Re-silvering Sextant mirrors? Better to aluminize them

Sooner or later the silvering on the mirror of your sextant will start fading away, eventually making the instrument unusable (and worthless). Even a well maintained and lubricated sextant, the nature of the marine environment makes the issue (of fading mirror silvering) the most serious problem encountered in its use. When you see small pieces of the silvering start to detach, the moment has arrived to do something.

In an emergency situation, during a long navigation, you can always make what is known as ‘emergency silvering’. This achieved by removing the original paint and silver from the mirror with a solvent and smooth mechanical abrasion to avoid scratching the (delicate) glass surface and applying some black synthetic paint, taking care to (accurately) mask half of the horizon mirror with tape. It may seem strange, but even with reduced luminosity; the mirror will perform better than using a mirror with damaged silvering; and for sun sights, it may be even better than using a silvered mirror.

By the way, if you ever wondered if you could cut a conventional mirror for sextant use, you’d better put the weird idea straight out of your mind. Only original sextant mirrors are manufactured with perfectly parallel flat surfaces to avoid introducing diffraction errors and are expressly built for this purpose.

At one time, there were many craft laboratories capable of mirror re-silvering at a reasonable price, but today these services have all but disappeared, particularly for such small jobs. Having experienced such difficulties myself, I was forced make the decision to try the job myself.

Getting the necessary chemicals (isn’t a problem) but bear in mind that handling toxic substances always requires correct use of appropriate personal protection equipment and a well ventilated working environment. After a few experimental attempts, anyone can achieve good results, but no matter the care and accuracy of the application, the lifespan of these mirrors will struggle to be at best the same as the original.

Looking closely at modern sextants, I noticed that the reflecting surface of these mirrors was far superior to their older counterparts; being more durable and discovered they are no longer silvered but aluminized (i.e. coated with a thin layer of aluminum). A quick search on the internet returned contact details for a number craft laboratories specializing in aluminizing mirrors for telescopes and amateur astronomers. I contacted one and explained my situation; that I needed to use the reflecting surface through the crystal, not the external one (in order to respect the original level of the mirror in its housing) and that the horizon mirror needed to be aluminized only on half its surface. I dispatched the mirrors from some old (but very good!) sextants by post and waited anxiously for their return. After only ten days, they were returned duly aluminized.

The next step was to apply two layers of protective paint to prevent the corrosive action of the salty environment and abrasion of the calibration screws by painting the screw heads. Before starting, I protected the areas not to be painted with masking tape. In the accompanying pictures you can see some of the mirrors and how to carefully remove the masking tape with the aid of a razor blade from the horizon mirror. It is good practice to allow the protective paint on the horizon mirror to slightly exceeding the extent of aluminized portion while maintaining a sharp profile (in order to avoid the reflective surface coming into contact with maritime mist. You can also see an old sextant (a Carl Plath) returned back to its original glory – and with more durable mirrors.

Last but not least, don’t forget that for a navigator who is respectful of sea life and nature, as we all claim to be, the best lubricant to replace whale oil in sextant maintenance, is Jojoba oil which you can purchase in any herbalist shop for a few bucks(US)/quid(UK)

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