c.  Small    quantities    of    lead   are    frequently    found    in
bronze.      The  lead  is  detected  as   triple  nitrite   (§    127).
ii  EXAMINATION  OF  ALLOYS  207

Zinc  is  often  present   in  great  quantity.      See  Brass   and
Statuary  Bronze,  §  130,  §  132.
§  130.  Copper  and  Zine,  Common  Brass

These  alloys  are  more  homogeneous  than  true  bronzes.
Spots  of  white  alloy  are  never  found.  By  etching,  a  yellow
lattice-work  on  a  reddish  ground  is  laid  bare  in  alloys  con-
taining from  3  to  12  per  cent  of  zinc,  or  a  yellow  weft,
filled  in  with  darker  yellow  (alloys  with  20  to  50  per  cent
of  zinc).  Caustic  ammonia  will  give  good  etchings;  its
only  drawback  is  a  broad  dark  seam  of  precipitated  copper.
This  is  very  characteristic  of  brass,  but  at  the  same  time
it  looks  somewhat  unworkmanlike.  As  it  is  difficult  to
remove,  it  is  avoided  by  immersing  the  specimens  in
ammonia  instead  of  pouring  on  the  solvent.  Heating,
followed  by  quenching  in  weak  sulphuric  acid,  will  give
good  characteristic  etchings  :  on  bronze,  a  red  field  with
whitish  threads;  on  brass,  an  orange  field  with  yellow  threads.
Strong  nitric  acid  will  produce  a  marked  relief  on  bronze  ;
on  brass  the  relief  will  be  insignificant,  the  colour  of  the
etched  specimen  being  a  brilliant  gold  yellow.
The  hardness  of  brass  is  inferior  to  that  of  bronze.  It
remains  stationary  between  3.1  and  3.2,  from  8  per  cent  up
to  50  per  cent  of  zinc.  No  sensible  difference  of  hardness
is  to  be  found  between  crystals  and  matrix.
