RE: [ATM] 'Commercial' annealed, rather than 'fine' annealed

[Mark Cowan <toolontop@yahoo.com>]

Hi all:

Art's description of machining stress reduction on
materials (I'm reading metals) is quite apt, but the
case with glass has another factor that comes into
play...

Annealing whether commercial, fine, or precision (all
measurable using polarimeters) in glass affects how
good a surface one can reliabily apply to the mirror. 
Newport's catalog indicates that "fine annealead piece
of glass should be more than adequate for a surface
quality not exceeding a quarter wave".  Indeed, years
ago when I first ordered a precision annealed piece,
they asked "are you sure you can make the surface good
enough to have that matter?"  Well the short answer is
yes.  

By their notes, commercial anneal is rated as greater
than 10nm stress, fine anneal 10nm or less, and
precision anneal 6nm or less.  Finer grades take
longer - much longer.  Precision annealing takes about
2-1/2 weeks in their ovens. 

The sheet stock comes in huge sheets the size of
semi-trailers from Corning, it's already commercial or
fine annealed and they cut large blanks from it to
either 1-5/8, 1-7/8, or 2-1/8 thick.  To get a 24x3"
blank they would mold it instead, then they do the
annealing.

I've only ever had one (rushed) piece arrive from
Newport that showed strain under the polarizer, but it
made a fine mirror, though I wouldn't trust it beyond
tenth wave.  Why?

OK - when the temp changes the strains in the glass
can shift around - think of it as a homogeneous solid
with no long range structure (ie, classical glass). 
Ignore that Pyrex is a mixture of stuff that isn't
necessarily mixed completely.

If the temp differs enough from the temp at which the
glass was worked and finished, the figure can shift in
unpredictable ways.  I've read ample stories about
this effect.  The strain that's left in the glass is
enough to warp the surface, obviously, otherwise
nobody would care about annealing at all.  Break a
piece of tempered glass if you want to see strain
release in action. ;)

Use the mirror at your workshop temp and there's no
change in the surface.  Change the temp and it can
change.

That's factor one (sorry, I lied).  Factor two is the
surface reacting to working itself - the admonition to
grind the back flat before finishing the front, for
one, and that as you work the front you can pass
through different layers of stress accomodation now
brought to the surface.  These will cause nasty
effects on the surface regularity as they try to
reduce their stress upon exposure.

Your mileage may vary, but the better annealing helps
to get a better result, IMHO.

Cheers,
Mark Cowan
Salem, OR


	
		
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