WORKING  IN  ALUMINUM. 
553 
Aluminum,  like  silver,  is  susceptible  of  a  very  fine  matting," 
which  is  not  affected  by  exposure  to  the  air,  or  by  any  of  the 
impurities  usually  present  in  the  atmosphere  of  towns.  To  ob- 
tain this  matting,  the  aluminum  objects  (being  previously  washed 
in  benzole  or  essence  of  turpentine)  must  be  plunged  into  a  weak 
solution  of  caustic  soda,  thoroughly  well  washed,  and  exposed 
to  the  action  of  strong  nitric  acid.  When  the  desired  matting 
has  been  obtained,  it  must  be  well  washed  again,  and  dried  in 
sawdust. 
Aluminum  is  easily  polished  or  burnished.  To  do  this,  it  is 
necessary  to  use  a  mixture  of  equal  parts  of  rum  and  olive  oil, 
as  an  intermediate  substance  between  it  and  the  polishing  stone 
or  powder  used.  The  polishing  stone  is  steeped  in  this  mixture, 
and  will  then  burnish  aluminum  in  the  same  manner  as  gold  and 
silver  is  burnished,  care  being  taken  not  to  press  too  heavily 
upon  the  burnishing  instrument. 
Aluminum  can  be  beaten  out,  either  hot  or  cold,  to  the  same 
extent  and  as  perfectly  as  gold  or  silver  ;  and  it  is  susceptible 
of  being  rolled  in  much  the  same  way  as  either  of  the  above 
metals.  Leaves  as  thin  as  those  used  for  gilding  and  silvering 
can  be  made  of  aluminum.  Covered  ingot  moulds  of  iron  answer 
best  for  receiving  aluminum  intended  to  be  used  in  the  rolling 
mill.  Aluminum  quickly  loses  its  temper,  and  therefore  requires 
frequent  reheating.  The  temperature  of  this  reheating  is  a  dull 
red  heat,  and  when  the  plates  become  very  thin,  this  demands 
the  greatest  attention. 
Aluminum  is  easily  drawn  into  wire.  For  this,  the  ingots  are 
run  into  an  open  mould,  so  as  to  form  a  kind  of  quadrangular 
shape  of  a  little  less  than  half-an-inch  section,  which  is  then 
beaten  upon  the  edges  by  the  hammer  very  regularly  ;  the  oper- 
ation of  drawing  out  is  then  commenced  on  a  horizontal  bench, 
by  very  gradually  reducing  the  diameter  of  the  metal  intended 
to  be  drawn  into  wire,  and  by  frequent  reheating,  and  then  the 
ordinary  process  of  wire-drawing  can  be  proceeded  with.  When 
the  threads  are  required  extremely  fine — as,  for  example,  for 
the  manufacture  of  lace — the  heating  becomes  a  very  delicate 
operation,  on  account  of  the  fineness  of  the  threads  and  the  fusi- 
bility of  the  metal.    The  heat  of  the  current  of  air  issuing  from 
