutta 
percha  at  the  corners,  and  the  space  between  the  surface  of  the 
glass  and  the  bottom  is  then  filled  with  the  silvering  fluid.  The 
author  admits  that  these  arrangements  are  very  imperfect,  and 
that  many  improvements  might  be  introduced,  but  the  glass 
should  always  be  suspended  at  the  surface  of  the  fluid. 
The  reduction  of  the  silver  takes  place  instantly  upon  the  mix- 
ture of  the  alkaline  solution  of  silver  with  the  sugar  of  milk  ;  the 
mixture  immediately  acquires  a  dark  color.  In  a  few  minutes  the 
glass  plate  appears  black ;  in  a  quarter  of  an  hour  it  becomes 
bright,  and  the  reduction  is  complete  when  the  fluid  between  the 
edge  of  the  glass  and  the  wall  of  the  vessel  is  covered  with  a  white 
specular  coat  of  silver.  Of  course  the  whole  of  the  silver  in  the 
solution  is  precipitated,  and  only  a  very  small  portion  of  it  goes 
to  form  the  mirror.  The  quantity  of  silver  attached  to  a  sur- 
face of  226  square  centims.  was  49  milligrms.  The  silvering  of 
a  mirror  of  1  metre  square  would  consequently  take  2.210  grms. 
