Re: Are Aluminum Mirrors in reach of the ATMs?

["John Lynch" <john_p_lynch@hotmail.com>]

Malcolm,

Out of curiosity after seeing numerous other posts regarding the
aluminum mirror question, I went back and more carefully read the
chapter of the book that I referred to in my last E-mail message.
Oddly enough, it reinforced many points that some of the folks on
this listserv have already made.  Summarizing this article, the
author points out that:

1.  It is not suggested that better mirrors can be made of metal
    than of glass, and a comparison shows that glass is the         
preferred material.
2.  Glass held a decisive advantage over metal in the days when
    mirrors were silvered, and a tarnished silver mirror coating
    can be removed and replaced at home in a single evening
    (remember this was written in 1963!).  Now aluminum has replaced
    silver, but requires a protective film of oxide.  However,         
applying an aluminum coating requires an apparatus beyond the         reach 
of most amateurs, who must send their mirrors away to be        coated.  A 
small solid mirror can be repolished at home in an         evening.
3.  The polished aluminum still requires the protective oxide
    coating (I thought this also required the vacuum chamber
    process, which kind of defeats the argument).
4.  Rough shaping of the mirror and tool can be done by hand
    using either a scraper or file, or a lathe can be used.
5.  Progressive fine grinding is done in the usual manner with
    only 220 and 320 carborundum, and polishing is accomplished
    using a HARD pitch lap with, initially, 320 carborundum,
    then pumice (the author claims that AJAX powered cleaner
    works wonderfully for this purpose), and then a new lap is
    made and rouge is used for the final polishing process.  He         
strongly indicates that this process can be successful only
    if HARD pitch is used for the faceted polishing lap.
6.  Once polishing with rouge begins, the mirror should never be         
removed from the lap for extended periods.
7.  The last point refers to the thermal properties of both
    substrates (glass and aluminum).  He states that glass retains
    its shape except during periods of changing temperature, but a     solid 
metal mirror retains its optical figure during changes
    of temperature that would put a glass mirror out of business.

That sort of sums up the points of the article.  In my opinion,
sounds like a neat project if only for the novelty.  Otherwise,
it seems as if it's too much trouble for the work you have to
put into it, and, like I said before, he limits the discussion
to small solid metal mirrors with long focal lengths, i.e.,
spherical.  He also stated that he's made plenty of mirrors using stainless 
steel.

Regards,

John Lynch
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