NavList:

Resilvering with tin foil at sea from Bowditch 1962

ong period, the arc should be protected with a thin coat of petroleum jelly. If the mirrors need resilvering, they are best taken to an instrument shop where a professional job can be done . However, on rare occasions it may be necessary to re silver the mirrors of a sextant at sea. In anticipation of this possibility, the navigator should obtain the necessary materials in advance, as makeshift substitutes cannot be relied upon to do the job adequately . The required materials are xylene ( available in most pharmacies) , dilute nitric acid ( optional ) , alcohol , cotton , tin foil about 0.005 inch thick , a small amount of mercury, a clean blotter, and some tissue paper. Do not sub stitute aluminum foil commonly used in packaging candy and cigarettes. First, remove the protective coating with alcohol (or better, acetone) from the back of the mirror to be resilvered , and clean the glass with xylene or acid . If the old silvering is difficult to remove, soak it in water. Place the blotter on a flat surface and turn up and seal the edges to form a tray . This will serve to contain the mercury if the vessel should roll during the operation. Using cotton, clean and smooth out both sides of a piece of tin foil slightly larger than the glass to be silvered , first with alcohol and then with xylene (do not use acid) . Make certain that no lint adheres to the foil, and place it on the blotter. Clean the mercury by squeezing it through cheese cloth, and apply a drop to the foil. Carefully spread it over the surface with a finger, making sure that none of the mercury gets under the foil. Add a few more drops of mercury until - the entire surface of the foil is covered and tacky . The mercury combines with some of the tin to form an amalgam . Place the chemically cleaned glass on a piece of clean tissue paper with the side to be silvered face down . Then place the glass and the paper on the amalgam . Apply slight pressure to the glass and withdraw the tissue paper. Following this, grasp the edge of the tin foil and lift it and the mirror from the blotter . Invert the glass and the tin foil and place in an inclined position, silvered side up. Any mercury remaining on the blotter is no longer pure and should be disposed of. Five or six hours later any loose foil may be scraped from the sides of the mirror, and the following day a coat of commercial varnish or lacquer should be applied to the silvered INSTRUMENTS FOR CELESTIAL NAVIGATION 411 surface. Should the mirrored half of the horizon glass require silvering, the clear half may be protected by a strip of cellulose or adhesive tape. 1509. Sextant adjustments . There are at least sev