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According to Bill Morris,
it is not worth resilvering old mirrors, because good quality
mirrors are available for small price, and the only thing you
have to do is to cut them to fit.

Old sextants usually have rectangular mirrors, so cutting should
not be a problem. Bill also describes in his blog on sextant mirrors how
to cut round mirrors,
but for this some non-trivial tools are needed.

Speaking of the prisms, they should be fine without silvering.

I am curious what sort of sextant with prisms you have?
An aircraft sextant?

The mirror will also work without silvering, if you remove the
remains of the silvering and paint that back surface black.

Alex.

> Dennis Peterson, you asked:
> "When is silvering so old that it needs to be replace?"
>
> If there are patches missing or tarnished, it doesn't really matter. Since
> the observer's eye is focused at infinity when looking through a sextant
> while the mirrors are just a few inches away, even large missing patches
> are only a minor inconvenience. They may look bad, but they don't
> necessarily affect performance. The only case I had where the silvering
> failed was a modern sextant. The silvering actually bubbled and peeled
> away. That mirror became useless as soon as the silvering began to lift
> away from the glass since it was no longer a flat surface.
>
> -FER
>
>
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