Re: [ATM] Does glass retain moisture?

["john sherman" <atm@mail.johnspics.com>]

>Most glass has a hygroscopic surface. 


Hi Mark,

Thank you for the great post. I look forwards to the times you let loose with these long posts. I seem to learn a lot from you. 

>Pyrex, to my knowledge, does not absorb much water 
>internally, though it 
>does have surface adsorbed water

Maybe that could turn out to be the answer to my question?

Richard pointed out that if the glass absorbs water equally the figure should not change. And his explanation sounds quite similar to the explanation about why the temperature doesn't affect the figure, just the focal length. 

I remember once I had a nice flat piece of Rosewood. I put some polyurethane one one side to see how it'd look. The next day I learned my mistake. The coated side was now concave. The uncoated side was convex, presumably it had absorbed moisture and expanded, the other had not. 

Perhaps the aluminized side of our Pyrex disks perform the same function? The back side and the edge of the glass could absorb more surface moisture than the front side. If it works the same as my flimsy example, in the summer when there is more moisture the figure on the mirror would be less corrected, more concave.

Does that sound feasible?

Thanks,

John




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