                        Silvering Telescope Mirrors

                               by Don Barry

   After a  few years  with our  telescopes, most of us feel as if they are
our children  -- we  know how  to deal with every tantrum and fix every cut
and bruise.   Yet  when the  mirrors age  and tarnish  into senescence,  we
grumble as  we dismantle our behemoths and dispatch the mirrors in gigantic
excelsior boxes  to the factory for a face-lift.  But we don't have to send
them to  foster care:   with a little patience and the right chemicals, the
following recipe will restore a mirror to pristine youth, and even increase
its reflectivity some 8 percent over an aluminum coating.

   First the  old coating must be removed.  This can be done with a wash in
pure nitric  acid in the case of an old silver mirror, or a teaspoon of lye
in a  cup of  water for  an aluminum  coating.   After this  and a thorough
rinse, a  second and third wash in nitric acid is mandatory in either case,
applied (using  gloves!!) with  cotton balls,  and rubbed with considerable
pressure, followed  by rinsing until the characteristic squeak of cotton on
wet glass  is heard.   Following  a final  rinse in  distilled  water,  the
impeccably clean glass is kept under water until silvered as follows:


   A: Silver Nitrate          6 grams
      distilled water         to make 100 ml.
   B: Ammonium Nitrate        9 grams
      distilled water         to make 100 ml.
   C: Sodium Hydroxide        10.5 grams
      distilled water         to make 100 ml.
   D: Glucose       10 grams
      Denatured alcohol       15 ml.
      distilled water         to make 100 ml.


   Wrap the mirror face-up about the edge with waxed paper (never foil) and
hold with  a rubber  band in order to make a dish supporting the chemicals.
Alternately, lay  the mirror  face-down on  two wooden  dowels  in  a  non-
metallic pan.   Calculate  the amount  of fluid  in each  case to cover the
surface thoroughly  or to  reach halfway up the mirror side - this is about
30 to  40 ml. total volume for a 6" mirror.  Divide this by 3 to obtain the
quantity of  solution A  required.   Now mix  separately equal  volumes  of
solutions A, B, and C, adding C last, stirring to note that the last bit of
C added causes the solution to turn brown to black.  Stop just at the point
that precipitate  begins to  form - the ideal solution should be a slightly
turbid tea-color.   Next  add D  in quantity  one third  to that  of A, and
immediately pour upon the mirror.

   The solution  will turn  black and  begin to  deposit the  coating.  The
mirror will  appear invitingly  bright after  only a minute or two, but the
deposit must  be thickened  by allowing silvering to proceed for at least 5
to 8  minutes so that the mirror is not overly transparent.  After the coat
is deposited,  immediately rinse the mirror in copious amounts of distilled
water, then wash down with alcohol and allow to dry.  After a day of aging,
the film  can be  polished somewhat  if necessary  by rubbing  lightly with
lens-cloth with perhaps a little rouge.

   It is impossible to damage a mirror in any way by improper deposition of
silver.  If the film produced is too thin, it may be thickened by repeating
the process.   If  too thick  and rough,  it may be removed and the process
repeated.  The ideal film will be just thick enough so that a lamp filament
is barely  visible by  transmitted light,  yet thin enough that the surface
requires but little polishing to bring out a full lustre.

   Perhaps the only disadvantage to a silver coat is the inevitable tarnish
that results and limits the useful life of a silvered mirror to only one or
two observing  seasons.   Even heavy  tarnish,  however,  will  not  impact
viewing of  most deep  sky objects, whose emission is in the red end of the
spectrum.   Blue objects,  however, will  be affected early in the mirror's
life as  tarnish gradually  extinguishes the  mirror's ability  to  reflect
short-wave light.   A useful way to extend the mirror's tarnish-free heydey
is by  lining its  holder and  telescope cap  with alum-paper,  prepared by
soaking paper in a saturated solution of common alum.  This paper scavenges
from  the   air  the   agent(hydrogen  sulfide)   responsible  for  tarnish
deposition.  This paper can also protect your finest sterlingware!

   Remember to use only the finest chemicals available and distilled water.
As with all toxic and corrosive chemicals, use appropriate precautions such
as protective  eye-wear, gloves,  aprons, etc.   Never  use a  metal pan or
metal supports  for the  mirror.   Flush all end products down a drain with
copious amounts  of water.  If black silver spots are deposited in unwanted
places, they  may be  removed with a dilute solution of nitric acid.  Don't
worry about these spots on the skin - they aren't toxic, but may take a day
or two  to wear off.  Wash your hands thoroughly immediately after handling
the chemicals, and keep them out of reach of children.

   It is  sometimes difficult  to  find  local  sources  of  the  chemicals
required, but  they are always available by mail-order from chemical supply
corporations such  as Fisher  or Cenco.  If there is sufficient interest, a
workshop in  mirror-silvering can  be conducted  at a  future club meeting.
Bonne Chance!


               --------------------------------------------------

    Don Barry is a member of the Atlanta Astronomy Club.  This article appeared
in the February, 1987 issue of Ad Astra, the Atlanta Astronomy Club's monthly
bulletin.  Don may be reached in care of Leonard Abbey, CIS #72277,566.
