98 lines
5.6 KiB
Plaintext
98 lines
5.6 KiB
Plaintext
Silvering Telescope Mirrors
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by Don Barry
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After a few years with our telescopes, most of us feel as if they are
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our children -- we know how to deal with every tantrum and fix every cut
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and bruise. Yet when the mirrors age and tarnish into senescence, we
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grumble as we dismantle our behemoths and dispatch the mirrors in gigantic
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excelsior boxes to the factory for a face-lift. But we don't have to send
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them to foster care: with a little patience and the right chemicals, the
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following recipe will restore a mirror to pristine youth, and even increase
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its reflectivity some 8 percent over an aluminum coating.
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First the old coating must be removed. This can be done with a wash in
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pure nitric acid in the case of an old silver mirror, or a teaspoon of lye
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in a cup of water for an aluminum coating. After this and a thorough
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rinse, a second and third wash in nitric acid is mandatory in either case,
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applied (using gloves!!) with cotton balls, and rubbed with considerable
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pressure, followed by rinsing until the characteristic squeak of cotton on
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wet glass is heard. Following a final rinse in distilled water, the
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impeccably clean glass is kept under water until silvered as follows:
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A: Silver Nitrate 6 grams
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distilled water to make 100 ml.
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B: Ammonium Nitrate 9 grams
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distilled water to make 100 ml.
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C: Sodium Hydroxide 10.5 grams
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distilled water to make 100 ml.
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D: Glucose 10 grams
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Denatured alcohol 15 ml.
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distilled water to make 100 ml.
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Wrap the mirror face-up about the edge with waxed paper (never foil) and
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hold with a rubber band in order to make a dish supporting the chemicals.
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Alternately, lay the mirror face-down on two wooden dowels in a non-
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metallic pan. Calculate the amount of fluid in each case to cover the
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surface thoroughly or to reach halfway up the mirror side - this is about
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30 to 40 ml. total volume for a 6" mirror. Divide this by 3 to obtain the
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quantity of solution A required. Now mix separately equal volumes of
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solutions A, B, and C, adding C last, stirring to note that the last bit of
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C added causes the solution to turn brown to black. Stop just at the point
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that precipitate begins to form - the ideal solution should be a slightly
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turbid tea-color. Next add D in quantity one third to that of A, and
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immediately pour upon the mirror.
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The solution will turn black and begin to deposit the coating. The
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mirror will appear invitingly bright after only a minute or two, but the
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deposit must be thickened by allowing silvering to proceed for at least 5
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to 8 minutes so that the mirror is not overly transparent. After the coat
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is deposited, immediately rinse the mirror in copious amounts of distilled
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water, then wash down with alcohol and allow to dry. After a day of aging,
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the film can be polished somewhat if necessary by rubbing lightly with
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lens-cloth with perhaps a little rouge.
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It is impossible to damage a mirror in any way by improper deposition of
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silver. If the film produced is too thin, it may be thickened by repeating
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the process. If too thick and rough, it may be removed and the process
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repeated. The ideal film will be just thick enough so that a lamp filament
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is barely visible by transmitted light, yet thin enough that the surface
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requires but little polishing to bring out a full lustre.
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Perhaps the only disadvantage to a silver coat is the inevitable tarnish
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that results and limits the useful life of a silvered mirror to only one or
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two observing seasons. Even heavy tarnish, however, will not impact
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viewing of most deep sky objects, whose emission is in the red end of the
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spectrum. Blue objects, however, will be affected early in the mirror's
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life as tarnish gradually extinguishes the mirror's ability to reflect
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short-wave light. A useful way to extend the mirror's tarnish-free heydey
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is by lining its holder and telescope cap with alum-paper, prepared by
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soaking paper in a saturated solution of common alum. This paper scavenges
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from the air the agent(hydrogen sulfide) responsible for tarnish
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deposition. This paper can also protect your finest sterlingware!
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Remember to use only the finest chemicals available and distilled water.
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As with all toxic and corrosive chemicals, use appropriate precautions such
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as protective eye-wear, gloves, aprons, etc. Never use a metal pan or
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metal supports for the mirror. Flush all end products down a drain with
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copious amounts of water. If black silver spots are deposited in unwanted
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places, they may be removed with a dilute solution of nitric acid. Don't
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worry about these spots on the skin - they aren't toxic, but may take a day
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or two to wear off. Wash your hands thoroughly immediately after handling
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the chemicals, and keep them out of reach of children.
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It is sometimes difficult to find local sources of the chemicals
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required, but they are always available by mail-order from chemical supply
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corporations such as Fisher or Cenco. If there is sufficient interest, a
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workshop in mirror-silvering can be conducted at a future club meeting.
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Bonne Chance!
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Don Barry is a member of the Atlanta Astronomy Club. This article appeared
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in the February, 1987 issue of Ad Astra, the Atlanta Astronomy Club's monthly
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bulletin. Don may be reached in care of Leonard Abbey, CIS #72277,566.
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