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marnixR
Post  Post subject: thermos flask  |  Posted: Sat Apr 15, 2017 8:57 am
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not sure whether this topic belongs here, but i couldn't think of another part of the forum where it would, so here goes nothing :

the standard thermos flask has a glass inner body with a metallic reflective coating on the outside
i was just curious what metal(s) are being used or have been used to establish this reflective layer

i've been unable to find anything on-line that goes beyond describing it as a "metallic layer" (in all cases unspecified which metal(s) are involved)

does anyone out there know ?

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geordief
Post  Post subject: Re: thermos flask  |  Posted: Sat Apr 15, 2017 9:46 am

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marnixR wrote:
not sure whether this topic belongs here, but i couldn't think of another part of the forum where it would, so here goes nothing :

the standard thermos flask has a glass inner body with a metallic reflective coating on the outside
i was just curious what metal(s) are being used or have been used to establish this reflective layer

i've been unable to find anything on-line that goes beyond describing it as a "metallic layer" (in all cases unspecified which metal(s) are involved)

does anyone out there know ?

Very difficult to find a specific metal cited in my searches but surely it has to be aluminium as it is so cheap and already used for so many other purposes.
The closest description I could find was "silvered".

Also the fact that simple unsilvered aluminium flasks are also made seems to show that they may well be using aluminium for the silvering.

I always assumed it was mercury and perhaps it (or some equivalent ) is used in more efficient types of vacuum flasks.

What do they use on reflective surfaces in optical surfaces, I wonder (like the Hubble mirrors ,perhaps) ?


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g o r t
Post  Post subject: Re: thermos flask  |  Posted: Sat Apr 15, 2017 10:17 am

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Almost all second surface mirrors are now aluminum.
Silver used to be used but is more prone to oxidation (origin of term silvered).
Gold is mostly used for first surface mirrors in critical applications.
Aluminum is also used for optical spectrum first surface mirrors but is usually coated.


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marnixR
Post  Post subject: Re: thermos flask  |  Posted: Sat Apr 15, 2017 4:12 pm
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my wife is under the impression that at one point mercury was used for mirrors - is that correct ?

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geordief
Post  Post subject: Re: thermos flask  |  Posted: Sat Apr 15, 2017 6:25 pm

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marnixR wrote:
my wife is under the impression that at one point mercury was used for mirrors - is that correct ?


I have quite a few old mirrors with the coating very weathered . I was advised not to put in a new "pane" as it would detract from its aesthetic value.

That is not to say they are made with mercury but I too was under the impression that mercury was used in mirrors. Again that might just be a misunderstanding on my part.


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Dywyddyr
Post  Post subject: Re: thermos flask  |  Posted: Sat Apr 15, 2017 8:22 pm
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Gotta go with the answers already given: aluminium.
It's still called silvering though.
Re the mercury: early mirrors (C16) used an amalgam of tin and mercury as the silvering. Later mirrors used actual silver.


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marnixR
Post  Post subject: Re: thermos flask  |  Posted: Sat Apr 15, 2017 9:36 pm
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how do they make the aluminium so shiny ? my first impression of aluminium as a metal is that looks dull, but maybe that's because of surface oxides

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geordief
Post  Post subject: Re: thermos flask  |  Posted: Sat Apr 15, 2017 10:05 pm

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marnixR wrote:
how do they make the aluminium so shiny ? my first impression of aluminium as a metal is that looks dull, but maybe that's because of surface oxides

Alu foil is not dull (it is one one side ,though)
On the other hand it doesn't seem easy to shine/polish up an aluminium saucepan to that degree of brightness.

Maybe with a fine enough abrasive and buffing you might get my aluminium kettle to sparkle like a mirror :-)

If , during the manufacture the aluminium is cooled and solidified from molten I imagine it will at first have an extremely shiny,reflective surface.


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Dywyddyr
Post  Post subject: Re: thermos flask  |  Posted: Sun Apr 16, 2017 1:22 am
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marnixR wrote:
how do they make the aluminium so shiny ? my first impression of aluminium as a metal is that looks dull, but maybe that's because of surface oxides

Tsk, you haven't seen aluminium as a metal (unless very lucky/ quick).
Ally oxidises extremely rapidly, but only to a depth of a couple of molecules or so and that oxide layer protects the rest of the metal.
Pure ally is very shiny (and then it's gone before you can say "My god, that's bright), but even with the (normal) oxide coating it can clean up quite nicely: https://www.google.co.uk/search?q=alumi ... 06&bih=844


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marnixR
Post  Post subject: Re: thermos flask  |  Posted: Sun Apr 16, 2017 10:26 am
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admittedly, i'm only a ferrous metallurgist - to me aluminium is the stuff you add to liquid steel to kill it
and don't talk to me about aluminium drinks cans - the word alone leaves a dirty taste in the mouth to anyone who's worked in the tinplate business

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