ATM Re: coatings & such + question ( long )

[hclaus@clausing.com]

    As of late,  there have been numerous postings regarding optical coatings.  This is 
understandable since even in the world of ATM,  coatings still remain a somewhat arcane 
science.  The recent complaint from Mr. Bock has brought attention to a number of issues. 
 Granted,  the ATM listserv is no place for personal bickering,  but I  think a general 
reply may be informative to the ATM community at large.   

    Mr. Bock's 20" was brought in for re-coating in 2/96 .   There was substantial 
deterioration in some sections of the coated surface.  This, in fact, was precisely why 
he was having it re-coated.  After we chemically removed the coating, there were stains 
remaining in the glass, in the very same regions where the coating had most seriously 
deteriorated.   

    Most of the time coatings may be simply removed & the mirror perfectly reclaimed.  In 
the above case,  the process of coating deterioration had also stained the glass.  This 
situation is totally beyond my ( or any other vacuum coater's ) control.  This type of 
damage was occurring long before the mirror was ever submitted to me for re-coating. 

    I can not & will not be responsible for damage that I did NOT do.

    The problem is that the above situation leaves the illusion that something went wrong 
in the coating process,  when in fact,  nothing went wrong at all !

    The lessons to be learned from the above situation are:

    1)  Never let moisture stand for prolonged periods of time on your mirror.

    2)  Never let moisture air dry on the surface of your mirror.

    3)  Never let a coating deteriorate so severely that either the glass is exposed,  or 
the chemical reactions involved stain the glass. 

    
    Most of the time, coatings come off beautifully,  even on very badly deteriorated 
surfaces.  But there is always the occasional exception.

    I know there exists an extremely deep talent pool on this listserv,  so I'd like to 
pose some open questions to all of you chemists out there .....

    ( Please reply via private email as not to clutter the listserv )

    The staining left behind appeared to be some type of Aluminum Oxide.  The oxide was 
non reactive in HCl or Ferric Chloride.

    1)  What conditions oxidize Aluminum to this state ?

    2)  Are there any chemicals that will safely remove this stable oxide ?


    If I get a workable solution,  I will gladly re-coat Mr. Bock's mirror.  ( And do 
some coating for the bright chemist who solves the problem :-)    )  

    As for you,  Mr. Bock,  I am sorry the situation turned out as it did.  I will not 
use aggressive methods to remove a coating or it's subsequent stains.  That is beyond the 
scope of our service.

    Thank you all for your indulgence.


Regards,

H.C.

http://www.clausing.com