From: dsc@gemini.tmc.edu (Doug S. Caprette Bldg. 28 W191 x3892)
Subject: Re: Cleaning a mirror
Organization: NASA GSFC CDP VLBI
Date: Fri, 28 Aug 1992 14:27:38 GMT

In article <27AUG199221480457@rigel.tamu.edu> wdb3926@rigel.tamu.edu (BRUTON, WILLIAM DANIEL) writes:
>In article <BtnJ6A.6yJ@zorch.SF-Bay.ORG>, mike@zorch.SF-Bay.ORG 
>(Mike Smithwick) writes...
>> 
>>What is the recommended way to clean a mirror? I have an Astroscan
>>and the mirror is getting coated with a layer of crud. Can I use
>>camera lens cleaning fluids or would those damage the surface?
>> 
>>mike
>> 
>>Mike Smithwick - ames!zorch!mike
>> 
>	Not sure about the camera lens cleaning solution....
>This is not a recomended way to clean a mirror, but if all other techniques
>do not work then I use metal polish.....yes metal polish.  Sounds crazy
>because it removes a lot of metal.  But if one is careful one can get an
>ok finish.  I only do this if it's a very old mirror and when there
>is a lot of metal oxidation.
>	This is a last resort technique for us.  If it is not successful
                  ^^^^^^^^^^^^^^^^^^^^^
                         No shit!

>then I suppose one can remove the metal and re-silver the mirror.
>
>Has anyone ever re-silvered a mirror before?  How big of a pain was it?
>

Silvered mirrors have been obsolete for about 60 years or so, with the
exception of a few specialized application.  No wonder the ones you've 
been cleaning are heavily oxidized!  Aluminum is the most common mirror
coating for astronomical use.  There are several firms who advertise
in S&T and Astronomy who realuminize mirrors.  Most (probably all) will 
remove the old aluminum first.  Call and ask about packing and shipping
instructions.  I have had good results from Evaporated Metals, Ithaca, NY.

I have heard of three methods for removing old aluminum coatings on mirrors:

1) Dilute hydrocloric acid. (Concentrated Nitric acid does NOT work.)  This
   has been used for decades.  It is sometimes ineffective on overcoated
   mirrors.  There is some speculation that it can leach the aluminum out 
   from under the coating and leave a porous surface on the mirror.  I have 
   done this myself, it seemed to work fine.

2) Dilute sodium hydroxide.  This is reputed to work well, even for overcoated
   mirrors.  However, prolonged exposure to sodium hydroxide etches Pyrex,
   so don't let it soak.  I recommend rinsing the mirror in dilute acid 
   (Vinegar should be fine) immediately after the coating is removed.

3) Rubbing the old coating off with cheesecloth and rouge.  Egads!!!!
   I shudder to think of this being done to a fine mirror!!!!

PLEASE, whenever working with acids and/or bases (caustcs) wear gloves,
eye protection (a face shield is good!) etc.  And please don't mess with
the strong chemicals unless you've had the labratory experience to provide
you with the necessary safety techniques.

>***********************************************************************
>Dan
>Texas A&M Observatory 
 ^^^^^^^^^^^^^^^^^^^^^
 Are you sure you don't mean Lord Rosse's observatory?

--
dsc@gemini.gsfc.nasa.gov  
 |  Regards,         |   Hughes STX                |    Code 926.9 GSFC        |
 |  Doug Caprette    |   Lanham, Maryland          |    Greenbelt, MD  20771   |
 -------------------------------------------------------------------------------
"Alas for the wailing of the gulls!  Did not the Lady tell me to beware 
of them?"                         -- Legolas son of Thranduil
Newsgroups: sci.astro