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Yesterday at a library I came across "The Sextant Simplified" by Oswald
M. Watts ("Master Mariner, F.R.A.S, F.R.I.N., Assoc. R.I.N.A., editor of
Reed's Nautical Almanac"), 1976.

Watt uses a curious term I haven't seen before. "The Hezzanith
rectangular horizon glass is provided hermetically sealed (and therefore
impervious to salt water or spray) in the more expensive types, and can
be fitted as an extra in the cheaper models."

And, "A Hezzanith rectangular index glass may be made hermetically
sealed and impervious to the action of salt spray, and therefore the
mirror never needs re-silvering."

And, "Figure 7 shows the Husun circular index mirror, which hermetically
seals the mirror from the entry of salt spray, and thus entirely
prevents the deterioration of the mirrors from this cause."

So how do you hermetically seal a mirror? I got a clue from — of all the
unlikely places — a product description at restroomdirect.com: "Standard
glazing is No 1 quality, 1/4" (6.4mm) thick plate/float glass, silver
coated and hermetically sealed with uniform copper plating, and
warranted against silver spoilage for 15 years. Mirrors meet Federal
Specification DD-M-411C, ASTM C1036-91."

   

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