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!
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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.