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Thread: Pre-Silvered Mirrors

  1. #1
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    Pre-Silvered Mirrors

    I was thinking about the history of mirrors.

    Before it was discovered how to apply silver to glass, they were made of polished speculum metal. Just one problem: it tarnishes, and mirrors don't last so long before they have to be repolished. Why didn't they clear-coat such mirrors? They had resin varnishes.

    Why weren't mirrors created using mercury? Picture an aquarium, but in one dimension it's only around a millimeter wide on the inside, fill with mercury, glue a lid on it.
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    Quote Originally Posted by SkepticJ View Post
    Why weren't mirrors created using mercury? Picture an aquarium, but in one dimension it's only around a millimeter wide on the inside, fill with mercury, glue a lid on it.
    Expensive, very fragile, messy?

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    I would venture a guess that someone tried varnishing a speculum mirror and found that the optical quality was degraded. The coating would have to be very thin and optically perfect. With modern techniques using substances that can be evaporated onto the surface in a vacuum chamber, we now apply clear coating to aluminized mirrors. In principle that could be done with speculum, but glass is easier to polish and figure.

    Experimental mirrors have been made by putting a thin layer of mercury on a slowly rotating paraboloidal substrate. The centrifugal action causes the surface of the mercury to form the desired paraboloid. The drawback in addition to the toxic vapor is that such a device can only aim at the zenith.

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    I don’t know how successfully, but at least one (https://www.newscientist.com/article...ality-at-last/) has been built and put into service. One wonders if an alternative liquid to mercury. could be used; Gallistan, perhaps?

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  5. #5
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    Quote Originally Posted by SkepticJ View Post
    I was thinking about the history of mirrors.

    Before it was discovered how to apply silver to glass, they were made of polished speculum metal. Just one problem: it tarnishes, and mirrors don't last so long before they have to be repolished. Why didn't they clear-coat such mirrors? They had resin varnishes.

    Why weren't mirrors created using mercury? Picture an aquarium, but in one dimension it's only around a millimeter wide on the inside, fill with mercury, glue a lid on it.
    Varnishes yellow over time.
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    Quote Originally Posted by Swift View Post
    Varnishes yellow over time.
    Yes, but the process is slow. Varnishes are also easily removable, and new varnish can be reapplied--paintings were/are varnished to protect them-- later removal and replacement is part of the design.

    Another option is sun-bleached beeswax, which doesn't go yellow as far as I know. Get the layer thin enough (e.g. using heat or solvents) and it's clear. Sure, it would cut down on the reflectivity a few percent. If in a telescope, couldn't you just use a slightly larger primary mirror to compensate for the slightly diminished reflectivity?
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    Quote Originally Posted by SkepticJ View Post
    Yes, but the process is slow. Varnishes are also easily removable, and new varnish can be reapplied--paintings were/are varnished to protect them-- later removal and replacement is part of the design.
    No, it's not always a slow process. It varies depending upon the type of varnish you're talking about. Many have a warm/amber cast right out of the can. Acrylics are among the clearest but they're typically waterborne so adhesion can be problematic on a highly polished surface. Easy removal would not be a selling point for me. I want to touch a delicate optical surface as little as possible.

    Another option is sun-bleached beeswax, which doesn't go yellow as far as I know. Get the layer thin enough (e.g. using heat or solvents) and it's clear. Sure, it would cut down on the reflectivity a few percent. If in a telescope, couldn't you just use a slightly larger primary mirror to compensate for the slightly diminished reflectivity?
    But it wouldn't just be a question of reflectivity, would it? What about diffraction/diffusion effects? Even when polished out, beeswax takes on a dull sheen. Left to it's own devices it's decidedly more matte.
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  8. #8
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    It's the scattering. Varnish looks shiny because it produces a specular reflection, but that's not the same as producing a mirror image. When I was a teenager I varnished a reflective metal surface - an ID bracelet - that was intended to be engraved. But I couldn't afford engraving, so I applied Letraset characters to the surface. I could see a slightly blurry reflection in the metal before I varnished it, but afterwards there was no recognizable reflection at all - just bright specular scattering that actually obscured my Letraset. And it very quickly started to flake and peel.

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  9. #9
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    Quote Originally Posted by DaveC426913 View Post
    Expensive, very fragile, messy?
    And poisonous.

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